Dietary cholesterol caused modification in the structure and function of rat hepatic microsomes, studied by fluorescent probes.

A 4% cholesterol diet fed to rats for four weeks was found to increase the phospholipid and cholesterol contents and the activities of drug metabolizing enzymes in rat liver microsomes. Microsomes from rats on a high cholesterol diet were able to enhance the fluorescence of membrane bound 1-anilinonaphthalene 8-sulphonate (1,8-ANS) and ethidium bromide more than microsomes from rats on a standard diet. In the case of 1,8-ANS, the enhanced fluorescence was found to be due to the increased affinity of the molecules for microsomes. In the case of ethidium bromide the fluorescence increased partly because of the larger amount of binding sites and partly because of the enhanced quantum yield of the molecules. P-nitrophenol was found to compete with 1,8-ANS for the same binding sites in microsomes. On the other hand, 1,8-ANS lowered the rate of drug metabolism when present in the incubation mixture. In vitro treatments of microsomes with trypsin, phospholipase A or digitonin altered the binding properties of 1,8-ANS and ethidium bromide to microsomes. It is concluede that the binding sites of 1,8-ANS in microsomes are important for the activity of drug-metabolizing enzymes. The mechanisms of dietary cholesterol in enhancing the drug metabolism and the role of microsomal phospholipids in regulating the activity of drug-metabolizing enzymes are discussed.