Effect of cholesterol concentration on organization of viral and vesicle membranes. Probed by accessibility to cholesterol oxidase.

The membrane of vesicular stomatitis virus could be readily enriched with exogenous cholesterol by exposing virion suspensions to serum lipoproteins forming complexes with cholesterol or to a carrier of polyvinylpyrrolidone/bovine serum albumin mixed with cholesterol. These procedures increased the cholesterol content of the virion membrane f rom -35 to -60% of the total lipid. Endogenous cholesterol in the virion membrane was not oxidized by cholesterol oxidase, but supplementation with exogenous cholesterol to levels of 46 mol 5 % or greater exposed the &OH group of virion cholesterol to oxidation by the enzyme. At virion membrane levels of 54% cholesterol, 70 to 75% of total cholesterol (endogenous + exogenous) was rendered accessible to cholesterol oxidase. Similar results were obtained with a model membrane system of mixed unilamellar vesicles prepared by ultrasonication. Cholesterol present in vesicles composed wholly or predominantly of choline phospholipids became accessible to cholesterol oxidase only when the cholesterol concentration exceeded 42 to 46 mol 96. When concentrations of cholesterol reached 50 mol %, cholesterol was oxidized 95% in vesicles containing egg phosphatidylcholine and 63% in vesicles containing sphingomyelin. However, even at concentrations as low as 20 mol %, cholesterol present in phosphatidylserine vesicles was readily accessible to cholesterol oxidase. Oxidizability of cholesterol in phosphatidylcholine vesicles was not influenced by phase transitions. It appears likely that the accessibility of membrane cholesterol to cholesterol oxidase is determined not only by the size but also by spacing of bulky headgroups of choline phospholipids dispersed by intercalated cholesterol.