Abstract
Uroporphyrinogen is an intermediate of the heme biosynthetic pathway. The oxidation of uroporphyrinogen to uroporphyrin (UROX) has been demonstrated to be catalyzed by mammalian CYP1A2. This reaction has an important role in uroporphyria caused by halogenated aromatic compounds. Two CYP enzymes induced by Ah receptor ligands were purified recently from chick embryo liver. One, designated CYP1A5, was preferentially active in arachidonic acid epoxygenation and the other, designated CYP1A4, in 7-ethoxyresorufin deethylase (EROD) and aryl hydrocarbon hydroxylase (AHH), reactions mainly catalyzed by CYP1A1 in rodents. The amino acid sequences of both CYP1A5 and CYP1A4 are more similar to CYP1A1 than to 1A2, and neither can be classified as an ortholog of mammalian CYP1A1 or 1A2. Here we report that reconstituted purified CYP1A5 was eight times more active than CYP1A4 in catalyzing UROX. The stimulation of UROX by 3,4,3′,4′-tetrachlorobiphenyl that has been observed in microsomes was also observed with the reconstituted enzymes. Similar dose response relationships were found for induction of UROX and EROD in both chick embryo liver microsomes and in cultured chick hepatocytes, indicating coinduction of CYP1A5 and CYP1A4. UROX was induced by the Ah receptor ligand, 3-methylcholanthrene, in chicken kidney as well as liver. The findings reported here and other evidence that CYP1A4 and CYP1A5 tend to exhibit CYP1A1 and 1A2-like enzyme activites, respectively, indicate that the division of some enzyme activities among CYP1A enzymes applies to different vertebrate classes.
Footnotes
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Send reprint requests to: P. Sinclair, VA Medical Center (151), White River Junction, VT 05009-0001. E-mail:psinc{at}Dartmouth.edu.
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This study was funded by Grants ES03606 (A.B.R.) and ES06203 (P.R.S.) from the National Institutes of Health and by research funds from the Department of Veterans Affairs.
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This work was presented as a poster at the XthInternational Symposium on Microsomes and Drug Oxidations, Toronto, Canada, July, 1994.
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↵2 Sinclair, P, Gorman, N and Lambrecht, R., unpublished observations (1993).
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↵3 Hirada, K., C. A. Lee, and A. B. Rifkind, unpublished observations (1994).
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↵4 Gilday, D. and A. B. Rifkind, unpublished observations (1995).
- Abbreviations used are::
- TCDD
- 2,3,7,8-tetrachlorodibenzo-p-dioxin
- CYP
- cytochrome P450
- EROD
- ethoxyresorufin deethylase
- UROX
- uroporphyrinogen oxidation
- TCB
- 3,4,3′,4′-tetrachlorobiphenyl
- CYP1A5 and CYP1A4
- CYP1A enzymes isolated from TCDD-treated chick embryos previously known as TCDDAA and TCDDAHH, respectively (11)
- AHH
- aryl hydrocarbon hydroxylase
- AA
- arachidonic acid
- Received August 16, 1996.
- Accepted March 27, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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