The Ca2+-dependent incorporation of nitrogenous bases into brain microsomal phospholipid subspecies in vitro.
The specificity of the Ca2+-stimulated choline and ethanolamine incorporation into the molecular subspecies of the correspondent choline and ethanolamine phosphoglycerides has been investigated in vitro in rat brain microsomes. In the presence of 5.0 mM Ca2+-ions and at pH 8.1, choline was incorporated 6 times faster into the tetraenoic diacyl-glycero-3-phosphorylcholines (diacyl-GPCs or lecithins) than into the saturated subspecies. The specific activities of the other species were intermediary, and decreased with increasing saturation. Hexaenoic species of lecithins were however weakly labelled. The rate of labelling of diacyl-GPC molecular subspecies was affected noticeably by changing the pH and the Ca2+-ion concentration of the incubation medium. Ethanolamine was incorporated in the presence of 2.5 mM Ca2+-ions and at pH 8.1 preferentially into the monoenoic species of total ethanolamine phosphoglycerides of rat brain microsomes. The rate of incorporation into the monoenoic species was twice that into the trienoic, tetraenoic and hexaenoic and 4 times faster that into the dienoic species. When the pattern of labelling was determined specifically for the molecular subspecies of diacyl-glycero-3-phosphorylethanolamines (diacyl-GPEs or phosphatidylethanolamines), the rate of incorporation of ethanolamine into the hexaenoic species resulted three times faster that into the saturated and monoenoic species and about twice that into the trienoic and tetraenoic species, in accordance with data for liver microsomes. The pattern of labelling of the molecular subspecies of ethanolamine phosphoglycerides and of diacyl-GPEs was not influenced by changing the pH and the Ca2+-ion concentration of the incubation medium.