The role of an equine nutritionist in equine health, performance and wellbeing: Ideas stemming from an equine science society symposium workshop.

In the United States, there is little clarity on the qualifications and availability of equine nutritionists. Currently, no regulatory body exists for formal credentialing outside of veterinary medicine. Most equine nutritionists are not veterinarians but do have advanced scientific degrees (Master of Science and/or Doctor of Philosophy) in the field of Animal Science. However, not all reporting to be equine nutritionists have formal education in the field of equine nutrition. To discuss this, a workshop was held at the 2023 Equine Science Society (ESS) meeting. The purpose of this discussion was to share ideas among equine nutrition professionals about how best to provide support for the inclusion of the specialty as part of a horse's health team, alongside the veterinarian, farrier and other equine health specialists. In human, small animal and livestock practices, the importance of nutrition as part of an overall health, production (livestock) and well-being plan has been documented. However, surveys of veterinarians, the top source of information for horse owners, reveal a lack of confidence in the area of nutrition after graduating from veterinary school and a lack of available continuing education opportunities to learn more. Further, it appears that many horse owners may unknowingly be obtaining nutrition information from unverified sources (such as the internet). The discussion included formal and informal education of equine nutritionists, as well as avenues to open lines of communication with the veterinary community to provide nutrition resources for horse owners, managers and veterinarians.

Outcomes of complex abdominal wall reconstruction in patients with connective tissue disorders: a single center experience.

INTRODUCTION: Individuals diagnosed with connective tissue disorders (CTD) are known to be predisposed to incisional hernia formation. However, there is a scarcity of data on outcomes for these patients undergoing hernia repair. We sought to describe our outcomes in performing abdominal wall reconstructions in these complex patients. METHODS: Adult patients with CTD undergoing open, elective, posterior component separation with permanent synthetic mesh at our institution from January 2018 to October 2022 were queried from a prospectively collected database in the Abdominal Core Health Quality Collaborative. We evaluated 30-day wound morbidity, perioperative complications, long-term hernia recurrence, and patient-reported quality of life. RESULTS: Twelve patients were identified. Connective tissue disorders included Marfan's n = 7 (58.3%), Loeys-Dietz syndrome n = 2 (16.7%), Systemic Lupus Erythematosus n = 2 (16.7%), and Scleroderma n = 1 (8.3%). Prior incisions included three midline laparotomies and nine thoracoabdominal, mean hernia width measured 14 cm, and 9 were recurrent hernias. Surgical site occurrences (SSOs) were observed in 25% of cases, and 16.7% necessitated procedural intervention. All twelve patients were available for long-term follow-up, with a mean of 34 (12-62) months. There were no instances of reoperation or mesh excision related to the TAR procedure. One patient developed a recurrence after having his mesh violated for repair of a new visceral aneurysm. Mean HerQLes scores at 1 year were 70 and 89 at ≥ 2 years; Mean scaled PROMIS scores were 30.7 at 1 year and 36.3 at ≥ 2 years. CONCLUSION: Ventral hernia repair with TAR is feasible in patients with connective tissue disorder and can be a suitable alternative in patients with large complex hernias.

Association of active smoking on 30-day wound events and additional morbidity and mortality following inguinal hernia repair with mesh: an analysis of the ACHQC database.

BACKGROUND: To date, there is limited data on the association of active smoking and 30-day wound events following inguinal hernia repair (IHR) with mesh. We aimed to determine if active smoking at the time of IHR with mesh was associated with worse 30-days wound events and additional morbidity outcomes using the Abdominal Core Health Quality Collaborative (ACHQC) database. METHODS: All adult patients undergoing elective, IHR with mesh who had 30-day follow-up data available were identified within the ACHQC database. Smokers were defined as having used nicotine within the 30 days prior to surgery. A 1:1 propensity score matched analysis was performed comparing smokers to non-smokers, controlling for factors previously shown to be associated with postoperative wound events. The effect of smoking on 30-day wound events and additional morbidity outcomes following IHR with mesh was investigated using Chi-square or Fisher's exact test for categorical data and Wilcoxon ranked test for continuous data. RESULTS: A total of 17,543 patients met inclusion criteria; 1855 (11%) were active smokers at the time of minimally invasive IHR with mesh. A total of 3694 patients were used for the matched analysis. There were no statistically significant differences between the non-smokers and smokers with respect to the incidence of surgical site infection (p = 0.10), surgical site occurrences (p = 0.22), or surgical site occurrences requiring procedural intervention (p = 0.64). Non-smokers were significantly more likely to be readmitted to the hospital and had significantly less improvement in all pain domains following IHR with mesh. CONCLUSIONS: Active smoking at the time of IHR with mesh is not associated with worse 30-day wound or additional morbidity and mortality outcomes. Based on these results, preoperative smoking cessation for all patients undergoing IHR may not reduce 30-day morbidity.

Comparing rates of bowel injury for laparoscopic and robotic ventral hernia repair: a retrospective analysis of the abdominal core health quality collaborative.

PURPOSE: Bowel injury during laparoscopic and robotic ventral hernia repair is a rare but potentially serious complication. We sought to compare bowel injury rates during minimally invasive approaches to ventral hernia repair using a national hernia registry. METHODS: Patients undergoing elective laparoscopic and robotic ventral hernia repair (including cases converted-to-open) between 2013 and 2021 were retrospectively identified in the Abdominal Core Health Quality Collaborative registry. The primary outcome was bowel injury, which included partial- and full-thickness injuries and re-operations for missed enterotomies. Statistical analysis was performed using multivariate logistic regression. RESULTS: Overall, 10,660 patients were included (4116 laparoscopic, 6544 robotic). The laparoscopic group included more incisional hernias (68% vs 62%, p < 0.001) and similar rates of recurrent hernias (23% vs 22%, p = 0.26). A total of 109 bowel injuries were identified, with more occurring in the laparoscopic group (55 [1.3%] laparoscopic vs. 54 [0.8%] robotic; p = 0.01). Specifically, there were more full-thickness and missed enterotomies in the laparoscopic group (29 laparoscopic vs. 20 robotic; p = 0.012). Bowel injury resulted in higher rates of wound morbidity and major post-operative complications including sepsis, re-admission, and re-operation. Following adjustment for recurrent and incisional hernias, prior mesh, patient age, and hernia width, bowel injury during laparoscopic repair remained significantly more likely than bowel injury during robotic repair (OR 1.669 [95% C.I.: 1.141-2.440]; p = 0.008). CONCLUSION: In a large registry, laparoscopic ventral hernia repair is associated with an increased risk of bowel injury compared to repairs utilizing the robotic platform. Knowing the limitations of retrospective research, large national registries are well suited to explore rare outcomes which cannot be feasibly assessed with randomized controlled trials.

Early repair of ventral incisional hernia may improve quality of life after surgery for abdominal malignancy: a prospective observational cohort study.

PURPOSE: Recent work has shown that over 40% of patients undergoing surgery for abdominal malignancy develop ventral incisional hernias (VIH) within 2 years. We hypothesized that early repair of VIH for cancer survivors could improve long-term quality of life (QoL). METHODS: All patients presenting with a history of surgery for abdominal malignancy and a VIH were prospectively enrolled. QoL was assessed at baseline and 3-, 6-, 12-, 18-, and 24-month follow-up using abdominal wall-specific (HerQLes) and cancer-specific (FACT-G) instruments. At the study's conclusion, patients were divided into 2 groups-those that underwent VIH repair during the study's course (Repair Group) and those that did not (Control Group). Categorical variables were analyzed using Pearson's Chi-square and continuous variables with Wilcoxon rank sum test. RESULTS: Eighty-four patients were enrolled. Overall, 46 patients (55%) underwent VIH repair, with 36 repairs (78%) occurring within 3 months of initial evaluation. Sixty-six (79%) had complete 1-year follow-up data, and 30 (36%) had 2-year data, with a median follow-up duration of 15.6 months. At baseline, both groups were similar with respect to demographics, cancer stage, and HerQLes/FACT-G scores. Compared to the Controls, the Repair Group showed greater improvements over baseline HerQLes Summary Scores at the 3-, 6-, 12-, and 18-month time points (median increase, 37 vs. 26 points), and in FACT-G total scores at the 3-, 6-, and 12-month time points (median increase, 6 vs. 4 points). CONCLUSIONS: Repair of VIH after surgery for abdominal malignancy may improve abdominal wall-specific and cancer-specific QoL, making post-resection abdominal wall reconstruction an important aspect of cancer survivorship.

Frequency of endoscopic surveillance for Barrett's esophagus is influenced by health insurance status: results from a population-based analysis.

Factors that influence the frequency of surveillance endoscopy for nondysplastic Barrett's esophagus are not well understood. The objective of this study is to assess factors which influence the frequency of endoscopic surveillance for Barrett's esophagus, including health insurance/third-party payer status. Cases of nondysplastic Barrett's esophagus undergoing esophagogastroduodenoscopy with biopsy were identified using longitudinal data from the Healthcare Utilization Project database in 2005-2006 and followed through 2011. The threshold for appropriate surveillance utilization was defined as two to four surveillance esophagogastroduodenoscopies over a standardized 5-year period. Patients' insurance status was designated as either Medicare, Medicaid, private, or noninsured. 36,676 cases of nondysplastic Barrett's esophagus were identified. Among these, 4,632 patients (12.6%) underwent between two and four surveillance esophagogastroduodenoscopies in 5 years of follow-up versus 31,975 patients (87.3%) who underwent fewer than two esophagogastroduodenoscopies during follow-up. Multivariate analysis found that Barrett's patients insured through Medicaid (OR 1.273; 95% CI = 1.065-1.522) or without insurance (OR = 2.453; 95% CI = 1.67-3.603) were at increased likelihood of being under-surveilled. This study identified a difference in frequency of surveillance esophagogastroduodenoscopy for Barrett's esophagus by payer status. Patients without health insurance and those whose primary insurance was Medicaid were at increased odds for under-surveillance. These data suggest that a more robust system for tracking and ensuring longitudinal follow-up of patients with Barrett's esophagus, with attention to the uninsured and underinsured population, may be needed to ensure optimal surveillance.

Norepinephrine transporter expression is inversely associated with glycaemic indices: a pilot study in metabolically diverse persons with overweight and obesity.

OBJECTIVE: The objective of this study was to examine the cross-sectional relationship between the expression of norepinephrine transporter (NET), the protein responsible for neuronal uptake-1, and indices of glycaemia and hyperinsulinaemia, in overweight and obese individuals. METHODS: Thirteen non-medicated, non-smoking subjects, aged 58 ± 1 years (mean ± standard error of the mean), body mass index (BMI) 31.4 ± 1.0 kg m-2, with wide-ranging plasma glucose and haemoglobin A1c (HbA1c, range 5.1% to 6.5%) participated. They underwent forearm vein biopsy to access sympathetic nerves for the quantification of NET by Western blot, oral glucose tolerance test (OGTT), euglycaemic hyperinsulinaemic clamp, echocardiography and assessments of whole-body norepinephrine kinetics and muscle sympathetic nerve activity. RESULTS: Norepinephrine transporter expression was inversely associated with fasting plasma glucose (r = -0.62, P = 0.02), glucose area under the curve during OGTT (AUC0-120, r = -0.65, P = 0.02) and HbA1c (r = -0.67, P = 0.01), and positively associated with steady-state glucose utilization during euglycaemic clamp (r = 0.58, P = 0.04). Moreover, NET expression was inversely related to left ventricular posterior wall dimensions (r = -0.64, P = 0.02) and heart rate (r = -0.55, P = 0.05). Indices of hyperinsulinaemia were not associated with NET expression. In stepwise linear regression analysis adjusted for age, body mass index and blood pressure, HbA1c was an independent inverse predictor of NET expression, explaining 45% of its variance. CONCLUSIONS: Hyperglycaemia is associated with reduced peripheral NET expression. Further studies are required to identify the direction of causality.

Evaluation of long-term surgical site occurrences in ventral hernia repair: implications of preoperative site independent MRSA infection.

PURPOSE: Previous work demonstrated that prior MRSA infection [MRSA(+)] is associated with 30-day surgical site infection (SSI) following ventral hernia repair (VHR). We aimed to determine the impact of MRSA(+) on long-term wound outcomes after VHR. PARTICIPANTS: A retrospective cohort study was performed at a tertiary center between July 11, 2005, and May 18, 2012, of patients undergoing elective VHR with class I wounds. Patients with documented preoperative MRSA infection at any site (urinary, bloodstream, SSI, etc.) were considered MRSA(+). Primary outcome was 2-year surgical site occurrence (SSO), defined as SSI, cellulitis, necrosis, nonhealing wound, seroma, hematoma, dehiscence, or fistula. SSOs were subdivided into those that required procedural intervention (SSOPI) and those that did not. RESULTS: Among 632 patients, 46 % were female with average age 53 ± 13 years. There were 368 SSOs in 193 patients (31 %); an SSOPI occurred in 9.8 % (n = 62). The most common SSOs were cellulitis (91/632), seroma (91/632), and serous drainage (58/632). The rate of 2-year SSO was higher with MRSA(+) compared to those without (46 vs. 29 %, p = 0.023), attributed to increased soft tissue necrosis, purulent drainage, serous drainage, cellulitis, and fistula. In multivariable analysis, MRSA(+) was not associated with 2-year SSO (HR 1.5, 95 % CI 0.91-2.55, p = 0.113); factors associated with SSO included obesity, immunosuppression, mesh repair, and operative times. CONCLUSIONS: This study is the first to evaluate long-term SSOs and SSOPIs after VHR, highlighting the importance of long-term follow-up. Though not independently associated with SSOs, MRSA(+) may be a marker of hernia complexity.

Survival Benefit of Adjuvant Radiation Therapy for Gastric Cancer following Gastrectomy and Extended Lymphadenectomy.

Purpose. Although randomized trials suggest a survival benefit of adjuvant chemotherapy and radiation therapy (XRT) for gastric adenocarcinoma, its use in patients who undergo an extended lymphadenectomy is less clear. The purpose of this study was to determine if a survival benefit exists in gastric cancer patients who receive adjuvant XRT following resection with extended lymphadenectomy. Methods. The SEER registry was queried for records of patients with resected gastric adenocarcinoma from 1988 to 2007. Multivariable Cox regression models were used to assess independent prognostic factors affecting overall survival (OS) and disease-specific survival (DSS). Results. Of 15,060 patients identified, 3,208 (21%) received adjuvant XRT. Adjuvant XRT was independently associated with improved OS (HR 0.67, CI 0.64-0.71) and DSS (HR 0.69, CI 0.65-0.73) in stages IB through IV (M0). This OS and DSS benefit persisted regardless of the extent of lymphadenectomy. Furthermore, lymphadenectomy with >25 LN resected was associated with improved OS and DSS compared with <15 LN or 15-25 LN. Conclusion. This population-based study shows a survival benefit of adjuvant XRT following gastrectomy that persists in patients who have an extended lymphadenectomy. Furthermore, removal of >25 LNs results in improved OS and DSS compared with patients who have fewer LNs resected.

Blood pressure as a predictor of cardiovascular events in the elderly: the William Hale Research Program.

This study evaluates the association between blood pressure (BP) and the risk of developing cardiovascular disease (CVD) events in the elderly. The Morton Plant Mease Foundation has followed 4,008 elderly patients >64 years of age for at least 5 years. Systolic and diastolic blood pressure (SBP and DBP) was divided into categories. Cardiovascular disease events were classified as myocardial infarction, stroke, and CVD-related deaths reported from the National Death Index. Cox proportional hazard ratios were used to assess the relationship between BP and CVD events and controlled for weight, gender, smoker, and alcohol use. Ages <75 and >or=75 years were assessed separately. After 11.1 years of follow-up, elevated SBP (P=<0.0001) is strongly associated with developing a future CVD event; the relationship is linear and graded and holds for ages above and below 75 years. The frequency of CVD events was lowest in the SBP <120 mm Hg group. In subjects <75 years of age, DBP elevations were not a significant risk factor for CVD events. (relative risk (RR): DBP 70 to <80 mm Hg=0.92; DBP 80 to <90 mm Hg=0.88; DBP >or=90 mm Hg=1.02.) With subjects >or=75 years of age, a DBP between 80 and 90 is associated with the lowest significant risk for CVD (RR: DBP 70 to <80 mm Hg=0.74; DBP 80 to <90 mm Hg=0.59; DBP >or=90=0.71). In conclusion, these findings support the Joint National Committee on Hypertension recommendations for SBP in the elderly. Further studies are warranted to identify optimal DBP for the elderly at various ages.

Structural and kinetic studies of a series of mutants of galactose oxidase identified by directed evolution.

Galactose oxidase (GO; E.C. 1.1.3.9) is a copper- containing enzyme that oxidizes a range of primary alcohols to aldehydes. This broad substrate specificity is reflected in a high K(M) for substrates. Directed evolution has previously been used to select variants of GO that exhibit enhanced expression and kinetic properties. In assays using unpurified enzyme samples, the variant C383S displayed a 5-fold lower K(M) than wild-type GO. In the present study, we have constructed, expressed, purified and characterized a number of single, double and triple mutants at residues Cys383, Tyr436 and Val494, identified in one of the directed evolution studies, to examine their relative contributions to improved catalytic activity of GO. We report kinetic studies on the various mutant enzymes. In addition, we have determined the three-dimensional structure of the C383S variant. As with many mutations identified in directed evolution experiments, the availability of structural information does not provide a definitive answer to the reason for the improved K(M) in the C383S variant protein.

Cofactor processing in galactose oxidase.

Galactose oxidase (GO; EC 1.1.3.9) is a monomeric 68 kDa enzyme that contains a single copper and an amino acid-derived cofactor. The mechanism of this radical enzyme has been widely studied by structural, spectroscopic, kinetic and mutational approaches and there is a reasonable understanding of the catalytic mechanism and activation by oxidation to generate the radical cofactor that resides on Tyr-272, one of the copper ligands. Biogenesis of this cofactor involves the post-translational, autocatalytic formation of a thioether cross-link between the active-site residues Cys-228 and Tyr-272. This process is closely linked to a peptide bond cleavage event that releases the N-terminal 17-amino-acid pro-peptide. We have shown using pro-enzyme purified in copper-free conditions that mature oxidized GO can be formed by an autocatalytic process upon addition of copper and oxygen. Structural comparison of pro-GO (GO with the prosequence present) with mature GO reveals overall structural similarity, but with some regions showing significant local differences in main chain position and some active-site-residue side chains differing significantly from their mature enzyme positions. These structural effects of the pro-peptide suggest that it may act as an intramolecular chaperone to provide an open active-site structure conducive to copper binding and chemistry associated with cofactor formation. Various models can be proposed to account for the formation of the thioether bond and oxidation to the radical state; however, the mechanism of prosequence cleavage remains unclear.

Copper amine oxidase: cunning cofactor and controversial copper.

Copper amine oxidases have a complex reaction cycle that converts a primary amine and molecular oxygen into the aldehyde, ammonia and hydrogen peroxide. Coupling structural studies of freeze-trapped reaction intermediates in crystals with kinetic and spectroscopic experiments in solution has generated a detailed molecular picture of catalysis. Although dioxygen has been directly observed bound to the copper at a late stage in the reaction cycle, whether copper is the initial binding site remains controversial.

Crystal structure of the precursor of galactose oxidase: an unusual self-processing enzyme.

Galactose oxidase (EC ) is a monomeric enzyme that contains a single copper ion and catalyses the stereospecific oxidation of primary alcohols to their corresponding aldehydes. The protein contains an unusual covalent thioether bond between a tyrosine, which acts as a radical center during the two-electron reaction, and a cysteine. The enzyme is produced in a precursor form lacking the thioether bond and also possessing an additional 17-aa pro-sequence at the N terminus. Previous work has shown that the aerobic addition of Cu(2+) to the precursor is sufficient to generate fully processed mature enzyme. The structure of the precursor protein has been determined to 1.4 A, revealing the location of the pro-sequence and identifying structural differences between the precursor and the mature protein. Structural alignment of the precursor and mature forms of galactose oxidase shows that five regions of main chain and some key residues of the active site differ significantly between the two forms. The precursor structure provides a starting point for modeling the chemistry of thioether bond formation and pro-sequence cleavage.

Conserved tyrosine-369 in the active site of Escherichia coli copper amine oxidase is not essential.

Copper amine oxidases are homodimeric enzymes that catalyze two reactions: first, a self-processing reaction to generate the 2,4,5-trihydroxyphenylalanine (TPQ) cofactor from an active site tyrosine by a single turnover mechanism; second, the oxidative deamination of primary amine substrates with the production of aldehyde, hydrogen peroxide, and ammonia catalyzed by the mature enzyme. The importance of active site residues in both of these processes has been investigated by structural studies and site-directed mutagenesis in enzymes from various organisms. One conserved residue is a tyrosine, Tyr369 in the Escherichia coli enzyme, whose hydroxyl is hydrogen bonded to the O4 of TPQ. To explore the importance of this site, we have studied a mutant enzyme in which Tyr369 has been mutated to a phenylalanine. We have determined the X-ray crystal structure of this variant enzyme to 2.1 A resolution, which reveals that TPQ adopts a predominant nonproductive conformation in the resting enzyme. Reaction of the enzyme with the irreversible inhibitor 2-hydrazinopyridine (2-HP) reveals differences in the reactivity of Y369F compared with wild type with more efficient formation of an adduct (lambda(max) = 525 nm) perhaps reflecting increased mobility of the TPQ adduct within the active site of Y369F. Titration with 2-HP also reveals that both wild type and Y369F contain one TPQ per monomer, indicating that Tyr369 is not essential for TPQ formation, although we have not measured the rate of TPQ biogenesis. The UV-vis spectrum of the Y369F protein shows a broader peak and red-shifted lambda(max) at 496 nm compared with wild type (480 nm), consistent with an altered electronic structure of TPQ. Steady-state kinetic measurements reveal that Y369F has decreased catalytic activity particularly below pH 6.5 while the K(M) for substrate beta-phenethylamine increases significantly, apparently due to an elevated pK(a) (5.75-6.5) for the catalytic base, Asp383, that should be deprotonated for efficient binding of protonated substrate. At pH 7.0, the K(M) for wild type and Y369F are similar at 1.2 and 1.5 microM, respectively, while k(cat) is decreased from 15 s(-1) in wild type to 0.38 s(-1), resulting in a 50-fold decrease in k(cat)/K(M) for Y369F. Transient kinetics experiments indicate that while the initial stages of enzyme reduction are slower in the variant, these do not represent the rate-limiting step. Previous structural and solution studies have implicated Tyr369 as a component of a proton shuttle from TPQ to dioxygen. The moderate changes in kinetic parameters observed for the Y369F variant indicate that if this is the case, then the absence of the Tyr369 hydroxyl can be compensated for efficiently within the active site.

Phosphatidylinositol transfer proteins and protein kinase C make separate but non-interacting contributions to the phosphorylation state necessary for secretory competence in rat mast cells.

Mast cells permeabilized by streptolysin O undergo exocytosis when stimulated with Ca(2+) and guanosine 5'-[gamma-thio]triphosphate but become progressively refractory to this stimulus if it is delayed. This run-down of responsiveness occurs over a period of 20-30 min, during which the cells leak soluble and tethered proteins. We show here that withdrawal of ATP during the process of run-down is strongly inhibitory but that as little as 25 microM ATP can extend responsiveness significantly; this effect is maximal at 50 microM. When phosphatidylinositol transfer proteins (PITPs) are provided to cells at the time of permeabilization, run-down is retarded. We conclude that in the presence of ATP they convey substrates for phosphorylation that are essential for exocytosis and thus interact with the regulatory machinery. Furthermore, we show that PITPalpha and PITPbeta have additive effects in this mechanism, suggesting that they are not functionally redundant. Alternatively, secretion from run-down cells can be inhibited by the aminoglycoside antibiotic neomycin, which is understood to bind to phosphoinositide headgroups, and by a PH (pleckstrin homology) domain polypeptide that binds phosphoinositides. The apparent displacement of neomycin by exogenous PITPs suggests that these proteins screen essential lipids. Secretion from run-down cells is also inhibited by 1-O-hexadecyl-2-O-methyl-rac-glycerol (AMG-C(16)), an inhibitor of protein kinase C. The lack of synergy between neomycin and AMG-C(16) suggests that protein kinase C independently provides a second essential component through protein phosphorylation and that there are two independent phosphorylation pathways necessary for secretion competence.

The active site of the junction-resolving enzyme T7 endonuclease I.

Endonuclease I is a junction-resolving enzyme encoded by bacteriophage T7, that selectively binds and cleaves four-way DNA junctions. We have recently solved the structure of this dimeric enzyme at atomic resolution, and identified the probable catalytic residues. The putative active site comprises the side-chains of three acidic amino acids (Glu20, Asp55 and Glu65) together with a lysine residue (Lys67), and shares strong similarities with a number of type II restriction enzymes. However, it differs from a typical restriction enzyme as the proposed catalytic residues in both active sites are contributed by both polypeptides of the dimer. Mutagenesis experiments confirm the importance of all the proposed active site residues. We have carried out in vitro complementation experiments using heterodimers formed from mutants in different active site residues, showing that Glu20 is located on a different monomer from the remaining amino acid residues comprising the active site. These experiments confirm that the helix-exchanged architecture of the enzyme creates a mixed active site in solution. Such a composite active site structure should result in unilateral cleavage by the complemented heterodimer; this has been confirmed by the use of a cruciform substrate. Based upon analogy with closely similar restriction enzyme active sites and our mutagenesis experiments, we propose a two-metal ion mechanism for the hydrolytic cleavage of DNA junctions.

Crystal structure of the Holliday junction resolving enzyme T7 endonuclease I.

We have solved the crystal structure of the Holliday junction resolving enzyme T7 endonuclease I at 2.1 A resolution using the multiwavelength anomalous dispersion (MAD) technique. Endonuclease I exhibits strong structural specificity for four-way DNA junctions. The structure shows that it forms a symmetric homodimer arranged in two well-separated domains. Each domain, however, is composed of elements from both subunits, and amino acid side chains from both protomers contribute to the active site. While no significant structural similarity could be detected with any other junction resolving enzyme, the active site is similar to that found in several restriction endonucleases. T7 endonuclease I therefore represents the first crystal structure of a junction resolving enzyme that is a member of the nuclease superfamily of enzymes.

The tyrosine kinases p53/56lyn and p72syk are differentially expressed at the protein level but not at the messenger RNA level in nonreleasing human basophils.

Within the general population, individuals can be found whose basophils do not secrete after stimulation through the immunoglobulin (Ig) E receptor. In this study we compared two groups of donors, those whose basophils responded with 65+/-16% histamine release to an optimal concentration of anti-IgE antibody and those whose basophil response was not statistically different from nonstimulated release (1+/-1%). We show that these so-called nonreleasing basophils have at least 10-fold lower expression of the tyrosine kinases, lyn and syk, but normal expression of the tyrosine kinase Btk when compared with the panel of releasing basophils. Indeed, maximum histamine release correlated with expression of both syk (Spearman rank correlation coefficient [Rs] = 0.98) and lyn (Rs = 0.93). In contrast, equivalent levels of messenger RNA (mRNA) for lyn and syk kinase were found for both groups. By sequencing a critical region in the syk mRNA, our results also demonstrate that the frame shift mutation in syk leading to a premature stop codon which has been observed in other cell types is not present in nonreleasing human basophils. Our results suggest that there may be translational or post-translational regulatory mechanisms specific to the expression of two important FcepsilonRI-associated signaling elements in basophils.

Direct and indirect readout in mutant Met repressor-operator complexes.

BACKGROUND: The methionine repressor, MetJ, represses the transcription of genes involved in methionine biosynthesis by binding to arrays of two to five adjacent copies of an eight base-pair 'metbox' sequence. Naturally occurring operators differ from the consensus sequence to a greater extent as the number of metboxes increases. MetJ, while accommodating this sequence variation in natural operators, is very sensitive to particular base changes, even where bases are not directly contacted in the crystal structure of a complex formed between the repressor and consensus operator. RESULTS: Here we report the high-resolution structure of a MetJ mutant, Q44K, bound to the consensus operator sequence (Q44Kwt19) and two related sequences containing mutations at sites believed to be important for indirect readout at non-contacted bases. The overall structure of the Q44Kwt19 complex is very similar to the wild-type complex, but there are small variations in sugar-phosphate backbone conformation and direct contacts to the DNA bases. The mutant complexes show a mixture of direct and indirect readout of sequence variations, with differences in direct contacts and DNA conformation. CONCLUSIONS: Comparison of the wild-type and mutant repressor-operator complexes shows that the repressor makes sufficiently strong interactions with the sugar-phosphate backbone to accommodate some variation in operator sequence with minor changes in direct bases contacts. The reduction in repressor affinity for the two mutant repressor complexes can be partially attributed to a loss in direct contacts to the DNA. In one case, however, the replacement of a flexible TA base-step leads to an unfavourable DNA conformation that reduces the stability of the repressor-operator complex.