Rat-liver cholesterol 7 alpha-hydroxylase. 1. Development of a new assay based on the enzymic exchange of the tritium located on the 7 alpha position of the substrate.

A new assay is described to measure the activity of cholesterol 7alpha-hydroxylase and compared to the conventional 14C method used by other investigators. This method is based on the mechanism of the enzymic hydroxylation, i.e. a direct and stereospecific substitution of the 7alpha-hydrogen by a hydroxyl group. [7alpha-3H]Cholesterol is incubated at 37 degrees C and in the presence of molecular O2, in a medium buffered by postassium phosphate at pH 7.4 and containing liver microsomes (or 9000 X g supernatant), NADPH, MgCl2 and cysteamine. Tween-80 (1.5 mg/ml) is used to introduce enough substrate (300 muM) in the incubation mixture to saturate the enzyme (Km = 100 muM). Under these conditions the tritiated water released into the incubation medium reflects accurately the enzymic activity. The results obtained with this method are similar to the one obtained with a [4-14C]cholesterol technique (r = 0.96; P less than 0.001). The main advantage of the [7alpha-3H]cholesterol method is a complete independence from further metabolism of the first enzymic product, the 7alpha-hydroxycholesterol, the tritiated water representing the entire cholesterol 7alpha-hydroxylase activity.