In vivo effect of cholesterol feeding on the short term regulation of hepatic hydroxymethylglutaryl coenzyme A reductase during the diurnal cycle.

Light-dark-cycled rats were fed a 3% cholesterol-supplemented diet at the beginning of the dark phase. Cholesterol-fed and control animals were taken at intervals throughout the following 12 h and the microsomal and solubilized hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase was isolated. Immunotitrations of this microsomal and solubilized enzyme were performed with a monospecific antibody to 3-hydroxy-3-methylglutaryl coenzyme A reductase. In contrast to the specific activity of the enzyme, which differs extremely during the diurnal cycle, the immunotitrations obtained from cholesterol-fed and control animals, yielded in identical antisera equivalence points. On the other hand, when the enzyme was phosphorylated in vitro, the antisera equivalence points corresponded to the alterations of the specific activity. In contrast to the results published by Higgins and Rudney ((1973) Nature New Biol. 246, 60-61), our data prove that even the in vivo short term changes in enzyme activity are due to changes in the quantity of enzyme rather than to a modulation of the catalytic activity.