Half-life of cholesterol 7alpha-hydroxylase activity and enzyme mass differ in animals and humans when determined by a monoclonal antibody against human cholesterol 7alpha-hydroxylase

A monoclonal antibody against human cholesterol 7alpha-hydroxylase was produced, and the half-life of the enzyme was studied. Both the activity and protein mass of the enzyme were measured at timed intervals during microsomal incubation. The enzyme activity dropped rapidly; the half-life was 1, 1.7 and 3h in humans, rats and rabbits, respectively. In contrast, the protein mass, measured by immunoblotting, declined slowly; its half-life was 5h in the human and 9h in the rat and rabbit enzymes. This suggests that there may be vulnerable sites responsible for the enzyme activity. Addition of dithiothreitol (DTT) restored the decreased activity considerably, indicating that at least one sulfhydryl group is involved in the vulnerability. These results show considerable decrement in cholesterol 7alpha-hydroxylase activity due to sulfhydryl groups.