PROPERTIES AND FUNCTION OF THE CALCIUM‐DEPENDENT INCORPORATION OF CHOLINE, ETHANOLAMINE AND SERINE INTO THE PHOSPHOLIPIDS OF ISOLATED RAT BRAIN MICROSOMES¹

Abstract

Abstract— The calcium‐dependent incorporation of choline, ethanolamine and L‐serine into the phospholipids of isolated rat brain microsomes has been studied in vitro, and various properties of the incorporation have have been examined. The optimum pH for the incorporation of each base was found to vary inversely with the Ca 2‐ concentration. Conversely, the optimal Ca 2 + concentration for the exchange of the bases increased with decreasing pH values. The enzymic system for the incorporation of ethanolamine appeared to be saturated by two substrate concentrations, i.e. 0‐2 and 1‐7‐2‐0 mM. At low ethanolamine concentration (0‐2 mM] much less incorporation of the base occurred into the alkenylacyl‐ and alkylacyl‐derivatives of ethanolamine phosphoglycerides compared to that into the diacyl species, whereas the difference becomes smaller at a high substrate concentration (1‐7 mM). At pH 81 and 2 mM‐Ca 2+ the apparent K m of ethanolamine at low substrate concentration was 80 × 10 ‐5 M, and this value increased to 16‐2 × 10 ‐4 .viat 10mM‐Ca 2+ concentration. At similar pH the K m values for choline and L‐serine were 5.88 × 10 ‐4 M and 40 × 10 ‐4 M at 2 mM‐ and 10mM‐Ca 2 + concentrations, respectively. The properties of the enzyme system show differences for the three substrates when various factors are changed during incubation. These and other results indicate that more than one enzyme is probably involved in the Ca 2+ ‐medialed exchange of nitrogenous bases.