The Organization of Cholesterol Esters in Membranes of Mycoplasma capricolum

The organization of cholesterol esters in Mycoplasma capricolum membranes was studied by differential scanning calorimetry. Cells grown in the presence of horse serum incorporated large amounts of cholesterol esters into their membranes. The cholesterolester‐containing membranes after incubation at low temperature showed an endotherm characteristic of a cholesterol ester crystalline→isotropic liquid transition that was identical in membranes both before and after thermal protein denaturation. This transition was not observed in membranes of cells grown in medium in which the horse serum was replaced by bovine albumin, fatty acids and unesterified cholesterol unless cholesterol esters were added to the growth medium. In membrane preparations obtained both from cells grown in horse serum and from cells grown with bovine albumin plus cholesterol and fatty acids, the free cholesterol content was sufficient to eliminate the bilayer order/disorder transition observed in isolated membrane phospholipids. Our studies indicate that the majority of cholesterol esters in M. capricolum membranes is not present in attached serum lipoprotein particles, nor is intimately associated with membrane protein, but exists as relatively large cholesterol ester droplets or pockets tightly associated with the membrane. The cholesterol esters in these pockets appear relatively pure, although the presence of small amounts of other membrane components is likely.