Circadian Rhythms of Sterol 12α-Hydroxylase, Cholesterol 7α-Hydroxylase and DBP Involved in Rat Cholesterol Catabolism

Circadian rhythms of important enzymes involved in the conversion of cholesterol to bile acids [sterol 12α-hydroxylase (12α-hydroxylase) and cholesterol 7α-hydroxylase (7α-hydroxylase)] and an albumin site D-binding protein (DBP) were examined in rats. When the animals were fed freely, they usually ate in the dark and the circadian rhythms of activities of 12α-hydroxylase and 7α-hydroxylase showed the same peaks (at 10 p.m.) and lows (at 2 p.m.). Their mRNA levels were determined at four timepoints: 3 a.m., 10 a.m., 3 p.m. and 10 p.m. A maximum of the rhythm of 12α-hydroxylase was observed at 3 p.m. and the minimum at 3 a.m. These results are distinct from those of 7α-hydroxylase, whose maximum point was at 10 p.m. and minimum at 3 p.m. When the rats were fed only in the daytime (from 9 a.m. to 5 p.m.), a marked shift of the activity and mRNA rhythms was observed with both enzymes. The circadian rhythms of the activities of both enzymes showed the same peaks (at 3 p.m.), but the mRNA levels of 12α-hydroxylase were distinct from those of 7α-hydroxylase, whose maximum point was at 3 a.m. and minimum at 10 p.m. Differences between the maximum and the minimum points of each enzyme mRNA level were statistically significant ( P < 0.01 for 12α-hydroxylase and 0.05 for 7α-hydroxylase). Moreover, circadian rhythms of DBP were also markedly shifted with the change of feeding period. The maximum mRNA level was observed at 10 p.m. instead of 10 a.m. and the minimum was at 10 a.m. instead of 10 p.m.