SYNCHRONOUS CIRCADIAN-RHYTHMS OF MESSENGER-RNA LEVELS AND ACTIVITIES OF CHOLESTEROL 7-ALPHA-HYDROXYLASE IN THE RABBIT AND RAT

Cholesterol 7 alpha-hydroxylase, the key enzyme in a series of metabol ic changes for the production of bile acids from cholesterol, shows ci rcadian rhythms. The mechanism of the circadian rhythms is the subject of controversy; moreover, few pieces of information are presently ava ilable. Because the rabbit is a good animal model for the study of hum an 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 pr obes, the circadian rhythms of this enzyme were studied in normal male rabbits and rats. The animals were maintained on a 12-h light-dark cy cle. Three subjects each were then killed every 3 h for 1 day. The liv ers were harvested, and the mRNA levels and the activities of the enzy me were measured. Both mRNA levels and activities of the enzyme showed remarkable circadian rhythms, with higher values during the dark phas e and lower Values reaching minimum during the light phase. The enzyme activities correlated well with the mRNA levels. There was no shift i n the cycles of the enzyme activities and the mRNA levels. These findi ngs suggest that pretranslational regulation is the most likely mechan ism for the circadian rhythms of cholesterol 7 alpha-hydroxylase. Beca use the rhythms of mRNA levels and activities of this enzyme are synch ronous, the turnover of the enzyme should be as fast as that of the mR NA, and we can hereafter measure the mRNA levels instead of the enzyme .