The esterification of cholesterol with palmitic acid by rat liver homogenates.

Although the enzymes involved in the synthesis and hydrolysis of cholesterol esters of fatty acids have been the subject of a number of communications (l-7), the sites and mechanisms of these reactions have not been definitely established. It has been suggested by Schramm and Wolff (8) that fatty acids are esterified in the intestinal lumen before absorption into the cells of the mucosa and that fatty acids are carried from the blood into tissue cells as cholesterol esters. The reported occurrence of a serum cholesterol esterase (9) has led to the suggestion that this enzyme may be involved in the maintenance of the cholesterol ester level of blood. Cholesterol esterases of pancreatic origin have been shown to have the required specificity and activity for the synthesis and hydrolysis of long chain fatty acid esters of cholesterol (10-12). The presence of a cholesterol-esterifying system in liver has also been demonstrated by several investigators (13-17). According to a recent report by Swell and coworkers (18), the activity in liver is due to a liver esterase which acts only on short chain fatty acid esters (e.g. butyrate); these esters do not occur normally in any appreciable amount in the animal body. It is apparent from the existing literature that cholesterol esterases occur in a number of tissues, but typical preparations of the enzymes have extremely low activity. Systems were often incubated for a period of 2 to 3 days to demonstrate appreciable synthesis or hydrolysis of the ester. The possibility exists that this low activity may indicate that the enzymatic reaction requires certain cofactors for the synthesis of fatty acid esters. In the present investigation it has been possible to verify the presence of an enzyme in rat liver which can bring about the synthesis of a cholesterol ester of a long chain fatty acid (palmitic acid). This enzymatic