METABOLISM AND CYTOCHROME P-450 BINDING SPECTRA OF (+)- AND (-)-HEXOBARBITAL IN RAT LIVER MICROSOMES

The mixed function oxygenation system in liver microsomes from rats was investigated as to its stereospecificity towards the two optical anti-jpodes of hexobarbital. Considerable differences in the reactions with (+)- and (-)-hexobarbital were found. The specific activity of the mixed function oxygenation of microsomes from untreated rats was 1.6 times higher for (+)-hexobarbital than for (-)-hexobarbital. After pretreatment with phenobarbital the ratio between the two specific activities increased to 2.2. The specific spectral changes in the cytochrome P-450 bidding difference spectra with (-+)- and (-)-hexobarbital yielded almost the same ratios: 1.7 in microsomes of untreated rats and 2.1 in those of phenobarbital treated rats. The affinities of cytochrome P-450 for the two antipodes were slightly different; the Ks for (+)-hexo-barbital being about 15% lower. The first rapid phase of the reduction of cytochrome P-450 by NADPH under anaerobic conditions was stimulated twofold in the presence of (+)-hexo-barbital compared to (-)-hexobarbital. The discussion deals with the possibility of several species of cytochromes P-450, with different affinities towards (+)- and (-)-hexobarbital, being unequally induced by phenobarbital. A linear relationship between the specific activity of the mixed function oxygenating system for a substrate and the spectral change in the cytochrome binding difference spectra is stated. It is possible that the cytochrome P-450 substrate complex is faster reduced than the cytochrome P-450 alone. © 1969, IEEE. All rights reserved.