Inhibition of substrate binding to the adrenal cytochrome P450C-21 by acrylamide and its implications for solvent accessibility of the binding site in the microsomes.

The present study offers evidence indicating that acrylamide, a highly polar molecule and an efficient quencher of tryptophanyl fluorescence, inhibits substrate binding to P450C-21 in bovine adrenocortical microsomes, in a competitive manner similar to that in the purified enzyme. Resolution of the fluorescence-quenching data revealed an acrylamide quenching constant (K2 = 9.9 M, that is, the association constant for the quencher-fluorophore complex) that was similar to the reciprocal of its inhibition constant (1/Ki = Ka = 8.3 +/- 0.9 M) for substrate binding. The substrate inhibited the fluorescence quenching by acrylamide as indicated by its concentration-dependent decrease in K2. The inhibition was in accordance with partial competition. These results are essentially similar to those previously observed in the purified lipid-free enzyme. In addition, the substrate dissociation, acrylamide inhibition, and fluorescence-quenching constants and the tryptophanyl fluorescence maximum (340-342 nm) were essentially the same in the microsomes and the lipid-free purified enzyme. These results indicate that the substrate-binding site of P450C-21 and the concerned tryptophan are accessible to the highly polar molecule in the microsomal membranes, similar to that in the lipid-free purified enzyme. This implies that the substrate-binding site is not shielded by lipids in such a way that only the substrate in the lipid phase can gain access to the binding site. This conclusion is consistent with the currently favored model, for membrane topology of mammalian P450 enzymes, in which P450 is anchored to the membrane through a short N-terminal sequence while the remaining portion of the molecule is exposed to polar environment.