Alterations in the activities of rabbit erythrocyte membrane-bound enzymes induced by cholesterol enrichment and depletion procedures.

The effect of cholesterol enrichment and depletion of rabbit erythrocytes on the activities of membrane-bound enzymes, namely (Na+,K+)-stimulated ATPase, NAD+ase and acetylcholinesterase was examined. The cholesterol content of erythrocyte membranes has been modified by incubation of intact cells with sonicated egg lecithin/cholesterol vesicles (cholesterol/phospholipid molar ratio approx. 2) and with egg lecithin vesicles for time intervals up to 10 hours. The cholesterol/phospholipid molar ratio (CH/PL) of untreated rabbit red blood cell membranes was 0.92-0.94. Linear increase (up to CH/PL molar ratio 1.72-1.9) or decrease (up to CH/PL molar ratio 0.27-0.43) in cholesterol content of erythrocyte membranes was observed over the 10 hours of incubation with egg lecithin/cholesterol and egg lecithin liposomes respectively. Fusion of liposomes to the membrane or their attachment to the membrane surface was not a significant factor in the alteration of CH/PL ratio. (Na+,K+)-stimulated ATPase, NAD+ase and acetylcholinesterase activities were measured as a function of membrane cholesterol. The specific activities of all three enzymes were progressively decreased with increase in cholesterol content. Partial reversibility of the inhibitory effect of cholesterol was demonstrated by measurement on cells depleted again after cholesterol enrichment. This was confirmed by the fact that a lowering in cholesterol content evoked an analogous activation of enzymes. The possible implications of physicochemical modifications of bulk and annular lipids of membrane-bound enzymes in the inhibition mechanism are discussed.