Physical state of membrane lipids of Mycoplasma capricolum

The physical state of the lipids 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. After incubation at a low temperature, the cholesterol ester-containing membranes showed an endotherm characteristic of a cholesterol ester transition from a crystalline state to an isotropic liquid 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 from both horse serum-grown cells 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 calorimetric studies indicate that the majority of cholesterol esters in M. capricolum membranes is not present in attached serum lipoprotein particles nor is it intimately associated with membrane protein but exists as relatively large droplets of cholesterol ester or as pockets in the membrane. The cholesterol esters in these pockets exist in a liquid-like state at growth temperature and appear to be relatively pure, although the presence of small amounts of other membrane components, especially glycerides, is likely. The existence of a low-temperature endotherm in membrane attributable to glycerides suggests there may be glyceride-rich regions in the membranes.