Regulation of hepatic dolichol synthesis by beta-hydroxy-beta-methylglutaryl coenzyme A reductase.

Dolichol is an isoprenoid lipid involved in the assembly of many membrane-bound and secreted glycoproteins. Dolichol biosynthesis can be considered as a branch of the cholesterol biosynthetic pathway subsequent to the reaction catalyzed by beta-hydroxy-beta-methylglutaryl coenzyme A reductase (hydroxymethylglutaryl-CoA reductase, EC 1.1.1.34), the major regulatory enzyme of cholesterol biosynthesis. Changes in reductase activity can also affect the rate of dolichol synthesis. Since the majority of plasma glycoproteins are synthesized by the liver, we have measured the rate of dolichol synthesis in mouse-liver slices after various treatments which alter hepatic beta-hydroxy-beta-methyl-glutaryl-CoA reductase activity in vivo. The rate of hepatic dolichol synthesis was decreased by dietary cholesterol and fasting, and increased by feeding cholestyramine. There is also a diurnal variation in the rate of dolichol synthesis. A plot of the rate of dolichol synthesis versus the rate of cholesterol synthesis suggests that, after the formation of isoprene units, the branch of dolichol biosynthesis is saturated at a lower concentration of isoprene intermediates than is required to saturate the branch of cholesterol biosynthesis. After 2 weeks of cholesterol feeding and the consequent depression of hepatic dolichol synthesis, the rate of [3H]mannose incorporation into liver and plasma glycoproteins was unchanged, indicating that the rate of dolichol biosynthesis was not rate-limiting for total glycoprotein synthesis under these conditions.