Preferential uptake of lipids by mycoplasma membranes from human plasma low-density lipoproteins

The binding and transfer of low-density lipoprotein (LDL) constituents to mycoplasma membranes were examined. Mycoplasma capricolum was found to bind more 125I-labeled LDL than Acholeplasma laidlawii. Free and esterified cholesterol uptake was 3-4 times higher in M. capricolum than in A. laidlawii. Cholesterol transfer to the membranes of both organisms far exceeded the amounts of cholesterol expected according to LDL protein found associated with the membranes. Trypsin digestion of the membranes prior to incubation with LDL decreased the binding of LDL and the transfer of free cholesterol to M. capricolum membranes but did not affect these processes with A. laidlawii membranes. These findings suggest the existence of protease-sensitive receptors on M. capricolum cell surface responsible for tighter contact with LDL. Analysis of LDL after incubation with large amounts of M. capricolum membranes revealed a loss of 60% of the lipoprotein-free cholesterol, 25.9% of the esterified cholesterol, and 20.7% of phospholipids without appreciable loss of protein. Rate zonal ultracentrifugation showed that the LDL separated as a single symmetrical peak, denser than the control. Electron microscopy showed that the lipoprotein particles retained their spherical shape but became about 10% smaller in diameter. These observations lead us to conclude that both free and esterified cholesterols are transferable from LDL to the membranes by a simple exchange process which does not involve prolonged contact or fusion of the particle with the membrane and does not result in degradation of the lipoprotein particle. We propose that similar mechanisms of free and esterified cholesterol transfer may also operate in vivo and contribute to the process of cholesterol exit from the plasma.