[Hepatic metabolism of cholesterol]

Cholesterol is an essential component of all tissues, as it is a part of the structure of cell membranes, and it is an immediate precursor of a series of essential substances such as vitamins, steroid hormones, and bile acids. Under physiologic conditions, the intake and output of cholesterol in the organism is coordinated and balanced with the aim of guaranteeing the availability of adequate amounts of cholesterol to satisfy the needs of the different tissues (fig. 1). Under pathological conditions there is an imbalance between these mechanism, which leads to an increase in the circulating levels of cholesterol, leading to pathological processes such as hyperlipemias, atherosclerosis and bile stones. The liver plays a central role in the regulation of the homeostasis of cholesterol. The molecule enters the liver in the form of chylomicrons and low density lipoproteins (LDL), through lipoprotein receptors, and this is also the most important organ for the de novo biosynthesis of cholesterol from acetyl coenzyme A, by means of a cascade enzyme reaction in which the enzyme 3-hydroxy-3 methyl glutaryl CoA reductase (HMG-CoA) is the key of the entire process. Cholesterol is found in the liver in the form of cholesterol esters or as free cholesterol. The two most effective ways of eliminating body cholesterol are found in the liver, with the degradation of the compound to bile acids and the biliary secretion of cholesterol. The conversion to bile acids takes place through a series of enzymatic steps in which the formation of 7-alpha-hydroxycholesterol by the enzyme cholesterol 7-alpha-hydroxylase is the key of the process. The biliary secretion of cholesterol is 600 mg/day. Both the abundance and the universality of cholesterol in living things as its clinical implications emphasize the importance and interest of this compound.