Molecular arrangement of phosphatidylcholine and sphingomyelin in sarcoplasmic reticulum membranes

The distribution of phosphatidylcholine and of sphingomyelin in sarcoplasmic reticulum membranes was studied by using phospholipases. Treatment of intact membranes with phospholipase A from Vipera russeli, at 35 °C, causes breakdown of about 50–55% of the total phosphatidylcholine present in the sarcoplasmic reticulum, whereas about 90–95% degradation is obtained under the same conditions in membranes disrupted by sodium deoxycholate. On the other hand, in intact membranes, sphingomyelinase hydrolyzes only 20% of the sphingomyelin, which is largely hydrolyzed by the enzyme after disrupting the membranes with deoxycholate. The results suggest that phosphatidylcholine is similarly distributed on both layers of the membrane (~50% on each side), whereas most of the sphingomyelin (~80%) is internally localized and, therefore, asymmetrically distributed in the sarcoplasmic reticulum membranes.