Overview of Mammalian Cholesterol Metabolism

The metabolism of cholesterol in mammals is extremely complex. A summary sketch (fig. 20.24) helps to draw the major metabolic interrelationships together. Cholesterol is biosynthesized from acetate largely in the liver (fig. 20.24a) or taken in through the diet (fig. 20.24b). From the intestine, dietary cholesterol is secreted into the plasma mainly as a component of chylomicrons. The triacylglycerol components of chylomicrons are quickly degraded by lipoprotein lipase, and the remnant particles are removed by the liver. Apoproteins and lipid components of the chylomicrons and remnants appear to exchange with HDL. Cholesterol made in the liver (fig. 20.24a) has several alternative fates. It can be (1) secreted into plasma as a component of VLDL, 

Conversion of a cholic acid into glycocholate. Glycocholate is an example of a bile salt. The bile salts solubilize lipids in the small intestine so they can be degraded by lipases. 

Biosynthesis of progesterone. Desmolase is found in the mitochondria of steroid-producing tissues. Cholesterol is converted to pregnenolone in the mitochondria and then transferred to the endoplasmic reticulum, where it is converted to progesterone. Progesterone is a hormone as well as the precursor of several other hormones. 

Inactivated steroid hormones are conjugated to glucuronic acid prior to excretion in the urine. 

Precursor of other steroids prepares uterus for implantation of an egg prevents ovulation during pregnancy 

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