Cholesterol

Cholesterol is a sterol with the structure shown in Figure 1. Daily cholesterol intake is 0.5-1.0 g, half of which is absorbed. On a low-cholesterol diet (<300mg/day) the body synthesizes approximately 800 mg of cholesterol per day, mainly in the liver and, to a lesser extent, in the intestine.

The rate-limiting step in synthesis is highly sensitive to cellular levels of cholesterol, themselves sensitive to circulating levels of cholesterol. This feedback regulation occurs through changes in the amount and activity of an enzyme called 3-hydroxy-3-methylglutaryl CoA reductase (HMG-CoA reduc-tase), which catalyzes the formation of mevalonate, the rate-limiting step in cholesterol biosynthesis. The rate of synthesis of HMG-CoA reductase mRNA is

21 H3C.

18 1 ch3

21 H3C.

18 1 ch3

.27 CH3

Cholesterol Cholesteryl ester

Figure 1 Structure of cholesterol and cholesteryl ester.

23 CHS

.27 CH3

Cholesterol Cholesteryl ester

Figure 1 Structure of cholesterol and cholesteryl ester.

controlled by the sterol regulatory element binding protein (SREBP). SREBP in its inactive state is attached to the endoplasmic reticulum or nuclear membrane, but when cholesterol levels decline the amino-terminal domain is released from its association with the membrane by proteolytic cleavage; it migrates to the nucleus and binds to the sterol regulatory element (SRE) on the 5' side of the reductase gene to enhance transcription. As cholesterol levels increase, the proteolytic release of SREBP is blocked, SREBP in the nucleus is rapidly degraded, and cholesterol synthesis is switched off.

Cholesterol is found in the body largely as free cholesterol in membranes, but in the plasma it is two-thirds esterified, mainly as cholesterol linoleate and cholesterol oleate. Free cholesterol in plasma exchanges freely with cholesterol in membranes. The major route of cholesterol excretion is through the bile, directly as cholesterol or after conversion to bile salts, some of which are reabsorbed from the terminal ileum in the enterohepatic circulation.

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