The plasma membrane of Malpighian cells from pig epidermis: isolation and lipid and protein composition.

A plasma membrane fraction from Malpighian cells has been isolated by differential and density gradient centrifugation of a pig epidermal homogenate. It was enriched in the marker enzymes 2-naphthylamidase, 5'-nucleotidase, phosphodiesterase I and acid phosphatase and depleted of NADH-ferricyanide reductase and cytochrome c oxidase. It had a protein to lipid ratio of 3:2 by weight. The protein composition was complex with compounds ranging from a molecular weight of 150,000 down to 13,000. Major components with molecular weights 120,000 to 90,000 were glycoproteins. Two other components had molecular weights of 39,000 (actin ?) and 24,000. There were minor components with molecular weights from 63,000 to 46,000. About 76% of the total lipid was present as phospholipid, which was enriched in sphingomyelin. Most of the neutral lipids were accounted for by cholesterol, triacylglycerols and fatty acids: very little glycosphingolipid was present. The preparation was probably derived from non-desmosomal areas of the plasma membrane of Malpighian cells, as desmosomes were not seen in the preparation.