Binding of Polysomes In Vitro with Endoplasmic Membrane Prepared from Rat Liver

Abstract

Using an in vitro polysome-membrane binding system from rat liver developed by us, the effect of structural alteration of the microsomal membrane on its polysome-binding capacity was examined. Treatments of rats with phenobarbital (PB) or 3-methyicholanthrene (3-MC), inducers of cytochrome P450 in rat liver, enhanced the binding capacity of the liver membrane. This effect was especially marked with 3-MC, which induced a new type of cytochrome P450, called cytochrome P448. On the other hand, treatment with CoCl₂in vivo reduced not only the amount of cytochrome P450, but also the polysome-binding capacity of the membrane. When NADPH was added to the incubation mixture, the binding capacity of the membrane was reduced. MnCl₂ prevented this reduction. Spectrophotometric analysis showed that the amount of (cytochrome P450+cytochrome P420) decreased in the presence of NADPH, while addition of MnCl₂ inhibited this decrease. Addition of ethanol in vitro enhanced the binding capacity, consistent with results described previously. The inhibitory effect of NADPH and the stimulatory effect of ethanol on the binding were also found with PB- or 3-MC-treated rat liver membranes. Partially purified cytochrome P448 from 3-MC-treated rat liver was shown to interact with polysomes in vitro. These results are discussed from the standpoint of an involvement of cytochrome P450 in the binding of polysomes to microsomal membrane structure.