The ring structure of cholesterol cannot be metabolised to CO2 and H2O in humans. The steroid ring is eliminated from the body by the following ways:

The drugs compactin and lovastatin are fungal products. They are used to decrease the serum cholesterol level in patients with hypercholesterolemia. Compactin and lovastatin are competitive inhibitors of the enzyme HMG-CoA reductase and, therefore, reduce cholesterol synthesis. About 50%–60% decrease in serum cholesterol level has been reported by a combined use of these two drugs.…

The enzyme HMG-CoA reductase exists in two interconvertible forms. The dephosphorylated form of HMG-CoA reductase is more active while the phosphorylated is less active. The hormones exert their influence through cAMP by a series of reactions, which are comparable with the control of the enzyme glycogen synthase. The net effect is that glucagon and glucocorticoids…

The end product cholesterol controls its own synthesis by a feedback mechanism. Increase in the cellular concentration of cholesterol reduces the synthesis of the enzyme HMG-CoA reductase. This is achieved by decreasing the transcription of the gene responsible for the production of HMG-CoA reductase.

Cholesterol biosynthesis is controlled by the rate-limiting enzyme HMG-CoA reductase at the beginning of the pathway. HMG-CoA reductase is found in association with endoplasmic reticulum and is subjected to different metabolic controls.

Synthesis of Squalene

The biosynthesis of cholesterol may be divided into five steps: