A novel option for dosing of proton pump inhibitors: dispersion of lansoprazole orally disintegrating tablet in water via oral syringe.

BACKGROUND: A new formulation of lansoprazole, the lansoprazole orally disintegrating tablet, rapidly disintegrates in water eliminating the need for swallowing whole pills. AIM: To assess the effect that dispersing the lansoprazole orally disintegrating tablet in water would have on lansoprazole pharmacokinetics. METHODS: Forty healthy adult men and women (18-43 years) received two single 15 mg lansoprazole orally disintegrating tablet doses separated by 3 days (one administered directly onto the tongue without water and one dispersed in water and administered orally via syringe) in a randomized, crossover fashion. Serial plasma samples were determined from 0 to 12 h for each dose. Ratios of central values for peak plasma exposure (C(max)) and mean overall extent of exposure (area under the plasma concentration) were used to compare the bioavailability. RESULTS: The two dosing regimens were bioequivalent, with the point estimate for area under the plasma concentration equalling 1.080 (confidence interval 1.012-1.152) and the point estimate for C(max) equalling 1.082 (confidence interval 0.961-1.218). CONCLUSIONS: Dispersing the 15 mg lansoprazole orally disintegrating tablet in water and administering the dose orally via syringe is bioequivalent to the 15 mg intact lansoprazole orally disintegrating tablet with respect to lansoprazole area under the plasma concentration and C(max). This dosing route provides an additional, convenient dosing option for lansoprazole.

Assessment of hepatic function in cystic fibrosis by lidocaine metabolism.

BACKGROUND: To determine hepatic drug metabolism in patients with cystic fibrosis, as measured by monoethylglycinexylidide formation after lidocaine injection and indocyanine green (ICG) clearance. METHODS: The following study is a case-control study, which included 19 patients with cystic fibrosis and 13 control subjects. Serum monoethylglycinexylidide concentration was measured after intravenous injection of 1 mg/kg (maximum, 50 mg) lidocaine. Indocyanine green (0.5 mg/kg) was injected concomitantly, and absorbance (805 nm) of serum was measured over time to determine its volume of distribution, serum half-life, and hepatic blood flow. RESULTS: Monoethylglycinexylidide formation was decreased in patients with cystic fibrosis compared with controls (39.4+/-16.9 microg/L versus 70.3+/-45.7 microg/L, mean +/- SD, respectively, P < 0.02). Indocyanine green half-life (4.6+/-2.7 min versus 3.0+/-1.0 min), volume of distribution (8.6+/-5.5 L versus 8.3+/-3.4 L), and hepatic blood flow (10.9+/-5.9 ml x kg(-1) x min(-1) versus 7.4+/-2.0 ml x kg(-1) x min(-1)) were similar in both groups. CONCLUSION: Monoethylglycinexylidide formation after lidocaine injection is impaired in patients with cystic fibrosis. This impairment may have clinical implications when using hepatically metabolized medications in patients with cystic fibrosis.

Lansoprazole: pharmacokinetics, pharmacodynamics and clinical uses.

Lansoprazole (Prevacid, TAP Pharmaceuticals, Inc.) is a substituted benzimidazole that inhibits gastric acid secretion. This agent is approved for the short-term treatment of erosive reflux oesophagitis, active gastric ulcer, active duodenal ulcer and the treatment of non-steroidal anti-inflammatory drug (NSAID)-induced gastric and duodenal ulcers. It is also approved for the long-term treatment of healed reflux oesophagitis, healed duodenal ulcer, the treatment of hypersecretory conditions such as Zollinger-Ellison syndrome and the eradication of Helicobacter pylori as a component of triple therapy with lansoprazole, clarithromycin and amoxicillin, or dual therapy with lansoprazole and amoxicillin. Its mechanism of action is to selectively inhibit the membrane enzyme H+/K+ ATPase in gastric parietal cells. In clinical trials, lansoprazole is more effective than placebo or histamine (H2)-receptor antagonists in the treatment of reflux oesophagitis. Lansoprazole administered at a dose of 30 mg daily produced faster relief of symptoms and superior healing rates in patients with gastric or duodenal ulcers or reflux oesophagitis than H2-receptor antagonists. A daily dose of 30 mg lansoprazole reduced epigastric pain faster than omeprazole 20 mg daily in patients with peptic ulcer disease but healing rates at 4 and 8 weeks were similar with both agents at these dosages. Lansoprazole was more effective than H2-receptor antagonists in patients with Zollinger-Ellison syndrome and produced similar treatment outcome to omeprazole. Lansoprazole in combination with clarithromycin and amoxicillin produced similar rates of eradication of H. pylori. In clinical trials, lansoprazole is well-tolerated and has a low frequency of side effects similar to that of H2-receptor antagonists or omeprazole.

The yeast mitochondrial citrate transport protein. Probing the roles of cysteines, Arg(181), and Arg(189) in transporter function.

Utilizing site-directed mutagenesis in combination with chemical modification of mutated residues, we have studied the roles of cysteine and arginine residues in the mitochondrial citrate transport protein (CTP) from Saccharomyces cerevisiae. Our strategy consisted of the sequential replacement of each of the four endogenous cysteine residues with Ser or in the case of Cys(73) with Val. Wild-type and mutated forms of the CTP were overexpressed in Escherichia coli, purified, and reconstituted in phospholipid vesicles. During the sequential replacement of each Cys, the effects of both hydrophilic and hydrophobic sulfhydryl reagents were examined. The data indicate that Cys(73) and Cys(256) are primarily responsible for inhibition of the wild-type CTP by hydrophilic sulfhydryl reagents. Experiments conducted with triple Cys replacement mutants (i.e. Cys(192) being the only remaining Cys) indicated that sulfhydryl reagents no longer inhibit but in fact stimulate CTP function 2-3-fold. Following the simultaneous replacement of all four endogenous Cys, the functional properties of the resulting Cys-less CTP were shown to be quite similar to those of the wild-type protein. Finally, utilizing the Cys-less CTP as a template, the roles of Arg(181) and Arg(189), two positively charged residues located within transmembrane domain IV, in CTP function were examined. Replacement of either residue with a Cys abolishes function, whereas replacement with a Lys or a Cys that is subsequently covalently modified with (2-aminoethyl)methanethiosulfonate hydrobromide, a reagent that restores positive charge at this site, supports CTP function. The results clearly show that positive charge at these two positions is essential for CTP function, although the chemistry of the guanidinium residue is not. Finally, these studies: (i) definitely demonstrate that Cys residues do not play an important role in the mechanism of the CTP; (ii) prove the utility of the Cys-less CTP for studying structure/function relationships within this metabolically important protein; and (iii) have led to the hypothesis that the polar face of alpha-helical transmembrane domain IV, within which Arg(181), Arg(189), and Cys(192) are located, constitutes an essential portion of the citrate translocation pathway through the membrane.

Irritable bowel syndrome and lactose maldigestion in recurrent abdominal pain in childhood.

BACKGROUND: The aim of this study was to evaluate the impact of irritable bowel syndrome (IBS) and lactose maldigestion in children with recurrent abdominal pain. METHODS: Children who had abdominal pain associated with defecation or change in bowel habit, disordered defecation, and distension were diagnosed with IBS, and lactose maldigestion was defined by lactose breath hydrogen testing. Children with IBS were managed with increased fiber intake, while those with lactose maldigestion restricted dietary lactose. A telephone survey was conducted to determine the response to treatment. RESULTS: The mean age of the 59 boys and 87 girls was 9.5 +/- 3.0 years. Children with IBS and lactose maldigestion had more frequent abdominal pain than children without these conditions, but they required less medication for relief of symptoms. CONCLUSIONS. Lactose maldigestion may be a contributory factor in children with IBS, and lactose avoidance in these patients may reduce medication use to relieve symptoms.

Hepatic function as assessed by lidocaine metabolism in sickle cell disease.

OBJECTIVE: To evaluate hepatic drug metabolism, as determined by the formation of monoethylglycinexylidide (MEGX) after lidocaine injection and indocyanine green (ICG) clearance, in patients with sickle cell disease. STUDY DESIGN: A case-control study including 19 patients with homozygous hemoglobin S, and 13 age- and sex-matched black control subjects. Serum MEGX concentration was measured after intravenous injection of 1 mg/kg (maximum 50 mg) lidocaine. ICG (0.5 mg/kg) was injected concomitantly and absorbance (805 nm) of serum was measured over time to determine its volume of distribution, serum half-life, and hepatic blood flow. RESULTS: MEGX formation at 15 minutes was decreased in patients with sickle cell disease compared with formation in the control subjects (39.9 +/- 18.0 vs 65.6 +/- 50.0 micrograms/L, respectively, p < 0.02). The volume of distribution of ICG was increased in patients with sickle cell disease compared with that in the control subjects (0.21 +/- 0.09 vs 0.11 +/- 0.03 L/kg, p < 0.01). This partly accounts for the decreased MEGX formation. The ICG half-life was similar in both groups (3.8 +/- 1.5 vs 3.1 +/- 1.0 min). Hepatic blood flow, derived from ICG clearance, was increased in sickle cell patients compared with that of the control subjects (12.2 +/- 4.5 vs 8.1 +/- 2.1 ml/kg/min, p < 0.01). CONCLUSION: Hepatic drug metabolism, as assessed by MEGX formation after lidocaine injection, is impaired in patients with sickle cell disease. This impairment may have clinical implications when using hepatically metabolized medications in patients with sickle cell disease.

Characterization of failure to imbibe in infants.

The term failure to imbibe is proposed to describe infants with failure to thrive due to poor feeding. Feeding assessment was performed in 128 patients: 43 healthy controls, 53 diseased controls, 12 with nonorganic failure to thrive, and 20 with failure to imbibe. Infants with failure to imbibe required a significantly longer time to feed compared with other infants. In contrast to other infants with nonorganic failure to thrive, patients with failure to imbibe were more likely to need pediatric subspecialty care and nasogastric or gastrostomy tube feeding. Since these patients may have treatable conditions, infants with failure to imbibe merit further investigation.

Conscious sedation with high-dose midazolam for pediatric gastrointestinal endoscopy.

BACKGROUND: Midazolam is used frequently to sedate children for gastrointestinal endoscopy. The sedative dosage of intravenous midazolam commonly reported in children is up to 0.3 mg/kg. We hypothesized that larger doses of midazolam could be used for pediatric endoscopy. METHODS: We retrospectively reviewed the medical records of 116 pediatric patients (aged 1 year to 18 years) who had endoscopy. The efficacy and side effects of sedation in 45 patients who received midazolam doses of > or = 0.3 mg/kg were compared with the same effects in 71 children who received < 0.3 mg/kg. RESULTS: All patients received approximately 1 mg/kg meperidine (up to 50 mg) intravenously. The blood pressure, pulse rate, respiratory rate, oxygen saturation, degree and duration of sedation, and incidence of side effects such as hypotension, hypoxia, or vomiting were similar in both groups. CONCLUSIONS: Intravenous doses of midazolam > 0.3 mg/kg can be used for conscious sedation in children.

Identification of a novel gene encoding the yeast mitochondrial dicarboxylate transport protein via overexpression, purification, and characterization of its protein product.

A gene encoding the mitochondrial dicarboxylate transport protein (DTP) has been identified for the first time from any organism. Our strategy involved overexpression of putative mitochondrial transporter genes, selected based on analysis of the yeast genome, followed by purification and functional reconstitution of the resulting protein products. The DTP gene from the yeast Saccharomyces cerevisiae encodes a 298-residue basic protein which, in common with other mitochondrial anion transporters of known sequence and function, displays the mitochondrial transporter signature motif, three homologous 100-amino acid sequence domains, and six predicted membrane-spanning regions. The product of this gene has been abundantly expressed in Escherichia coli where it accumulates in inclusion bodies. Upon solubilization of the overexpressed DTP from isolated inclusion bodies with Sarkosyl, 28 mg of DTP was obtained per liter of E. coli culture at a purity of 75%. The purified, overexpressed DTP was then reconstituted in phospholipid vesicles where both its kinetic properties (i.e. Km = 1. 55 mM and Vmax = 3.0 micro;mol/min/mg protein) and its substrate specificity were determined. The intraliposomal substrates malonate, malate, succinate, and phosphate effectively supported [14C]malonate uptake, whereas other anions tested did not. External substrate competition studies revealed a similar specificity profile. Inhibitor studies indicated that the reconstituted transporter was sensitive to inhibition by n-butylmalonate, p-chloromercuribenzoate, mersalyl, and to a lesser extent pyridoxal 5'-phosphate but was insensitive to N-ethylmaleimide and selective inhibitors of other mitochondrial anion transporters. In combination, the above findings indicate that the identified gene encodes a mitochondrial transport protein which upon overexpression and reconstitution displays functional properties that are virtually identical to those of the native mitochondrial dicarboxylate transport system. In conclusion, the present investigation has resulted in identification of a gene encoding the mitochondrial DTP and thus eliminates a major impediment to molecular studies with this metabolically important transporter. Based on both structural and functional considerations, the yeast DTP is assignable to the mitochondrial carrier family. Additionally, the development of a procedure that enables the expression and isolation of large quantities of functional DTP provides the foundation for comprehensive investigations into the structure/function relationships within this transporter via site-directed mutagenesis, as well as for the initiation of crystallization trials.

Bacterial overexpression of putative yeast mitochondrial transport proteins.

Thirty-two genes have been identified within the genome of the yeast Saccharomyces cerevisiae which putatively encode mitochondrial transport proteins. We have attempted to overexpress a subset of these genes, namely those which encode mitochondrial transporters of unknown function, and have succeeded in overexpressing 19 of these genes. The overexpressed proteins were then isolated and tested for five well-characterized reconstituted transport activities (i.e., the transport of citrate, dicarboxylates, pyruvate, camitine, and aspartate). Utilizing this approach, we have clearly identified the yeast mitochondrial dicarboxylate transport protein, as well as two additional lower-magnitude transport functions (i.e., tricarboxylate and dicarboxylate transport activities). The implications of these results and the considerations relevant to this approach are discussed.

Comparison of oral sodium phosphate to polyethylene glycol-based solution for bowel preparation for colonoscopy in children.

Thirty-four patients, aged 3 to 17 years, were randomized to receive oral sodium phosphate solution or a polyethylene glycol-based solution in preparation for elective colonoscopy. Nineteen patients received two doses of oral sodium phosphate solution (45 mL/1.7 m2/ dose) and 15 received polyethylene glycol-based solution (4 L/1.7 m2). Compliance with oral sodium phosphate solution was judged as easy or tolerable in 15 of 19 patients, but only in 5 of 15 who were given polyethylene glycol-based solution. The quality of colon cleansing was rated by an endoscopist who was blinded to the colon preparation method used. The bowel preparation was excellent or good (only liquid remaining in the colonic lumen) in 18 of 19 patients who received oral sodium phosphate solution and in 6 of 15 who received polyethylene glycol-based solution. The incidence of vomiting was similar in both groups, but abdominal pain occurred more frequently in the polyethylene glycol-based solution group. Hyperphosphatemia developed in patients who received oral sodium phosphate solution (serum phosphorus = 2.3 +/- 0.7 mmol/L (7.2 +/- 2.2 mg/dL; mean +/- SD), but only in 1 of 15 patients in the polyethylene glycol-based solution group. Patients did not exhibit symptoms of hyperphosphatemia and serum calcium concentrations were similar in both groups. In summary, oral sodium phosphate solution is better tolerated than polyethylene glycol-based solution for bowel preparation in children. However, hyperphosphatemia occurred frequently in patients who received oral sodium phosphate solution. Further studies are needed to determine the optimal dose for safety and efficacy for the use of these solutions in children.

Deletion of the nuclear gene encoding the mitochondrial citrate transport protein from Saccharomyces cerevisiae.

The nuclear gene encoding the mitochondrial citrate transport protein (i.e., CTP1) has been deleted from a haploid yeast strain. The stable yeast deletion strain was constructed by homologous recombination of the HIS3 gene at the CTP1 gene locus. Deletion of the CTP was confirmed by PCR. Immunoblot analysis provided the first quantitative estimate of the level of the CTP in wild-type yeast mitochondria and indicated the absence of expressed CTP in mitochondria isolated from the deletion strain. Deletion of CTP1 did not lead to a phenotype on any carbon source tested, indicating that CTP1 is not an essential gene. This suggests that either known alternative pathways are able to produce sufficient acetyl-CoA to support biosynthetic reactions, or there exists a second CTP gene. The ability of the deletion strain to serve as a host for the correct targeting and overexpression of a mutated CTP was then demonstrated. These studies provide a system which permits the use of site-directed mutagenesis to examine both CTP targeting to mitochondria, as well as the molecular basis underlying CTP function. Moreover, this system will not only facilitate the study of the yeast CTP, but also CTPs expressed from the cDNAs of higher eukaryotes.

Symptoms of gastritis due to Helicobacter pylori in children.

Infection due to Helicobacter pylori may be associated with gastritis and peptic ulcer disease in children. The aim of this study was to compare the presentation of gastritis due to H pylori with that of gastritis not associated with H pylori infection. The medical records of 296 children who had esophagogastroduodenoscopy were reviewed; 23 (8%) had H pylori gastritis, and 51 had primary gastritis without H pylori infection. Of patients with H pylori, 43% had antral nodularity and 17% had duodenal ulcers. The incidence of epigastric pain, nocturnal pain, postprandial pain, family history of peptic ulcer disease, water brash, vomiting, weight loss, fecal occult blood, and hematemesis was similar between both groups. Periumbilical pain was less common in children with gastritis than epigastric pain, and pain in the periumbilical region was present in only 4% of children with H pylori infection, compared with 31% of patients who had gastritis without H pylori infection. The presence of H pylori should be sought in children having endoscopy for evaluation of upper gastrointestinal mucosal disease.

Effectiveness of nasogastric rehydration in hospitalized children with acute diarrhea.

The American Academy of Pediatrics recommends oral rehydration and early refeeding for management of infants with diarrhea and mild to moderate dehydration. However, intravenous rehydration is still widely used for treatment of infants hospitalized for dehydration. The administration of oral rehydration solution via continuous infusion through a nasogastric tube facilitates its delivery in hospitalized children. The purpose of this study is to compare intravenous and nasogastric rehydration in children hospitalized for mild to moderate dehydration. Infants who failed attempts at oral rehydration and were hospitalized for dehydration due to acute diarrheal illness were randomized to receive intravenous or nasogastric rehydration. Following rehydration, infants received soy formula and a maintenance oral electrolyte solution to replace ongoing stool losses, as directed by the attending physician. Patients were discharged from the hospital once oral feeding was tolerated, and the vomiting and diarrhea resolved. Twenty-four patients, from 2 to 19 months of age, were enrolled in the study. Rehydration was successful in 11 of 12 patients in the nasogastric rehydration group and in all 12 patients who received intravenous rehydration. The degree of dehydration, severity of vomiting and diarrhea, and duration of rehydration were similar in both groups. The duration and cost of hospitalization were less for patients receiving nasogastric rehydration compared to those who were rehydrated intravenously. Rehydration by infusion of oral rehydration solution via a nasogastric tube is a safe and effective treatment for infants with mild to moderate dehydration. Rehydration with infusion of oral rehydration solution through a nasogastric tube should be considered for in-patient management of infants with diarrhea.

Lactose maldigestion and recurrent abdominal pain in children.

Our objectives were to evaluate children with recurrent abdominal pain for lactose maldigestion and to assess factors which might predict lactose absorption status. One hundred thirty-seven children were referred for specialty evaluation of recurrent abdominal pain of at least three months' duration. Study subjects were evaluated by history and physical examination, dietary interviews, hematologic and biochemical laboratory testing, stool parasite examination, and radiologic or endoscopic structural examinations, as indicated. Lactose hydrogen breath testing was performed after challenge with 1 g/kg lactose 10% aqueous solution). There were 53 males and 84 females, whose ages ranged from 6 to 18 years (9.64 +/- 2.9; mean +/- SD) Lactose maldigestion was detected in 33/137 patients (24%). The prevalence of abdominal pain, bloating, gas, flatulence, diarrhea, and constipation was similar in children with or without lactose maldigestion. The perception of symptoms related to the ingestion of dairy products was similar in both groups. No other clinical parameter predicted lactose maldigestion. However, children with lactose maldigestion had overall clinical improvement with a lactose-restricted diet. Clinical evaluation alone cannot adequately predict the presence of lactose maldigestion in children. Formal evaluation for lactose maldigestion using breath hydrogen testing methods should be considered in children with recurrent abdominal pain.

High level expression and characterization of the mitochondrial citrate transport protein from the yeast Saccharomyces cerevisiae.

The gene encoding the mitochondrial citrate transport protein (CTP) in the yeast Saccharomyces cerevisiae has been identified, and its protein product has been overexpressed in Escherichia coli. The expressed CTP accumulates in inclusion bodies and can be solubilized with sarkosyl. Approximately 25 mg of solubilized CTP at a purity of 75% is obtained per liter of E. coli culture. The function of the solubilized CTP has been reconstituted in a liposomal system where both its kinetic parameters (i.e. Km = 0.36 mM and Vmax = 2.5 mumol/min/mg protein) and its substrate specificity have been determined. Notably, the yeast CTP displays a stricter specificity for tricarboxylates than do CTPs from higher eukaryotic organisms. Dot matrix analysis of the yeast CTP sequence indicates the presence of three homologous sequence domains (each approximately 100 residues in length), which are also related to domains in other CTPs. Thus, the yeast CTP displays the tripartite structure characteristic of other mitochondrial transporters. Alignment of the yeast CTP sequence with CTPs from other sources defines a consensus sequence that displays 89 positions of amino acid identity, as well as the more generalized mitochondrial transporter-associated sequence motif. Based on hydropathy analysis, the yeast CTP contains six putative membrane-spanning alpha-helices. Finally, Southern blot analysis indicates that the yeast genome contains a single gene encoding the mitochondrial CTP. Our data indicate that, based on both its structural and functional properties, the expressed yeast CTP can be assigned membership in the mitochondrial carrier family. The identification of the yeast CTP gene, and the expression and purification of large quantities of its protein product, pave the way for investigations into the roles of specific amino acids in the CTP translocation mechanism, as well as for the initiation of crystallization trials.

High-yield bacterial expression, purification, and functional reconstitution of the tricarboxylate transport protein from rat liver mitochondria.

The rat liver mitochondrial tricarboxylate transport protein has been overexpressed in E. coli. The expressed transporter, which contains a 21 amino acid N-terminal fusion sequence, accumulates in inclusion bodies. Subsequent extraction of the tricarboxylate transporter from isolated inclusion bodies yields approximately 90 mg of transport protein per liter of E. coli culture at a purity of greater than 90%. Upon incorporation into phospholipid vesicles the purified, overexpressed transporter catalyzes a 1,2,3-benezenetricarboxylate-sensitive citrate/citrate exchange (i.e., the defining reaction of the mitochondrial tricarboxylate transporter). Kinetic characterization of the reconstituted transporter indicates a Km of 0.37 mM and a Vmax of 101 nmol/min/mg protein. The substrate specificity of the reconstituted, expressed transporter is virtually identical to that of the native transporter. These studies represent the first overexpression of the rat liver mitochondrial tricarboxylate transporter. By providing a large amount of highly-purified, functionally competent transporter this system will now enable a variety of structural studies, including site-directed mutagenesis, which heretofore could not be performed.

Sunflower seed bezoar leading to fecal impaction.

We report the cases of two children, ages 8 and 10 years, with painful defecation and constipation due to impaction of sunflower seed shells that did not respond to laxative and enema therapy. Physical examination revealed a prickly mass in the rectal vault. Proctoscopy revealed bezoars of sunflower seed shells, which were removed by endoscopic biopsy forceps followed by a combination of rectal irrigation and manual disimpaction. The rectal pain resolved, and bowel habits returned to normal with laxatives. Patients with rectal bezoars who fail to respond to enemas or colonic lavage may require endoscopic management for resolution of their symptoms.

Effect of pyridoxal 5'-phosphate on the function of the purified mitochondrial tricarboxylate transport protein.

The effect of pyridoxal 5'-phosphate (PLP), a lysine-selective reagent, on the function of the purified and reconstituted mitochondrial tricarboxylate transport protein has been studied. PLP caused a concentration-dependent inhibition of citrate transport with an IC50 value of 0.09 mM. At 10 mM PLP virtually complete inhibition of transport was observed (i.e. 97%), thereby indicating that modification of a residue(s) whose participation is essential to the translocation mechanism had occurred. Substrate protection studies demonstrated that substrates for the tricarboxylate transporter (i.e., citrate, isocitrate, phosphoenolpyruvate, and malate) effectively protected against PLP inhibition, whereas other organic anions which are not transported by the tricarboxylate carrier (i.e., malonate, alpha-ketoglutarate, phosphate, and succinate) afforded considerably less protection. Kinetic studies in which the transport rate was measured at varying citrate and PLP concentrations indicated that PLP caused an increase in the apparent Km of transport with little change in the Vmax, thereby resulting in a primarily competitive inhibition pattern. In combination, the above findings indicate that PLP interacts with the tricarboxylate transporter at a site(s) (i.e., a lysine residue(s) and/or the amino-terminal alanine residue) that is important in the translocation mechanism and may reside within or near the substrate binding site.