Date of Award:

1997

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Animal, Dairy, and Veterinary Sciences

Advisor/Chair:

Roger A. Coulombe Jr.

Abstract

Aflatoxin B1 (AFB1) is a potent dietary hepatocarcinogen and may be a lung carcinogen when inhaled. To study the relative ability of lung and liver to metabolize AFB1, a susceptible (Swiss-Webster rat) and resistant species (Syrian golden hamster) were pretreated with inducing agents in order to identify specific AFB1 metabolizing enzymes in each tissue.

Analysis of AFB1-exo-epoxide (AFBO) formation, O-dealkynation assays, and protein immunoblots demonstrated that cytochrome P450 (CYP) 1A proteins were overexpressed in both the lung and liver of hamsters pretreated with 3-methylchyolanthrene (3-MC). Only CYP1A1 was expressed in the lung and there was no indication that this protein was involved in AFB1 activation. CYP1A2, on the other hand, was induced in the liver and this correlated well with both increasing protein activity and AFBO formation. It would appear that CYP1A2 is important in activating AFB1 in hamster liver.

Although the hamster is resistant as compared to the rat, AFBO formation was higher in both the lung and liver of the hamster compared to the rat. Glutathione S-transferase (GST) Yc subunits were detected in the lung and liver of both species but were not induced by the inducing agents used in these experiments.

Following intratracheal injections of [3H]AFB1, in the rat, specific activity was localized in the liver. Only a fraction of the activity was detected in the lung. Of four inducing agents used, only pretratment with phenobarbital (PB) showed increased AFB1-DNA binding in either lung or liver. This correlated with increased CYP2B1 protein levels in both lung and liver, as well as increased CYP2B1 activity and AFBO formation in the liver.

Cooxidation of AFB1 by purified prostaglandin H-synthase was shown to produce AFBO but microsomal fractions from rabbit lung and liver failed to show detectable levels of AFBO formation by this cooxidative pathway. Neither purified 5-lipoxygenase or cytosolic fractions from rabbit lung or liver showed detectable levels of LOX mediated cooxidation of AFB1 to AFBO.

These studies demonstrate that hamster resembles the human in regard to AFB1 activation in the liver, but that a different as yet unknown enzyme is responsible for hamster lung AFB1 activation. Further evidence that the rat is a poor model for human AFB1 metabolism was demonstrated with the fact that rat activates AFB1 with CYP2B1, a protein unknown in humans.

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