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Abstract

The effects of modifying the lipids of catecholamine storage vesicles have been studied. The lipids of the storage vesicles are unequally distributed between the water-insoluble membrane fraction and the water-soluble moiety; protein:lipid ratios were 0.8 and 4.0, respectively. Digestion of the vesicles with phospholipase A enhanced the adenosine triphosphatase activity three-fold, liberated 90% of the stored catecholamine and nearly abolished the catecholamine influx. Similar results were obtained when the products of phospholipolysis—fatty acids and lysolecithin—were added to the vesicular preparation. Phospholipase A, however, had no stimulant effect on the membrane-bound adenosine triphosphatase activity, when the vesicular structure had been destroyed previously by sonication or by hypo-osmotic shock. While vesicles disrupted by sonication hydrolysed ATP with the same rate as intact vesicles did, the rate of ATP hydrolysis of vesicles subjected to hypo-osmotic medium was enhanced two- to three-fold, irrespective of whether phospholipase A was present or not. The vesicular membrane, when isolated from the water-soluble protein, did not respond to phospholipase A treatment or to hypo-osmotic media with a stimulation of the adenosine triphosphatase activity. The stimulant effect of hypo-osmotic media on the adenosine triphosphatase activity was not restored by the recombination of the membrane protein with the water-soluble protein fraction.