Mechanism of release of integral proteins from rat liver microsomal membranes

The release of three integral enzymatic activities (NADH- and NADPH-cytochrome c reductase and 5′-nucleotidase) and total protein from washed rat liver microsomal membranes, upon simple incubation at 37°C in aqueous media, was investigated. Release does not depend on contaminating proteases and is enhanced by alkaline pH. Total protein and enzyme release is consistent with a loss of phospholipids which are not recovered in the soluble phase. Following incubation at pH 9.0 large amounts of free fatty acids were recovered in the soluble phase, accounting for a ratio of 1 1 (w/w) with released protein. This evidence, together with the data available about densities (1.07-1.08 g/ml) and molecular weights (1 700 000-700 000) of the released enzymes, suggests that they are solubilized from microsomal membranes in the form of mixed micelles mostly formed by free fatty acids and integral proteins, probably owing to the activity of endogenous phospholipases on membrane lipids. Release of total protein and enzymatic activities is decreased by Ca²⁺, whose possible role in the phenomenon is discussed.