Item – Theses Canada

OCLC number
1006926401
Link(s) to full text
LAC copy
LAC copy
Author
Wong, Simon Gar Wah.
Title
Identification of the cytochrome P450 isozymes responsible for N-alkylprotoporphyrin IX formation after the administration of porphyrinogenic xenobiotics.
Degree
Ph. D. -- Queen's University, 2001
Publisher
Ottawa : National Library of Canada = Bibliothèque nationale du Canada, [2001]
Description
2 microfiches
Notes
Includes bibliographical references.
Abstract
Several xenobiotics produce porphyria, a derangement of heme biosynthesis, through mechanism-based inactivation (MBI) of cytochrome P450 resulting in the formation of N-alkylprotoporphyrin IX (N-alkylPP), a potent inhibitor of the terminal enzyme in heme biosynthesis, ferrochelatase. Because compounds such as 3,5-diethoxycarbonyl-4-ethyl-2,6-dimethylpyridine (4-ethylDDC), 3-[(arylthio)ethyl] sydnone (TTMS), and allylisopropylacetamide (AIA) depend upon selected P450 isozymes for mechanism-based inactivation and N-alkylPP formation, species differences in P450 isozyme expression are important for their porphyrinogenicity. Therefore, to assess the validity of animal models for predicting human drug-induced porphyria, the objective of this study was to determine the key P450 isozymes susceptible to MBI and N-alkylPP formation in chick embryo, rat, and human. Chicken CYP2H1/2 were major sources of TTMS-mediated N-alkylPP formation in glutethimide-treated chick embryos. Because of the lack of selective rat hepatic P450 isozyme inducers and inhibitors, a novel approach was introduced utilizing the marked gender differences in rat P450 isozyme expression and response to inducers to determine the P450 isozyme contribution to N-alkylPP formation. Male rats produced markedly more N-alkylPP than females for all three drugs, demonstrating that male specific P450 isozymes were major sources of N-alkylPP. In male rats the major P450 isozyme involved in (N-alkylPP), formation mediated by AIA and 4-ethylDDC was male specific CYP2C11, and the minor isozyme source was the non-gender specific CYP2C6. CYP3A2, which was not involved in N-alkylPP formation from 4-ethylDDC or AIA, was the major contributor to N-alkylPP formation from TTMS; CYP2C11 was a minor contributor. Thus, CYP2C11 was the only isozyme which contributed to (N-alkylPP), formation from all three drugs tested. Studies with rat hepatic microsomes showed that (N-alkylPP), formation occurs ' in vitro' following MBI, although in a lower yield than ' in vivo'. Due to the low sensitivity of the detection methods employed in these 'in vitro' studies, a more sensitive bioassay, utilizing human recombinant ferrochelatase was developed to detect N-alkylPP formation ' in vitro'. This bioassay was employed to determine whether hepatic microsomes containing predominantly CYP3A4 were a source of TTMS-mediated N-vinylPP formation. The inability to detect N-vinylPP formation was attributed to MBI of CYP3A4 by route(s) which do not include N-alkylPP formation.
ISBN
0612561089
9780612561083