Search PubMed⌕ Search

Biomedical subjects

B K Tang

Publications and source records attributed to B K Tang.

At least 37 records · Page 2Linked to original sources
PubMed1995

Interindividual variability in the glucuronidation of (S) oxazepam contrasted with that of (R) oxazepam.

Although conjugation with glucuronic acid is a major process for converting many xenobiotics into hydrophilic, excretable metabolites, relatively little has been reported concerning interindividual variability of glucuronidation in human populations. Oxazepam, a therapeutically active metabolite of diazepam, is one of a number of C3-hydroxylated benzodiazepines for which glucuronide conjugation is the predominant pathway of biotransformation. The drug is normally formulated as a racemic mixture of inactive (R) and active (S) enantiomers. In the present study we have investigated the use of oxazepam as a potential probe drug for studying the variability of glucuronide conjugation, and for demonstrating the extent to which genetic factors may be responsible. In preliminary studies we determined oxazepam pharmacokinetics metabolite profiles after administration of racemic (R,S) oxazepam to eleven human volunteers. The (S) glucuronide was preferentially formed and excreted in nine of the eleven subjects. The ratios of (S) to (R) glucuronide metabolites (S/R ratios) were 3.87 +/- 0.79 (mean +/- SD) and 3.52 +/- 0.60 in urine and plasma, respectively. However, both ratios were significantly lower in two subjects (p < 0.01). In these two atypical subjects, the half-life of (R,S) oxazepam was also markedly longer (14.7 and 15.9 h) than in the other subjects (8.1 +/- 3.2 h). A good correlation (rs = 0.90) between the S/R-glucuronide ratio in urine and the plasma clearance of (R,S) oxazepam suggested that a low S/R ratio may be a marker of poor elimination of oxazepam. In further investigations, the drug was administered to 66 additional subjects. The S/R-glucuronide ratio in 8 h pooled urine was bimodally distributed, with 10% of all subjects possessing ratios below an apparent antimode of 1.9. A survey of the in vitro formation of oxazepam glucuronides by microsomes from 37 human livers also showed that 10% of the livers displayed an abnormally high apparent Michaelis constant (Km) for the formation of the (S) glucuronide, but not of the (R) glucuronide. These results suggest that the glucuronidation of the pharmacologically active (S) enantiomer of oxazepam is decreased in a significant percentage (10%) of Caucasian individuals. The observed in vitro differences in apparent kinetics of the S-glucuronidation reaction may reflect defects at the genetic level, leading to structural changes in the isozyme(s) of UDP-glucuronyltransferase that catalyse this reaction.

Adult↗
PubMed1995

Induction of P-450 in workers exposed to dioxin.

OBJECTIVES: To examine the effects of occupational exposure to substances contaminated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on cytochrome P-4501A2 activity in a cross sectional medical survey. METHODS: The exposed workers had been employed at two chemical plants > 15 years earlier in the manufacture of 2,4, 5-trichlorophenol and its derivatives. The control group consisted of people with no occupational exposure to phenoxy herbicides and who lived within the communities of the exposed workers. A total of 58 workers and 125 unexposed controls participated in the analysis. Cytochrome P-450 activity was assessed with test that measures caffeine metabolites in the urine. A ratio of metabolites of caffeine (CMR) constituted a measure of P-4501A2 activity. RESULTS: Compared with the control group in multivariate logistic regression, raised non-significant associations were found for three of four categories of TCDD in exposed workers (TCDD < 20 pg/g, odds ratio (OR) 1.7, 95% confidence interval (95% CI) 0.6 to 5.0, TCDD 20-66, OR 0.3, 95% CI 0.0 to 1.7; TCDD 67-147, OR 2.3, 95% CI 0.6 to 8.8; TCDD > or = 148, OR 3.1, 95% CI 0.8 to 12.5). We found a strongly significant association of CMR and urinary cotinine, a measure of smoking, and urinary free ethanol. We found weak non-significant associations between P-4501A2 activity and increased serum TCDD among workers. CONCLUSIONS: The absence of an association between serum TCDD and cytochrome P-4501A2 may be due to the size of the study, insensitivity of the CMR to assess cytochrome P-4501A2 activity, or inadequate levels of exposure, although these were among the highest in human groups tested.

Adult↗
PubMed1995

A urinary marker of alcohol intake.

Previously, one of us (B. K. T.) developed an assay that measures levels of free ethanol and ethanol conjugates in urine and showed that the mean levels of these ethanol markers in confirmed alcoholics were at least 20-fold higher than those levels in control subjects. In this study, we assessed the relationship of these biomarkers with self-reported levels of alcohol intake in a multiethnic sample of Los Angeles County residents who were male and over the age of 35 years (n = 128; 40 non-Hispanic whites, 46 blacks, 17 Chinese, and 25 Japanese). Regardless of race, the mean levels of free, bound, and total (free plus bound) ethanol were lowest in nondrinkers, intermediate in weekly drinkers, and highest in daily drinkers (P = 0.0001 in all three statistical tests of differences in the three biomarkers). Stepwise discriminant analysis showed that of the three potential biomarkers, total ethanol best discriminates between the three classes of drinkers (non, weekly, and daily), and that additional inclusion of either free or bound ethanol in the discriminant function had negligible effect. Overall, mean level of total ethanol was 2.2 times higher in weekly than in nondrinkers; daily drinkers, in turn, showed a 4.2-fold increase in mean total ethanol relative to weekly drinkers. However, there was no correlation between any of the three biomarkers and self-reported level (in grams of ethanol) of average consumption in either weekly or daily drinkers whose mean intake was about 13 and 42 g of ethanol/day, respectively. As the level of urinary free ethanol and ethanol conjugates showed extraordinary differences among racial groups for a given level of self-reported ethanol intake, the data suggest possible interracial differences in the in vivo elimination rate of ethanol; this latter finding needs to be confirmed in larger studies.

Adult↗
PubMed1994

Caffeine biotransformation in human hepatocyte lines derived from normal liver tissue.

Caffeine biotransformation was demonstrated in three novel human hepatocyte cell lines established from normal liver tissue and cultured continuously for 19 to 30 months. Caffeine and its metabolites were identified and quantified by high performance liquid chromatography. Without induction, caffeine was metabolized to the four primary metabolites [theobromine (37X), paraxanthine (17X), theophylline (13X), 1,3,7-trimethylurate (137U)]. Under these basal conditions 137U was the predominant metabolite. The actual pattern of metabolite production was a reproducible characteristic of each line. After induction with dibenz(a,h)anthracene, the formation of 17X was increased 4-17 fold. Induction with phenobarbital did not change the metabolic profile. These human hepatocyte lines can reproduce in vitro metabolism of caffeine observed in man in vivo.

Adult↗
PubMed1994

Caffeine as a probe for CYP1A2 activity: potential influence of renal factors on urinary phenotypic trait measurements.

Two established caffeine-based urinary methods for measuring CYP1A2 activity were compared with each other, and also with the systemic clearance of caffeine which served as a standard of reference for such activity. Following a standardized dose, caffeine (137X) and its metabolites were measured in urine and plasma of 39 healthy subjects. The measurements allowed determinations of: (1) systemic caffeine clearance (CL(caff)); (2) the caffeine metabolite ratio (AFMU + 1X + 1U)/17U determined in an overnight-urine specimen and referred to as CMR, and (3) the ratio (17X + 17U)/137X measured in urine collected between 4 and 5 h after caffeine intake and referred to as PCUR for 'paraxanthine-caffeine urinary ratio'. The PCUR showed a bimodal distribution and a relatively wide variation, CL(caff) and CMR were both normally distributed. The correlation between CL(caff) and CMR was r = 0.77 (p < 0.001), between CLcaff and PCUR r = 0.46 (p < 0.01), and between CMR and PCUR r = 0.40 (p < 0.02). The difference between the correlation coefficients 0.77 and 0.46 was statistically significant (z-test; p < 0.05). The well established decrease of caffeine metabolism by oral contraceptive use was observed with both CL(caff) and CMR but not with PCUR. Examination of possible explanations for the differences between PCUR and CMR led to the finding of a correlation between PCUR and the renal clearance of caffeine (CLr) with r = -0.47 (p < 0.01). Further scrutiny demonstrated that a bimodal or non-normal frequency distribution as shown by PCUR was also shown by CLr and by urine flow rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Variability of acetaminophen metabolism in Caucasians and Orientals.

Acetaminophen (paracetamol) is extensively conjugated with glucuronic acid and sulfate prior to renal excretion. A minor metabolic route involves microsomal oxidation of acetaminophen to a hepatotoxic reactive intermediate, which subsequently undergoes glutathione (GSH) conjugation, yielding cysteine and mercapturate conjugates, both of which are excreted in the urine (Slattery et al., 1987). Data collected by de Morais et al. (1989) indicated that in comparison with normal subjects, glucuronidation of acetaminophen was impaired in subjects with Gilbert's syndrome, a genetically-based impairment of bilirubin glucuronidation. Thus, inter-subject and ethnic differences in acetaminophen disposition have pharmacogenetic and toxicological implications. This study was conceived to explore these differences. Urinary excretion of acetaminophen and its metabolites was observed in 125 Caucasian and 33 Oriental subjects. No appreciable difference was noted in the mean fraction of drug excreted as glucuronide between the two groups (51.5% in Caucasians vs 51.8% in Orientals). However, the data strongly indicated that the excretion of acetaminophen glucuronide was not normally distributed. Bimodality was apparent in both groups, with 20% of Caucasian and 33% of Oriental subjects displaying relatively extensive glucuronidation. In addition, glucuronidation displayed a strong negative correlation with sulfation (r = -0.97), suggesting the existence of a compensatory mechanism between the two metabolic pathways. The mean fractional excretions of cysteine and mercapturate conjugates did show significant differences between Caucasians and Orientals (p < 0.005). In addition, the ratio of mercapturate to total GSH-derived conjugates recovered appeared to be bimodal, indicating possible heterogeneity in the conversion of the cysteine conjugate to mercapturate via N-acetylation.

Acetaminophen↗
PubMed1992

Biotransformation of caffeine, paraxanthine, theobromine and theophylline by cDNA-expressed human CYP1A2 and CYP2E1.

Six human cytochrome P450s expressed in HepG2 cells using vaccinia virus cDNA-directed expression, were used to study the biotransformation of caffeine and its metabolites. CYP1A2 alone was responsible for caffeine 3-demethylation and paraxanthine 7-demethylation; in addition, 1A2 catalysed virtually all reactions related to caffeine and its metabolites. The metabolic profile of caffeine biotransformation by CYP1A2 averaged 81.5% for paraxanthine, 10.8% for theobromine and 5.4% for theophylline formation. It remained quite uniform when caffeine concentrations were varied. The most striking finding was that CYP2E1 (the ethanol-inducible form) had major influences upon caffeine metabolism: in particular, it catalysed the formation of theophylline and theobromine from caffeine. Thus, the in vivo metabolite profiling of caffeine may reveal CYP2E1 activities in addition to the previously documented activities of CYP1A2, polymorphic N-acetyltransferase and xanthine oxidase.

Biotransformation↗
PubMed1991

Use of caffeine metabolite ratios to explore CYP1A2 and xanthine oxidase activities.

Caffeine was used as a metabolic probe to screen healthy subjects for their activities of two enzymes, deduced to be CYP1A2 (an inducible cytochrome P450) and xanthine oxidase. A longitudinal study revealed modest effects of caffeine dose, ethanol intake, and time-of-day on the CYP1A2 index, without any effect on the xanthine oxidase index. The coefficients of intraindividual variation not accounted for were 5.0% for the xanthine oxidase and 17.2% for the CYP1A2 index. In a population study, both indexes showed a log normal distribution, with CYP1A2 values of most subjects covering a 6.3-fold range but only a 1.7-fold range with xanthine oxidase. The CYP1A2 index was 33% decreased in women who used oral contraceptives and substantially increased in cigarette smokers. Neither the CYP1A2 nor the xanthine oxidase index differed between volunteers of Chinese and European extraction. Four of 178 subjects showed unexplained low xanthine oxidase values (i.e., values several standard deviations below the mean).

Adult↗
PubMed1991

Prominence of slow acetylator phenotype among patients with sulfonamide hypersensitivity reactions.

Delayed hypersensitivity reactions are among the most severe adverse effects of the sulfonamides in current clinical use. These reactions appear to occur because of differences in the metabolism and detoxification of reactive metabolites of the sulfonamides. N-Acetylation is a major metabolic pathway for the sulfonamides. Slow acetylation phenotype might be a risk factor for the development of these reactions. We determined the acetylation phenotype of 21 patients who had suffered hypersensitivity reactions to the sulfonamides. There were 11 females and 10 males in the group, with a mean age of 15 years (age range, 1.8 to 50 years). Their acetylator phenotype was determined by determining the ratio of urinary caffeine metabolites (1-methylxanthine to 5-amino-6-formylmethyluracil after an oral dose of 50 mg caffeine). Nineteen (90%) of the patients were slow acetylators compared to a 55% incidence of slow acetylators in a race-matched control population (p less than 0.008). This suggests that a slow acetylator phenotype is a risk factor for the development of sulfonamide hypersensitivity reactions and provides further support for the role of imbalances in genetically determined pathways of metabolism and detoxification of the sulfonamides in the pathogenesis of these reactions.

Acetylation↗
PubMed1991

Caffeine as a metabolic probe: exploration of the enzyme-inducing effect of cigarette smoking.

It has been realized recently that the primary metabolism of caffeine in humans is catalyzed by P-450IA2 and that the rate of caffeine metabolism can be estimated from a metabolic ratio in a single urine sample. A population of 178 students including 19 smokers were subjected to this caffeine test to establish their P-450IA2 index. Both stated numbers of cigarettes smoked per day and urinary cotinine levels as a confirmatory measure correlated significantly with enzyme activity showing dose-effect relationships (r = 0.62 and 0.89, respectively). Nevertheless, more nonsmokers than smokers had the highest enzyme indexes, suggesting that dietary elements or other factors may determine P-450IA2 activities in populations. Because P-450IA2 is a monooxygenase that may be confined to the liver, caffeine reveals directly the Ah-receptor-dependent enzyme induction only in the liver, but it may also be a signal of induction elsewhere.

Caffeine↗
PubMed1991

Caffeine as a metabolic probe: validation of its use for acetylator phenotyping.

The use of two caffeine metabolite ratios for acetylator phenotyping was validated by demonstrating concordance with two sulfamethazine tests in 178 unrelated healthy subjects. The caffeine metabolites used for this purpose were 5-acetylamino-6-amino-3-methyluracil (AAMU), 1-methylxanthine (1X), and 1-methylurate (1U). The ratio AAMU/(AAMU + 1X + 1U), referred to as molar ratio or N-acetyltransferase, was compared with the ratio AAMU/1X. The results indicated that, for screening purposes, the acetylator phenotype can be determined by analysis of a 6-hour urine sample after a cup of coffee or strong tea or a can of caffeine-containing soft drink. The ratio AAMU/1X is the ratio of choice for the study of subjects in whom variability of xanthine oxidase can be neglected; use of the ratio AAMU/(AAMU + 1X + 1U) appears appropriate for special purposes. Gender, ethnic origin, habitual or moderate consumption of coffee, tea, soft drinks, or ethanol, or cigarette smoking have little if any effect on the caffeine tests for acetylator phenotyping.

Acetylation↗
PubMed1991

Urinary markers of chronic excessive ethanol consumption.

A gas chromatographic mass spectrometric procedure is described for the measurement of free ethanol and conjugates of ethanol and acetaldehyde. It was demonstrated that conjugates of ethanol and acetaldehyde were present in urine of alcoholics. Urinary levels of free ethanol and the conjugates in abstaining alcoholics were higher than those of social drinkers. On the average, alcoholics excreted equal amounts of free ethanol and ethanol conjugates while controls eliminated three parts of ethanol conjugates to one part of free ethanol. After 14 days of abstinence, 16/23 (70%) of alcoholics had levels of free ethanol and acetaldehyde conjugates higher than the mean +2 SD of social drinkers for these substances. When the molar ratio of ethanol conjugates/free ethanol was considered, five more subjects who were alcoholics could be classified as being alcoholics. There was one false positive among 46 control subjects.

Acetaldehyde↗
PubMed1990

Drug glucosidation.

A number of studies have shown that drug glucosidation occurs in vertebrates. This review summarizes information on the chemical, physical and biological properties of eleven drug glucosides. Three out of two hundred individuals exhibited a complete lack of amobarbital-N-glucoside formation. The data from a pair of identical twins who were deficient in this metabolic pathway suggested the presence of a recessive trait controlled by a single pair of autosomal genes which regulate N-glucoside formation. In addition, average values seemed to be subject to interethnic variation.

Animals↗
PubMed1990

Sequential metabolism of salicylamide exclusively to gentisamide 5-glucuronide and not gentisamide sulfate conjugates in single-pass in situ perfused rat liver.

Gentisamide (GAM), the hydroxylated metabolite of salicylamide (SAM), underwent sequential metabolism, and GAM-5-glucuronide was the only metabolite detected in the single-pass in situ rat liver preparation (10 ml/min/liver) perfused at varying SAM steady-state input concentrations (CIn). The exclusive formation of GAM-5-glucuronide was unexpected, because GAM, when administered to the rat liver, formed predominantly monosulfate conjugates at the 5- and 2-positions (Morris et al., J. Pharmacol. Exp. Ther. 245: 614-624, 1988). Kinetic parameters obtained from single-pass studies for SAM sulfation (CIn increasing from 31 to 347 microM) and glucuronidation and hydroxylation (CIn decreasing from 1383 to 130 microM, with 0.85 mM SO4(2-), and CIn increasing from 32 to 800 microM in an absence of SO4(2-), with 2,6-dichloro-4-nitrophenol, a sulfation inhibitor) revealed sulfation as a high-affinity, high-capacity pathway, glucuronidation as a lower-affinity, high-capacity pathway and hydroxylation as a low-affinity, low-capacity pathway. However, these parameters would not explain the exclusive glucuronidation of GAM when generated from SAM. Rather the zonal localization of metabolizing activities [a periportal sulfation, evenly distributed glucuronidation, and perivenous hydroxylation system (Xu and Pang, J. Pharmacokinet. Biopharm. 17: 645-671, 1989; Morris et al., J. Pharmacokinet. Biopharm. 16: 633-656, 1988)] could explain the complete lack of sulfation in SAM sequential metabolism. The different metabolite patterns arising from the administrations of a metabolite precursor (SAM) versus a preformed metabolite (GAM), due to the proximity of enzymes for formation and sequential metabolism, may serve to explain the differential toxic or pharmacologic effects observed in other precursor-metabolite pairs.

Animals↗
PubMed1988

Method to determine the enantiomers of ibuprofen from human urine by high-performance liquid chromatography.

By means of ethyl chloroformate, ibuprofen enantiomers were coupled to 4-methoxyaniline. The resulting amides were resolved from each other and from urinary constituents on a Pirkle column using an isocratic mobile phase with ultraviolet detection at 254 nm. Applicability of this method for the determination of inter-individual differences in urinary metabolic profiles of ibuprofen enantiomers is demonstrated. The chromatographic behavior of the corresponding amide derivatives of two ibuprofen metabolites is also described.

Biotransformation↗
PubMed1988

Competing pathways in drug metabolism. I. Effect of input concentration on the conjugation of gentisamide in the once-through in situ perfused rat liver preparation.

Sulfation and glucuronidation are two parallel pathways for the metabolism of phenolic substrates. Gentisamide (GAM) was used as a model compound to examine the effects of parallel competing pathways on drug disappearance and metabolite formation in the once-through perfused rat liver preparation. GAM was found to form one glucuronide (GAM-5G) and two sulfate (GAM-2S and GAM-5S) conjugates. These GAM conjugates were biosynthesized in recirculating rat liver preparations, and were isolated by preparative high-performance liquid chromatography. Specific incorporation of 35S-sodium sulfate and [14C]glucose into GAM sulfate and glucuronide conjugates revealed corresponding elution patterns as labeled GAM metabolites. Their identities were characterized by enzymatic and acid hydrolyses and by NMR spectroscopy. Gentisamide-5-sulfate (GAM-5S) and gentisamide-5-glucuronide (GAM-5G) are major metabolites, and gentisamide-2-sulfate (GAM-2S) is a minor metabolite. Single-pass rat liver perfusions were used to examine the effect of stepwise increases/decreases of input GAM concentration (CIn) on the extraction ratio (E) of GAM and formation of metabolites. The E of GAM remained constant (about 0.89) at input concentrations from 0.9 to 120 microM and decreased at CIn greater than 120 microM. Metabolite patterns, however, changed with GAM CIn, even when E was constant at CIn up to 120 microM. GAM-5S was present as the major metabolite of GAM at all GAM CInS in most liver preparations but the proportions of GAM-5S and GAM-2S decreased at increasing CIn; the proportion of GAM-5G, a minor metabolite at low CIn, increased with increasing CIn. Biliary excretion rates at steady state accounted for 5.3 +/- 2.7% (mean +/- S.D.) of the input rate: GAM-5G was the predominant metabolite found. Fitting the metabolic data (sum of the rates of efflux in bile and perfusate at steady state) and the logarithmic average drug concentration at the various CIn to the Michaelis-Menten equation furnished parameter estimates for the three metabolic pathways. The estimated Km and Vmax values were quite comparable: for GAM-2S formation, 22 microM and 287 nmol/min; for GAM-5S formation, 26 microM and 978 nmol/min; for GAM-5G formation, 71 microM and 1062 nmol/min, indicating that sulfation and glucuronidation are effective competing pathways of each other. In viewing the fate of GAM over the CIn range, a changing metabolic fate of GAM over a constant E was noted at CIn less than 120 microM: the sum of the rates of metabolite formation was proportional to CIn.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗