Studies on plasma membranes I. Chemical composition and enzyme content of plasma membranes isolated from rat liver

Abstract

Plasma membranes were isolated from rat liver. Electron microscopy showed that the preparations were not contaminated by other identifiable cellular components. Antibodies prepared against the isolated membranes were preferentially fixed by the plasma membranes of liver tissue. Chemically the plasma membranes consisted of a phospholipoglycoprotein core, mainly composed of components of a mol. wt. of about 10 000, to which saline-soluble proteins were attached. The latter were positively charged at neutral pH and contained four antigenically-active proteins. The protein, choline, phospholipid-P, cholesterol, hexose, hexosamine, sialic acid and sulphydryl content of the isolated membranes was measured and expressed in relation to the membrane-bound P, which consisted virtually completely of phospholipid P. Next to arylesterase (EC 3.1.1.2), NADH 2-cytochrome c reductase (EC 1.6.2.1) and a trace of NAD nucleosidase (EC 3.2.2.5) activity, the enzymes found present in the isolated plasma membranes were of phosphate-ester-splitting types, viz., alkaline and acid phosphomonoesterases (EC 3.1.3.1 and EC 3.1.3.2), phosphodiesterase (EC 3.1.3.2), ribonuclease(s) (EC 2.7.7.16), glucose 6-phosphatase (EC 3.1.3.9), NAD pyrophosphatase (EC 3.6.1.9), Mg 2+-ATPase (EC 3.6.1.4), Na +-K +-Mg 2+- ATPase, 5′-mononucleotidase (EC 3.1.3.5) and a trace of ADPase (EC 3.6.1.6). The specific activities of these enzymes were either much lower or much higher than those of the corresponding enzymes found in microsomal preparations. The possibility that the plasma-membrane preparations were contaminated by microsomal elements is discussed, and, on account of the available evidence, the conclusion is reached that there is no reason to assume that this was the case.