Synchronous circadian rhythms of mRNA levels and activities of cholesterol 7 alpha-hydroxylase in the rabbit and rat.

Cholesterol 7 alpha-hydroxylase, the key enzyme in a series of metabolic changes for the production of bile acids from cholesterol, shows circadian rhythms. The mechanism of the circadian rhythms is the subject of controversy; moreover, few pieces of information are presently available. Because the rabbit is a good animal model for the study of human cholesterol-bile acid metabolism, in the present study we isolated a complete cDNA encoding rabbit cholesterol 7 alpha-hydroxylase (3022 base pairs (bp) long and 1503 bp open-reading frame encoding 501 amino acid residues). Using this isolate rabbit cDNA and the rat cDNA as probes, the circadian rhythms of this enzyme were studied in normal male rabbits and rats. The animals were maintained on a 12-h light-dark cycle. Three subjects each were then killed every 3 h for 1 day. The livers were harvested, and the mRNA levels and the activities of the enzyme were measured. Both mRNA levels and activities of the enzyme showed remarkable circadian rhythms, with higher values during the dark phase and lower values reaching minimum during the light phase. The enzyme activities correlated well with the mRNA levels. There was no shift in the cycles of the enzyme activities and the mRNA levels. These findings suggest that pretranslational regulation is the most likely mechanism for the circadian rhythms of cholesterol 7 alpha-hydroxylase. Because the rhythms of mRNA levels and activities of this enzyme are synchronous, the turnover of the enzyme should be as fast as that of the mRNA, and we can hereafter measure the mRNA levels instead of the enzyme.