INFLUENCE OF CHOLESTEROL ON SPHINGOMYELIN METABOLISM AND HEMILEAFLET FLUIDITY OF RAT-LIVER PLASMA-MEMBRANES

The objective of this study was to examine the effect of dietary Chol supplementation on SM metabolism in rat liver plasma membranes, as wel l as on membrane leaflet fluidity characteristics. The membrane Chol c ontent increased significantly during the first 20 days of dietary fee ding, but returned to the level of the control group when the diet was continued for another ten days. The initially more fluid outer leafle t of the membrane rigidified as a result of the diet, obliterating the natural asymmetry in the fluidity of the membrane bilayer. Changes in the neutral SMase activity were also observed. These changes were in strong negative correlation (r = -0.978) with the Chol/Pr ratio and ar e consistent with the in vitro inhibition of SMase activity reported e arlier. In contrast, the SM synthesizing enzymes, PC:Cer-PCh and PE:Ce r-PEt transferase, were stimulated in course of the dietary Chol feedi ng. The activity of PC:Cer-PCh transferase was more strongly affected. Our results support the concept that SM metabolism is regulated coord inately with that of Chol. The present work could contribute to the be tter understanding of the parallel acumulation of SM and Chol observed in a variety of pathological conditions such as atherosclerosis and N iemann-Pick disease.