The rate of transmembrane movement of cholesterol in the human erythrocyte.

Cholesterol appears to be abundant on both sides of the human erythrocyte membrane, but its precise distribution and rate of transmembrane movement are undetermined. Although the cholesterol in the intact cell normally was resistant to cholesterol oxidase, enrichment of cells with exogenous cholesterol or preincubation at very low ionic strength rendered the entire cholesterol pool susceptibility to attack. Under these experimental conditions, all of the membrane cholesterol was oxidized in a strictly first order fashion with a half-time as short as 10 s at 37 degrees C. Since the enzyme had access only to the outer membrane surface, these data suggest that the transmembrane movement (flip-flop) of cholesterol is extremely rapid. From an error analysis, we estimate an upper bound on the half-time of the transmembrane movement of cholesterol of 3 s at 37 degrees C. A physiological function for rapid sterol flip-flop is suggested.