Comprehensive histological investigation of age-related changes in dermal extracellular matrix and muscle fibers in the upper lip vermilion.

OBJECTIVE: Few histological studies have directly examined age-related changes within the lips, although non-invasive investigations of such changes are increasing. Therefore, this study aimed to provide histological and molecular data on age-dependent alterations in the vermilion. METHODS: Upper vermilion specimens from 15 female Caucasian cadavers (age range, 27-78 years) were investigated histologically or immunohistochemically. RESULTS: Histologically, age-dependent decreases in areas occupied by hyaluronan and collagenous fibres in the dermis of upper vermilion were demonstrated. Elastic fibre content varied widely between individuals. The area occupied by muscle fibres in the orbicularis oris muscle region within the vermilion also correlated negatively with age. Immunohistochemically, signals of four proteins were attenuated in vermilion from older individuals compared with young individuals: procollagen type I, hyaluronan synthase (HAS)1, myosin heavy chain (MYH)2 (a component of fast-twitch oxidative muscle fibres) and MYH7 (a component of slow-twitch muscle fibres). In contrast, signals of cell migration inducing hyaluronidase 1 (CEMIP) were intensified in vermilion from older individuals. No marked differences between young and older individuals were seen in procollagen type III, HAS2, HAS3, hyaluronidase (HYAL)1, HYAL2, MYH1 or MYH4. CONCLUSION: Age-dependent decreases of hyaluronan in the dermis of vermilion were prominent, possibly due to both the decrease in synthesis (HAS1) and the increase in degradation (CEMIP). Furthermore, age-dependent decreases in collagenous fibres and two types of muscle fibre in the vermilion were also identified histologically. Type I collagen, MYH2 and MYH7 appear to represent the molecules responsible for these respective decrements.

OBJECTIF: Peu d'études histologiques ont examiné directement les changements liés à l'âge sur les lèvres, bien que les enquêtes non invasives de ces changements soient en augmentation. Par conséquent, cette étude visait à fournir des données histologiques et moléculaires sur les altérations liées à l'âge dans le vermillon. MÉTHODES: Des échantillons de vermillon supérieur provenant de 15 cadavres de femme Caucasiens (tranche d'âge, 27-78 ans) ont été étudiés histologiquement ou immuno-histochimiquement. RÉSULTATS: Histologiquement, des diminutions dépendant de l'âge dans les zones occupées par l'hyaluronane et les fibres de collagène dans le derme du vermillon supérieur ont été démontrées. La teneur en fibres élastiques variait considérablement entre les individus. La zone occupée par les fibres musculaires dans la région du muscle orbiculaire oris au sein du vermillon était également corrélée négativement avec l'âge. Immuno-histochimiquement, les signaux de quatre protéines ont été atténués dans vermillon des individus plus âgés que les jeunes: le procollagène type I, l'hyaluronane synthase (HAS) 1, la chaîne lourde de la myosine (MYH) 2 (un composant des fibres musculaires oxydatives à contraction rapide) et MYH7 (un composant des fibres musculaires à contraction lente). En revanche, les signaux du "cell migration inducing hyaluronidase 1 (CEMIP)" ont été intensifiés dans le vermillon des individus plus âgés. Aucune différence marquée entre les individus jeunes et âgés n'a été observée dans le procollagène type III, HAS2, HAS3, hyaluronidase (HYAL) 1, HYAL2, MYH1 et MYH4. CONCLUSION: Les diminutions dépendantes de l'âge du hyaluronane dans le derme du vermillon étaient importantes, probablement en raison à la fois de la diminution de la synthèse (HAS1) et de l'augmentation de la dégradation (CEMIP). En outre, les diminutions dépendantes de l'âge des fibres de collagène et de deux les types de fibres musculaires dans le vermillon ont également été identifiés histologiquement. Le collagène de type I, MYH2 et MYH7 semblent respectivement représenter les molécules responsables de ces diminutions.

Relationship between the retinacula cutis and sagging facial skin.

BACKGROUND: Sagging skin is one of the most concerning esthetic issues for elderly individuals. Although reduced skin elasticity has been reported as the cause of sagging skin, a loss of skin elasticity alone is insufficient to explain sagging facial skin. This study investigated the mechanisms underlying sagging skin, with a focus on the subcutaneous network of collagenous fibers known as the retinacula cutis (RC). METHODS: To evaluate the structure of the RC noninvasively, tomographic images of the face were obtained using magnetic resonance imaging (MRI). The RC was identified by comparing MRI results with histological specimens of human skin. A descriptive scale was used to evaluate the degree of sagging, and a device equipped with a 6-mm-diameter probe was used to measure the elasticity of deeper skin layers and evaluate the physical properties of the skin. RESULTS: The density of RC in subcutaneous tissue correlated negatively with sagging scores and positively with elasticity. CONCLUSION: These results imply that a sparse RC structure contributes to a reduction in the elasticity of subcutaneous tissue, resulting in a greater degree of sagging facial skin. These findings are expected to contribute to the understanding of the mechanisms underlying sagging skin.

Lactobacillus kunkeei YB38 from honeybee products enhances IgA production in healthy adults.

AIMS: To identify lactic acid bacterial isolates, which promote immunoglobulin A (IgA) production in honeybee products and honeybees (Apis mellifera). METHODS AND RESULTS: Pyrosequencing analysis of the microbiota of honeybee products and honeybees revealed the predominance of Lactobacillus kunkeei in honey, bee pollen, bee bread and royal jelly. Lactobacillus kunkeei was isolated from bee pollen, bee bread and honey stomach, and its effect on IgA production was evaluated in vitro. Heat-killed YB38 and YB83 isolates from bee pollen promoted IgA production in mouse Peyer's Patch cells and had little mitogenic activity or effect on IL-2 production in mouse spleen cells in comparison with Listeria monocytogenes, which does exhibit mitogen activity. A pilot study in 11 healthy adults showed that 4-week intake of 1000 mg day(-1) heat-killed YB38 increased secretory IgA (SIgA) concentrations and secretion in saliva with no adverse effects. CONCLUSION: Heat-killed Lact. kunkeei YB38 from bee pollen increases IgA production and may safely improve immune responsiveness. SIGNIFICANCE AND IMPACT OF THE STUDY: This is the first report of microbiota analysis of royal jelly and the immune efficacy of Lact. kunkeei from honeybee products in humans.

Comparison between chest digital tomosynthesis and CT as a screening method to detect artificial pulmonary nodules: a phantom study.

OBJECTIVES: The objective of this study was to evaluate the imaging capabilities of chest digital tomosynthesis (DT) as a screening method for the detection of artificial pulmonary nodules, and to compare its efficiency with that of CT. METHODS: DT and CT were used to detect artificial pulmonary nodules (5 mm and 8 mm in diameter, ground-glass opacities) placed in a chest phantom. Using a three-dimensional filtered back-projection algorithm at acquisition angles of 8°, 20°, 30° and 40°, DT images of the desired layer thicknesses were reconstructed from the image data acquired during a single tomographic scan. Both standard and sharp CT reconstruction kernels were used, and the detectability index (DI) valves computed for both the DT scan acquisition angles and CT reconstruction kernel types were considered. For the observer study, we examined 50 samples of artificial pulmonary nodules using both DT and CT imaging. On the basis of evaluations made by five thoracic radiologists, a jackknife free-response receiver operating characteristic (JAFROC) study was performed to compare and assess the differences in detection accuracy between CT and DT imaging. RESULTS: For each increased acquisition angle, DI obtained by DT imaging was similar to that obtained by CT imaging. The difference in the observer-averaged JAFROC figure of merit for the five readings was 0.0363 (95% confidence interval: -0.18, 0.26; F=0.101; p=0.75). CONCLUSION: With the advantages of a decreased radiation dose and the practical accessibility of examination, DT may be a useful alternative to CT for the detection of artificial pulmonary nodules.

Effect of position of electrodes relative to the innervation zone onsurface EMG.

We investigated the effect of the position of electrodes relative to the innervation zone (IZ) of the biceps brachii muscle during isometric elbow flexion using eight-channel surface array electrodes. We estimated the location of the IZ near the centre of the muscle in 20 male subjects. The pulse peaks from electromyogram (EMG) waveforms were detected for each channel and averaged, the triphasic pulse was determined, and the peak values of the first and third phases were compared. The results showed significantly greater pulse values for the first phase when the electrode placement was proximal to the estimated IZ, and for the third phase when the electrode placement was distal to the estimated IZ. Using this method, the positional relationship between electrodes and IZ can be determined using a surface EMG waveform recorded with a pair of bipolar electrodes. This method may be clinically useful in confirming the reliability of a recorded surface EMG.

Decay pattern of pygmy states observed in neutron-rich 26Ne.

Coulomb excitation of the exotic neutron-rich nucleus (26)Ne on a (208)Pb target was measured at 58 MeV/u in order to search for low-lying E1 strength above the neutron emission threshold. This radioactive beam experiment was carried out at the RIKEN Accelerator Research Facility. Using the invariant mass method in the 25Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV excitation energy. By performing a multipole decomposition of the differential cross section, a reduced dipole transition probability of B(E1)=0.49+/-0.16e(2) fm(2) is deduced, corresponding to 4.9+/-1.6% of the Thomas-Reiche-Kuhn sum rule. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is measured. The extracted decay pattern is not consistent with several mean-field theory descriptions of the pygmy states.

Evaluation of digital linear tomosynthesis imaging of the temporomandibular joint: initial clinical experience and evaluation.

OBJECTIVES: This study was performed to evaluate the potential clinical application of digital linear tomosynthesis systems in imaging of the temporomandibular joint (TMJ). METHODS: A volumetric X-ray digital linear tomosynthesis system (Sonialvision Safire; Shimadzu Co., Kyoto, Japan) was used for TMJ imaging. Images were reconstructed with a modified three-dimensional (3D) filtered backprojection (FBP) algorithm on this device. Our modified 3D FBP was first evaluated using simulated images of numerical phantoms. Next, patients with TMJ disease were evaluated with X-ray digital linear tomosynthesis. RESULTS: The results indicate that numerical phantom and TMJ visualization can be improved by the ability to produce sectional images that blur overlying structures and yield 3D information. The flexibility of digital linear tomosynthesis, as well as the fact that through an appropriate choice of modified FBP algorithms it can suppress streak artefacts, makes it a potentially appropriate approach for evaluating the TMJ. CONCLUSIONS: The utility of digital linear tomosynthesis for the evaluation of TMJ was demonstrated. Digital linear tomosynthesis may be considered as the imaging technique of choice in the investigation of bony changes of the TMJ.

Spectroscopic study of neutron shell closures via nucleon transfer in the near-dripline nucleus 23O.

Neutron single particle energies have been measured in 23O using the 22O(d,p)23O*-->22O+n process. The energies of the resonant states have been deduced to be 4.00(2) MeV and 5.30(4) MeV. The first excited state can be assigned to the nu d3/2 single particle state from a comparison with shell model calculations. The measured 4.0 MeV energy difference between the nu s1/2 and nu d3/2 states gives the size of the N=16 shell gap which is in agreement with the recent USD05 ("universal" sd from 2005) shell model calculation, and is large enough to explain the unbound nature of the oxygen isotopes heavier than A=24. The resonance detected at 5.3 MeV can be assigned to a state out of the sd shell model space. Its energy corresponds to a approximately 1.3 MeV sized N=20 shell gap, therefore, the N=20 shell closure disappears at Z=8 in agreement with Monte Carlo shell model calculations using SDPF-M interaction.

Development of a cone angle weighted three-dimensional image reconstruction algorithm to reduce cone-beam artefacts.

OBJECTIVES: Image reconstruction from cone-beam projections collected along a single circular source trajectory is commonly done using the Feldkamp algorithm, which performs well only with a small cone angle. In this report, we propose an algorithm to reduce cone-beam artefacts by increasing the cone angle by several fold to achieve satisfactory image quality at the same radiation dose. METHODS: To examine the factors involved in the occurrence of cone-beam artefacts, a microspheres-phantom was arranged longitudinally at different positions and a computer simulation was performed. Due to differences in projection angle, data projected onto the detector surface were projected along trajectories shown as different periodic functions depending on the distance and position from the mid-plane position. Therefore, projection along several detector channels based on different projection data resulting from different periodic functions is considered responsible for the increase in cone-beam artefacts associated with an increase in the distance of reconstruction planes from the mid-plane position. Our recommended algorithm to reduce such artefacts features a change in weighting with respect to projection data obtained at different projection angles, three-dimensional back-projection of corrected projection data. RESULTS: Numerical phantom simulation and real human head origin study (a prototype cone-beam CT) showed that the effect of the reduction in cone-beam artefacts of an object located at the edges was markedly enhanced at reconstruction planes at positions further from the mid-plane position. CONCLUSION: We propose a projection angle weight-based algorithm to increase the cone angle by several fold to achieve satisfactory image quality at the same radiation dose. These findings confirmed that this algorithm reduces cone-beam artefacts and generates high-quality reconstruction images.

Observation of strong low-lying E1 strength in the two-neutron halo nucleus 11Li.

An exclusive measurement has been made of the Coulomb dissociation of the two-neutron halo nucleus 11Li at 70 MeV/nucleon at RIKEN. Strong low-energy (soft) E1 excitation is observed, peaked at about Ex = 0.6 MeV with B(E1) = 1.42(18) e2fm2 for Erel < or = 3 MeV, which was largely missed in previous measurements. This excitation represents the strongest E1 transition ever observed at such low excitation energies. The spectrum is reproduced well by a three-body model with a strong two-neutron correlation, which is further supported by the E1 non-energy-weighted cluster sum rule.

Vanishing N = 20 shell gap: study of excited states in (27,28)Ne.

This Letter reports on the (1)H((28)Ne, (28)Ne) and (1)H((28)Ne, (27)Ne) reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first 2(+) excited state of (28)Ne, and using the previously determined BE(2) value, the neutron quadrupole transition matrix element has been calculated to be M(n)=13.8 +/- 3.7 fm(2). In the neutron knockout reaction, two low-lying excited states were populated in (27)Ne. Only one of them can be interpreted by the sd shell model while the additional state may intrude from the fp shell. These experimental observations are consistent with the presence of fp shell configurations at low excitation energy in (27,28)Ne nuclei caused by a vanishing N=20 shell gap at Z=10.

Development of diffusion-weighted image using a 0.3T open MRI.

The purpose of this study was to develop a new technique for diffusion-weighted MRI (DWI) with a low-field scanner. DWI is becoming important for assessment of acute stroke. Until recently DWI required expensive technology. We developed multishot-DWI sequence for 0.3T open type MR imager. We prospectively studied forty patients on this 0.3T MRI and compared this DWI to single-shot-DWI by 1.5T-MRI. Group A: Twenty-four patients with acute cerebral infarctions detected by 1.5T-DWI were re-examined using 0.3T-DWI within 24 hours. Sixteen patients with acute cerebral infarctions detected by 0.3T-DWI were re-examined using 1.5T-DWI within 24 hours. In 22 (92%) of 24 cases, 0.3T-DWI showed high signal. In the other two patients, motion artifact distorted 0.3T-DWI. Group B: In all 16 patients, all infarctions detected by 0.3T-DWI showed high signal on 1.5T-DWI. These preliminary data show that, as long as the patient is able to keep still, multishot-DWI can be acquired successfully on a 0.3T open type MRI system.

Anomalously hindered E2 strength B(E2;2+(1)-->0+) in 16C.

The electric quadrupole transition from the first 2(+) state to the ground 0(+) state in 16C is studied through measurement of the lifetime by a recoil shadow method applied to inelastically scattered radioactive 16C nuclei. The measured mean lifetime is 77+/-14(stat)+/-19(syst) ps. The central value of mean lifetime corresponds to a B(E2;2+(1)-->0(+)) value of 0.63e(2) fm(4), or 0.26 Weisskopf units. The transition strength is found to be anomalously small compared to the empirically predicted value.

Reconstitution and purification of cytolethal distending toxin of Actinobacillus actinomycetemcomitans.

Cytolethal distending toxin (CDT) has been found in various pathogenic bacterial species and causes a cell distending and a G2 arrest against eukaryotic cells. All the cdtABC genes, which encode CDT, are known to be required for the CDT activities although the CDT holotoxin structure has not been elucidated. We cloned the cdtABC genes of Actinobacillus actinomycetemcomitans and constructed an Escherichia coli expression system for them. We found that crude extracts from six deletion mutants (delta cdtA, delta cdtB, delta cdtC, delta cdtBC, delta cdtAC, and delta cdtAB) of recombinant E. coli, which showed very weak or no detectable CDT activities, restored the CDT activities when pre-mixing and pre-incubation of them were performed in combinations to contain all the CdtA, CdtB, and CdtC proteins. These results indicate that all the Cdt proteins are required for the CDT activities. We also found that the chimera CdtB protein, CdtB-intein-CBD (chitin binding domain) like CdtB protein itself assembled with CdtA and CdtC. The reconstituted CDT containing the chimera CdtB protein was specifically extracted by chitin beads and the only CDT portion was isolated from the chitin beads by a cleavage reaction of the intein. The purified reconstituted-CDT was found to consist of CdtA, CdtB, and CdtC proteins, and showed appreciable CDT activities, indicating that the CDT holotoxin structure is the CdtABC complex. To our knowledge, this is the first report succeeded in complete purification of an active CDT and may offer useful tools for elucidation of the toxic mechanism of CDT.

Urinary prostaglandin D synthase (beta-trace) excretion increases in the early stage of diabetes mellitus.

OBJECTIVE: Circulating levels of lipocalin-type prostaglandin D synthase (L-PGDS)/beta-trace reportedly increase in renal failure as well as in cardiovascular injuries. We investigated the alterations of L-PGDS in urine and plasma in the early stage of type-2 diabetic patients. METHOD: Thirty-six type-2 diabetic patients and 29 normal subjects were studied. Overnight spot urine and plasma samples were obtained in the morning. L-PGDS was measured by ELISA method using anti-L-PGDS antibody. Variables indicating renal function were determined. RESULTS: Plasma L-PGDS concentration was slightly higher in the patients with diabetes mellitus than in the control subjects, whereas the urinary L-PGDS excretion almost doubled in the diabetic patients as compared with that in the control subjects. Plasma L-PGDS was determined by plasma creatinine (Cr) concentration while urinary L-PGDS excretion was correlated solely with urinary protein excretion. There was no relationship between plasma L-PGDS concentration and urinary L-PGDS excretion. The averaged plasma concentration of L-PGDS in the diabetics with a normal Cr level in plasma, corresponding to that in the controls, was determined by the plasma Cr concentration. On the other hand, the urinary L-PGDS excretion was determined by the amount of proteinuria and greater in the diabetics with a normal Cr level in plasma than in the controls even when the patients exhibited urinary protein excretion equal to that in the control subjects. CONCLUSIONS: Urinary L-PGDS excretion increased in the early stage of kidney injury in patients with type-2 diabetes mellitus. The urinary excretion was correlated independently with urinary protein excretion even when there was no difference in urinary protein or albumin excretions, thereby suggesting that urinary L-PGDS excretion is possibly a more sensitive indicator of renal injuries than proteinuria. Urinary L-PGDS may thus predict the progression of renal injuries in diabetic patients.

Determination of hardness in tapwater and upland soil extracts using a long-term stable divalent cation selective electrode based on a lipophilic acrylate resin as a membrane matrix.

A divalent cation-selective electrode, which utilizes a lipophilic resin as a matrix for the sensing membrane, and which has long-term stability has been developed. The sensing membrane is a lipophilic acrylate resin which is impregnated with a solution of 1-decylalcohol and the calcium salt of bis[4-(1,1,3,3-tetramethylbutyl) phenyl] phosphate at concentrations of 0.08 g ml(-1) each. The electrode exhibited nearly equal selectivity to Ca(2+) and Mg(2+) ions and could be used as a water hardness sensor. The electrode shows a Nernstian response with a slope of 29 mV decade(-1) to both Ca(2+) and Mg(2+) ions in the concentration range from 10(-5) M to 10(-1) M and could be used in the pH range from 3 to 10 for the determination of 10(-3) M Ca(2+) and Mg(2+) solutions. The initial performance of the electrode could be maintained for 1 year, since the lifetime test of the electrode was conducted in tapwater at a continuous flow rate of 4 ml min(-1). The hardnesses of tapwater and upland soil extracts were determined using the developed electrode and the analytical results were in good agreement with those obtained by chelatometric titration using an EDTA solution as the titrant. A coefficient factor of correlation 0.998 was obtained between the electrode method and titrimetry. The long-term stability of the electrode was found to be due to strong affinity of 1-decylalcohol to the lipophilic acrylate resin.

Effects of antihypertensive treatment on platelet function in essential hypertension.

To evaluate the effect of antihypertensive therapy on platelet activation in essential hypertension, the plasma levels of beta-thromboglobulin (beta-TG) were examined in 45 patients with essential hypertension and 20 age-matched normotensive control subjects. Hypertensive patients were assigned to monotherapy with one of five different antihypertensive drugs for 6 months, and the change of plasma levels of beta-TG was reexamined after the completion of the monotherapy. The plasma beta-TG increased in hypertensive patients compared with levels in normotensive control subjects. Monotherapy with each drug resulted in sufficient blood pressure control in all hypertensive patients. The plasma beta-TG decreased significantly after monotherapy with an alpha-blocker or an angiotensin-converting enzyme inhibitor (ACEI). The plasma beta-TG increased with the use of a diuretic but did not change with the use of a beta-blocker or calcium antagonist. The platelet activation observed in patients with essential hypertension is reversed by monotherapy with an alpha-blocker or an ACEI. It is possible that these drugs reduce the development of hypertensive vascular complications due to suppression of platelet activation in patients with essential hypertension.

Effects of site-directed mutagenesis on structure and function of recombinant rat liver S-adenosylhomocysteine hydrolase. Crystal structure of D244E mutant enzyme.

A site-directed mutagenesis, D244E, of S-adenosylhomocysteine hydrolase (AdoHcyase) changes drastically the nature of the protein, especially the NAD(+) binding affinity. The mutant enzyme contained NADH rather than NAD(+) (Gomi, T., Takata, Y., Date, T., Fujioka, M., Aksamit, R. R., Backlund, P. S., and Cantoni, G. L. (1990) J. Biol. Chem. 265, 16102-16107). In contrast to the site-directed mutagenesis study, the crystal structures of human and rat AdoHcyase recently determined have shown that the carboxyl group of Asp-244 points in a direction opposite to the bound NAD molecule and does not participate in any hydrogen bonds with the NAD molecule. To explain the discrepancy between the mutagenesis study and the x-ray studies, we have determined the crystal structure of the recombinant rat-liver D244E mutant enzyme to 2.8-A resolution. The D244E mutation changes the enzyme structure from the open to the closed conformation by means of a approximately 17 degrees rotation of the individual catalytic domains around the molecular hinge sections. The D244E mutation shifts the catalytic reaction from a reversible to an irreversible fashion. The large affinity difference between NAD(+) and NADH is mainly due to the enzyme conformation, but not to the binding-site geometry; an NAD(+) in the open conformation is readily released from the enzyme, whereas an NADH in the closed conformation is trapped and cannot leave the enzyme. A catalytic mechanism of AdoHcyase has been proposed on the basis of the crystal structures of the wild-type and D244E enzymes.

Mechanisms for auto-inhibition and forced product release in glycine N-methyltransferase: crystal structures of wild-type, mutant R175K and S-adenosylhomocysteine-bound R175K enzymes.

Glycine N-methyltransferase (S-adenosyl-l-methionine: glycine methyltransferase, EC 2.1.1.20; GNMT) catalyzes the AdoMet-dependent methylation of glycine to form sarcosine (N-methylglycine). Unlike most methyltransferases, GNMT is a tetrameric protein showing a positive cooperativity in AdoMet binding and weak inhibition by S-adenosylhomocysteine (AdoHcy). The first crystal structure of GNMT complexed with AdoMet showed a unique "closed" molecular basket structure, in which the N-terminal section penetrates and corks the entrance of the adjacent subunit. Thus, the apparent entrance or exit of the active site is not recognizable in the subunit structure, suggesting that the enzyme must possess a second, enzymatically active, "open" structural conformation. A new crystalline form of the R175K enzyme has been grown in the presence of an excess of AdoHcy, and its crystal structure has been determined at 3.0 A resolution. In this structure, the N-terminal domain (40 amino acid residues) of each subunit has moved out of the active site of the adjacent subunit, and the entrances of the active sites are now opened widely. An AdoHcy molecule has entered the site occupied in the "closed" structure by Glu15 and Gly16 of the N-terminal domain of the adjacent subunit. An AdoHcy binds to the consensus AdoMet binding site observed in the other methyltransferase. This AdoHcy binding site supports the glycine binding site (Arg175) deduced from a chemical modification study and site-directed mutagenesis (R175K). The crystal structures of WT and R175K enzymes were also determined at 2.5 A resolution. These enzyme structures have a closed molecular basket structure and are isomorphous to the previously determined AdoMet-GNMT structure. By comparing the open structure to the closed structure, mechanisms for auto-inhibition and for the forced release of the product AdoHcy have been revealed in the GNMT structure. The N-terminal section of the adjacent subunit occupies the AdoMet binding site and thus inhibits the methyltransfer reaction, whereas the same N-terminal section forces the departure of the potentially potent inhibitor AdoHcy from the active site and thus facilitates the methyltransfer reaction. Consequently GNMT is less active at a low level of AdoMet concentration, and is only weakly inhibited by AdoHcy. These properties of GNMT are particularly suited for regulation of the cellular AdoMet/AdoHcy ratio.

Serine hydroxymethyltransferase and threonine aldolase: are they identical?

Serine hydroxymethyltransferase, a pyridoxal phosphate-dependent enzyme, catalyses the interconversion of serine and glycine, both of which are major sources of one-carbon units necessary for the synthesis of purine, thymidylate, methionine, and so on. Threonine aldolase catalyzes the pyridoxal phosphate-dependent, reversible reaction between threonine and acetaldehyde plus glycine. No extensive studies have been carried out on threonine aldolase in animal tissues, and it has long been believed that serine hydroxymethyltransferase and threonine aldolase are the same, i.e. one entity. This is based on the finding that rabbit liver serine hydroxymethyltransferase possesses some threonine aldolase activity. Recently, however, many kinds of threonine aldolase and corresponding genes were isolated from micro-organisms, and these enzymes were shown to be distinct from serine hydroxymethyltransferase. The experiments with isolated hepatocytes and cell-free extracts from various animals revealed that threonine is degraded mainly through the pathway initiated by threonine 3-dehydrogenase, and there is little or no contribution by threonine aldolase. Thus, although serine hydroxymethyltransferase from some mammalian livers exhibits a low threonine aldolase activity, the two enzymes are distinct from each other and mammals lack the "genuine" threonine aldolase.