Betamethasone-mediated activation of biphenyl 2-hydroxylation in rat liver microsomes. Studies on possible mechanisms

The addition of the steroid betamethasone to intact or detergent-solubilized rat liver microsomes caused a concentration-dependent increase in the rate of biphenyl 2-hydroxylation. Betamethasone (100 microM) increased the apparent Vmax for 2-hydroxybiphenyl formation 2- to 4-fold but had no effect on the apparent Km when either the biphenyl or NADPH concentration was varied. Betamethasone had little or no effect on the apparent Vmax or apparent Km of the 3- and 4-hydroxylations of biphenyl. The steroid did not enhance biphenyl 2-hydroxylation through a peroxidative mechanism. Betamethasone had little or no effect on the rate of the NADPH-dependent reduction of cytochrome c or total microsomal cytochrome P-450. The addition of purified NADPH-cytochrome P-450 reductase to cholate-solubilized liver microsomes increased the rate of hydroxylation of biphenyl in positions 2 and 4. Betamethasone (100 microM) decreased the apparent Km for purified cytochrome P-450 reductase by 48% and increased the apparent Vmax of 2-hydroxybiphenyl formation by 2-fold when the concentration of cytochrome P-450 reductase was varied. The steroid did not alter the Km or Vmax values for the 4-hydroxylation of biphenyl. The data suggest that betamethasone enhances the interaction between the reductase and the form(s) of cytochrome P-450 responsible for the 2-hydroxylation of biphenyl.