Effect of insulin, S-adenosylhomocysteine, phospholipase C, n-butanol and Triton X-114 on alkaline phosphatase from isolated rat adipocyte plasma membranes

Abstract

Alkaline phosphatase activity has been described in aqueous extracts of rat adipose tissue [l], in rat adipocyte plasma membrane fractions [2] and more recently in human adipose tissue [3]. Studies with hepatocytes have shown this to be a membrane-bound enzyme covalently linked to the polar head group of phosphatidylinositol (PI) [4]; this interaction is supported by the finding that PI-specific phospholipase C releases alkaline phosphatase from hepatocyte plasma membranes [5,6]. It has also been proposed that the butanol-extraction of alkaline phosphatase from plasma membranes is caused by an autolytic process derived from activation of a plasma membrane phospholipase, phospholipase D [7] or PI-specific phospholipase C [8]. Insulin stimulates phospholipid methylation in rat adipocyte plasma membranes [9,10] and releases a low molecular weight, acid-stable mediator which activates mitochondrial pyruvate dehydrogenase [ll]. Inhibition of insulin-stimulated phospholipid methylation by S-adenosylhomocysteine results in a significant increase in activity of the pyruvate dehydrogenase activator [12]. Saltiel et al [13] have suggested that insulin modulates the activity of both pyruvate dehydrogenase and low k, CAMP phosphodiesterase by activation of a PI-specific phospholipase C in plasma membranes, hydrolysis of a novel PI-glycan and release of a modulator of insulin action. Using polyacrylamide gel electrophoresis (PAGE), we describe the effect of phospholipase C, n-butanol, Triton X-114, insulin and S-adenosylhomocysteine on