Effect of phenobarbital and 3-methylcholanthrene on biphenyl hydroxylations and fluidity of rat liver microsomal membranes.

The metabolism of biphenyl by rat liver microsomes after administration of phenobarbital and 3-methylcholanthrene was studied. Phenobarbital increased the activity of biphenyl-4-hydroxylase and 3- methylcholanthrene increased the activity of both biphenyl-4-hydroxylase and biphenyl-2-hydroxylase as compared to non-treated (control) rats. Phenobarbital increased the lipid fluidity while 3-methylcholanthrene increased the lipid rigidity of microsomal membranes labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH), as indicated by the steady-state fluorescence anisotropy [(ro/r)-1]-1. Arrhenius plots of [ro/r)-1]-1 indicated that the lipid phase separation of the control membrane at 22.1 +/- 1.1 degrees was reduced in phenobarbital treated (14.5 +/- 0.8 degrees) and increased in 3-methylcholanthrene treated rats (32.7 +/- 2.2 degrees). Arrhenius plots of biphenyl-4-hydroxylase and biphenyl-2-hydroxylase activities exhibited a break point at 21.8 +/- 1.1 degrees and 32.1 +/- 2.1 degrees, respectively, suggesting differences in the interactions of the enzymes with their annular lipids. It is suggested that biphenyl-4-hydroxylase requires a liquid state of its lipid microenvironment to be fully active, while biphenyl-2-hydroxylase a gel state of its lipid microenvironment. These studies provide a basis for postulating that a "non-genomic" mechanism of phenobarbital and 3-methylcholanthrene induces cytochrome P-450 dependent monoxygenases.