Turnover Rate of Hepatic 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase as Determined by Use of Cycloheximide

Abstract Both rat liver cholesterol synthesis and the primary controlling enzyme in this synthesis, 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMG-CoA reductase, EC 1.1.1.34) are known to exhibit a circadian rhythm of activity. Rats fed ad libitum and housed under normal conditions (i.e. lights on at 6 a.m. and off at 6 p.m.) showed a maximum activity of HMG-CoA reductase at midnight and a minimum activity at noon. Changing the lighting conditions such that there was still a 12-hour light-dark cycle per day affected the timing but not the amplitude of the rhythm. The maximum reductase activity always occurred 6 hours after the onset of the dark period. Cycloheximide (1.3 µg per g body weight) caused an exponential decay of liver microsomal HMG-CoA reductase activity for a 9-hour period following the injection. On a normal lighting schedule this decay rate was the same when measured from 9 a.m., 9 p.m., or 2 a.m. The half-life (t½) of the enzyme was calculated to be approximately 4.2 hours. The rate of decay of enzyme activity measured from 2 a.m. was the same for untreated control rats fed ad libitum and for rats treated with cycloheximide. Consequently, the circadian rhythm can be explained as due solely to changes in the rate of synthesis of HMG-CoA reductase.