Reduction of aflatoxin B-1 dialdehyde by rat and human aldo-keto reductases

Oxidation of the mycotoxin aflatoxin (AF) B-1 yields the 8,9-epoxide, which nonenzymatically hydrolyzes rapidly to a dihydrodiol that in turn undergoe s slow, base-catalyzed ring opening to a dialdehyde [Johnson, W. W., Harris , T. M., and Guengerich F. P. (1996) J. Ant. Chem. Sec. 118, 8213-8220]. AF B(1) dialdehyde does not bind to DNA but can react with protein lysine grou ps. One enzyme induced by cancer chemopreventive agents is AFB(1) aldehyde reductase (AFAR), which catalyzes the NADPH-dependent reduction of the dial dehyde to a dialcohol. AFB(1) dialdehyde is known to convert, nonenzymatica lly to AFB(1) dihydrodiol at neutral pH, and we reinvestigated tile enzymat ic reaction by preparing AFB(1) dialdehyde at pH 10 and then used this to i nitiate reactions (at neutral pH) with rat and human AFAR isozymes. Two mon oalcohols were identified as products, and their identities were establishe d by (NaBH4)-H-2 reduction, chemical cleavage, and mass spectrometry. The m onoalcohol corresponding to reduction at C-8 formed first in reactions cata lyzed by either the rat or the human AFAR. This C-8 monoalcohol was further reduced to AFB(1) dialcohol by AFAR. The other monoalcohol (C-6a) was form ed but not reduced to the dialcohol rapidly. Steady-state kinetic parameter s were Estimated for the reduction of AFB(1) dialdehyde by rat and human AF AR to the monoalcohols. The apparent k(cat) and K-m values were not adequat e to rationalize the observed DeltaA(340) spectral changes in a kinetic mod el. Simulation fitting was done and yielded parameters indicative of greate r enzyme efficiency. A survey of 12 human liver cytosol samples showed a va riation of 2.3-fold in AFAR activity. Rats treated with AFB(1) excreted the dialcohol and a monoalcohol in urine. The results of these studies are con sistent with a role of(rat and human) AFAR in protection against AFB(1) tox icity.