Abstract
After describing the main steps of cholesterol biosynthesis the author recalls that the cholesterogenesis rate is feedback-induced by dietary cholesterol and examines the various processes of modulation. Hydroxy-3-methylglutaryl (HMG) CoA reductase. the key rate-limiting enzyme, is a 97 kDa endoplasmic reticulum glycoprotein, anchored 7-fold in this membrane. The N-terminal membrane-bound domain plays a fundamental role in the modulation of reductase activity. This modulation is essentially mediated by decreased gene transcription and enhanced degradation of the protein. The possible modulation by a bicyclic cascade system involving phosphorylation (inactivation) and dephosphorylation (activation) of reductase does not seem to play an essential role in vivo. Finally, recent data show that the lipid composition (C/P molar ratio) of some reticular membranes (fibroblasts, for example) can strongly modulate the activity of this ubiquitous enzyme.
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