Studies on the determination and induction of cholesterol 7alpha-hydroxylase

To determine the activity of cholesterol 7alpha-hydroxylase, different fractions from rat liver homogenates are incubated with glucose-6-phosphate, glucose-6-phosphate dehydrogenase and NADP as well as with cholesterol-4-14C. The 7alpha-hydroxycholesterol-4-14C formed is separated from the extract of the incubation preparation by means of thin-layer chromatography and its radioactivity is calculated as a percentage (conversion figure) of the total radioactivity on the plate. The highest conversion is found in the 18 000 X g supernatant. It is of the order of 1--4%. This activity decreases to about one-half when the homogenate or the cell fraction is stored, while cooling, for two days. It can be shown that repeated pretreatment of rats with phenobarbital increases the activity of hepatic cholesterol 7alpha-hydroxylase by more than three times over the control values. 1-(2'-Hydroxyphenyl-acetyl)-4-methyl-1,2,3,6-tetrahydropyridine (Bay a 4798) a choleretically active drug also increases the activity of this enzyme in the rat more than twofold. This effect is confirmed by reduced biliary excretion of radioactive cholesterol and increased biliary excretion of radioactive bile acids following i.v. administration of DL-mevalolactone-2-14C and continuous intraduodenal infusion of Bay a 4798.