Effects of extraction with isooctane upon the properties of liver microsomes.

When rat liver microsomes were extracted with isooctane (2,2,4-trimethylpentane), the resultant preparation was essentially devoid of the ability to produce a type I difference spectrum upon addition of hexobarbital. At high concentrations of hexobarbital, a type II spectral change was obtained with the extracted microsomes. The type II interaction of aniline with isooctane-extracted microsomes was not enhanced in the presence of hexobarbital, as it was in the case of untreated microsomes. Hexobarbital binding to cytochrome P-450 of untreated microsomes was inhibited in the presence of isooctane, but the amount of residual isooctane in the extracted microsomes was not sufficient to explain the loss of hexobarbital-binding activity. The isooctane extracts contained phosphatidylcholine and phosphatidylethanolamine. The results are discussed in terms of a model in which the type I binding form of cytochrome P-450 involves an association of hemoprotein with a phospholipid, or with some other microsomal constituent in an interaction involving phosphatides.