REDUCTIVE METABOLISM AND PROTEIN-BINDING OF CHROMIUM(VI) BY P450 PROTEIN ENZYMES

The cytochrome P450-dependent reduction of Cr(VI) using reconstituted phospholipid vesicles containing purified preparation of various forms of rabbit and rat liver microsomal cytochrome P450 has been investigated. The alcohol-induced form of the rat, P450IIE1, was the most efficient enzyme, 7.2 +/- 0.40 nmol Cr/nmol P450/min, whereas the corresponding rates for rat P450IA1, rat IIB1, rabbit IIB4, rabbit IA2 and rabbit IIE1 were 1.7 +/- 0.09, 2.5 +/- 0.08, 1.6 +/- 0.08, 2.5 +/- 0.15 and 1.6 +/- 0.08 nmol Cr/nmol P450/min respectively. NADPH-cytochrome P450 reductase had Cr(VI) reductase activity which was dependent on enzyme concentration. Below 0.15 nmol P450 reductase/ml the sp. act. was low and constant, while at a higher concentration the activity was markedly dependent upon the amount of enzyme present. In a quantitative binding assay it was shown that binding of [Cr-51]Cr(VI) to the catalytic enzymes was proportional to the enzyme concentration up to 0.8 nmol P450/ml, which caused binding of 70% of the total radioactivity. Analysis by SDS-PAGE and autoradiography exhibited binding to the individual catalytic proteins of [Cr-51]Cr. EDTA treatment removed the radioactivity from the bands matching P450 and P450 reductase, indicating that Cr(III) is bound to the proteins. The reducing activity of both P450 and P450 reductase was potently inhibited by oxygen. The inhibitory effect of oxygen is not due to reoxidation of the reduced Cr and redox cycling. Rat P450IA1 ethoxycoumarin O deethylase activity was inhibited after preincubation with chromate (CrO4(2-)). The P450 reductase inhibitor 2'-AMP stimulated the anaerobic P450 reductase dependent Cr(VI) reductase rate approximately 2-fold. Both CO and CCl4 inhibited the different P450 enzymes to various extents. With rabbit P450IIE1 CCl4 stimulated the Cr(VI) reduction approximately 4-fold, whereas the activity of the other enzymes was inhibited when the reconstituted system was incubated with CrO4(2-) and CCl4 prior to NADPH addition. Neither CO nor CCl4 affected the Cr(VI) reducing activity of the P450 reductase. The difference in CrO4(2-) reducing activity of the P450 enzymes and binding to the enzymes may be important for in vivo endoplasmic catalytic metabolism of CrO4(2-).