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Y Irshaid

Publications and source records attributed to Y Irshaid.

8 recordsLinked to original sources
PubMed1998

Variability of coumarin 7- and 3-hydroxylation in a Jordanian population is suggestive of a functional polymorphism in cytochrome P450 CYP2A6.

OBJECTIVE: To determine the variability of coumarin 7- and 3-hydroxylation in a human population and to evaluate the evidence for the existence of genetic polymorphism in these pathways. 7-Hydroxylation of coumarin is considered to be a detoxication pathway, whilst 3-hydroxylation, which predominates in rats, leads to hepatotoxicity in the rat. Coumarin metabolic phenotypes could aid in refining the risk evaluation for humans of dietary and environmental exposure to coumarin and for the chronic use of coumarin in high doses as a drug to treat lymphoedema and certain cancers. METHODS: Healthy male and female Jordanian volunteers (n = 103) were administered 2 mg coumarin by mouth and collected their 0-8-h urines. These, together with pre-dose blank urines, were analysed by selected-ion monitoring gas chromatography mass spectrometry for their content of the coumarin metabolites 7-hydroxycoumarin (70HC) and 2-hydroxyphenylacetic acid (2OHPAA), the latter arising from the 3-hydroxylation pathway. RESULTS: After coumarin administration, excretion of both 70HC and 2OHPAA was highly variable. A coumarin metabolic ratio (2OHPAA/7OHC) was suggestive of polymorphism. At least one subject had a metabolic response similar to an individual known to be both phenotypically and genotypically (CYP2A6 gene) 7-hydroxylation-deficient. CONCLUSION: In the light of the finding of high variability and possible polymorphism in both the 7- and 3-hydroxylation of coumarin in a human population. we recommend a reappraisal of the risk evaluation of human exposure to coumarin, particularly in pharmaceutical doses.

Adult↗
PubMed1996

Metabolic interactions of putative cytochrome P4503A substrates with alternative pathways of dapsone metabolism in human liver microsomes.

The cytochrome P4503A (CYP3A) subfamily of enzymes are responsible for the metabolism of a large number of endogenous and exogenous compounds, and activation of some procarcinogens; but the activity is not well understood. N-Hydroxylation of dapsone in human liver microsomes has been shown to be mediated largely by CYP3A4. We have also observed the formation of an as yet unidentified metabolite of dapsone, whose formation is inhibited by antibody to CYP3A4, by these microsomes. This study investigated the influence of various (22) CYP3A putative substrates on the formation of both metabolites of dapsone in human liver microsomes. The compounds fall into four different categories on the basis of the pattern of their inhibitory interaction with the formation of both metabolites: those that inhibited both metabolites; those that inhibited N-hydroxylamine alone; those that inhibited the unidentified metabolite alone; and those with no significant effect on either metabolite. Some others were stimulatory. These results are consistent with two alternative but not mutually exclusive hypotheses: 1) different isoforms of CYP3A are involved in the formation of the alternative metabolites and the pattern of interaction observed was caused by the particular isoform(s) that each compound interacted with; or 2) formation of the alternative metabolites is a result of dapsone's interaction with and orientation at the enzyme's active site and the pattern of interaction observed is a consequence of changes in orientation caused by these compounds. This study provides relevant observations that must be considered in understanding mechanisms of CYP3A-mediated metabolism.

Catalysis↗
PubMed1992

Acetylator phenotypes of Jordanian diabetics.

The acetylator phenotype was determined in 31 insulin-dependent (IDDM) and 110 noninsulin-dependent (NIDDM) Jordanian diabetics, and was compared to that of 160 healthy volunteers of the same ethnic group. Dapsone was used as the test drug. The rapid acetylator phenotype was slightly less frequent in IDDM and slightly more frequent in NIDDM. Neither of the differences was significant. When acetylator status in the two types of diabetes mellitus was compared, there was a significant difference among the two groups. Patients with IDDM had a higher percentage of the slow acetylator phenotype when compared to NIDDM patients. The association between acetylator status and IDDM in Jordanians, which agrees with that reported for the Saudi Arabian population, is the reverse of what is found in European populations. The results demonstrate ethnic differences in acetylator status among IDDM patients.

Acetylation↗
PubMed1992

Lack of effect of certain histamine H2-receptor blockers on the glucuronidation of 7-hydroxy-4-methylcoumarin by human liver microsomes.

Contrary to the general belief that cimetidine and ranitidine spare glucuronidation of drugs, some authors have indicated that both cimetidine and ranitidine could inhibit the glucuronidation of paracetamol (acetaminophen) by cultured rat hepatocytes. Thus, we tested the effect of three histamine H2-receptor blockers (cimetidine, ranitidine and famotidine) on the glucuronidation of 7-hydroxy-4-methylcoumarin (7-OH-4-MC) by human liver microsomes. None of the drugs studied produced significant inhibition of the glucuronidation of 7-OH-4-MC when used at concentrations up to 1.5 mM. Thus, even the new H2-receptor blocker, famotidine, spares glucuronidation. These findings further support the previous reports which suggest that glucuronide conjugation in humans is spared by H2-receptor blockers.

Cimetidine↗
PubMed1991

Glucuronidation of 7-hydroxy-4-methylcoumarin by microsomes from normal and diseased human livers.

Glucuronidation of 7-hydroxy-4-methylcoumarin (7-H-4-MC) by microsomal UDP-glucuronosyltransferases from 12 human liver biopsy samples was studied. Glucuronidation rates were highest for livers with mild cholestasis while those for livers with mild fatty changes were similar to normal livers. The lowest rates were exhibited by livers showing primary or metastatic carcinoma on histological examination. Thus, glucuronidation of not only bile acids but also of other substrates such as 7-H-4-MC may be elevated in cholestasis. These results are also consistent with other studies stating that glucuronidation might be spared in certain mild hepatic diseases.

Adult↗
PubMed1988

Endogenous substrates for UDP-glucuronosyltransferases.

1. Multiple forms of UDP-glucuronosyltransferase (UDPGTs) have been demonstrated in the livers of all mammalian species that have been studied. Rat liver possesses at least eight different isozymes and human liver has at least five different forms which have been identified. 2. Endogenous substrates (e.g., steroids) are helpful in distinguishing UDPGTs as they generally react with only a single form, whereas xenobiotic substrates (e.g., 4-methyl-umbelliferone, p-nitrophenol) react with several forms of the enzyme. 3. Human liver UDPGTs differ in physical properties and substrate specificity from these enzymes obtained from laboratory animals. Hence, it is necessary to study human liver UDPGTs to elucidate substrate specificity and to understand drug-endogenous substrate interaction in humans.

Animals↗
PubMedDate not supplied

Monoacetyldapsone inhibition of dapsone N-hydroxylation by human and rat liver microsomes.

Dapsone (DDS) is metabolized by N-hydroxylation and N-acetylation to DDS hydroxylamine (DDS-NOH) and monoacetyldapsone (MAD), respectively. The activities of these two alternative and independent reactions vary widely between individuals and show an inverse relationship during chronic DDS therapy. Toxicity observed during DDS therapy has been attributed to DDS-NOH. The observation of reduced toxicity in rapid acetylators, who are also poor hydroxylators, therefore, raised the possibility that MAD may be inhibiting DDS-NOH formation. This hypothesis was tested in human and rat liver microsomes. Human liver microsomes hydroxylated DDS with a lower affinity (KM 2-fold greater) and lower maximal catalytic activity (Vmax 12-fold lower) than that of the rat. The relative catalytic activity (Vmax/KM) was 22-fold higher in rat compared with human liver microsomes. Furthermore, MAD was a potent inhibitor of DDS N-hydroxylation by rat liver microsomes (52% inhibition at 0.01 mM MAD) compared with human liver microsomes (23% inhibition at 0.4 mM MAD). Human, but not rat, liver microsomes deacetylated MAD to DDS by an NADPH independent mechanism. These results show that substantial differences exist in DDS N-hydroxylase between rats and humans, with respect to substrate affinity, enzyme activity, and susceptibility to inhibition, such that information obtained from the rat should not be extrapolated to humans. We conclude that MAD is a potent inhibitor of DDS-NOH formation in rat liver microsomes. The degree of inhibition in human microsomes, however, suggest that MAD is unlikely to be a significant modulator of enzyme activity in vivo.

Animals↗