Pectin feeding influences fecal bile acid excretion, hepatic bile acid and cholesterol synthesis and serum cholesterol in rats.

This study was designed to investigate the effects of pectin on cholesterol and bile acid metabolism and to elucidate the mechanisms involved in its hypolipidemic effect in rats. The key regulatory enzymes in cholesterol and bile acid metabolism, 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase) and cholesterol 7alpha-hydroxylase, were determined. Circulating, hepatic, and biliary lipid concentrations and fecal bile acid excretion were also measured. Male Wistar rats were fed a fiber-free or a pectin-supplemented (7 g/100 g) diet for 4 wk. Bile flow and the biliary secretion of both bile acids and cholesterol were not significantly different than controls in pectin-fed rats. The addition of pectin to the diet resulted in lower serum and liver cholesterol concentrations (-27 and -17%, respectively). Fecal bile acid excretion (+168%) and the hepatic activity of cholesterol 7alpha-hydroxylase (+70%) were significantly higher in pectin-fed animals. HMG-CoA reductase activity was also significantly greater (+11%) in the presence of dietary pectin. Results of our study indicate that pectin, by enhancing fecal bile acid excretion, may cause increased hepatic synthesis of bile acids and liver depletion of cholesterol in rats, which results in a higher rate of cholesterol synthesis and reduced serum cholesterol concentrations.

[Hepatic metabolism of cholesterol]. / Metabolismo hepático del colesterol.

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.