Characterization of glycerophosphorylcholine, -ethanolamine, -serine, -inositol, and -glycerol hydrolytic activity in housefly larvae.

Homogenates of Musca domestica (housefly) larvae contain glycerophosphodiesterase activity, which is found in the supernatant fluid after centrifugation at 88,000 g. The phosphodiesterase is inhibited by EDTA and is stimulated by Mg(2+), Ni(2+), Co(2+), and Mn(2+). The pH optimum is 7.2. The enzyme is stable to heating at 50 degrees C for 15 min and is insensitive to sulfhydryl inhibitors. Glycerophosphoryl diesters of choline, ethanolamine, inositol, serine, glycerol, and beta-methylcholine are hydrolyzed to the common product, l-alpha-glycerophosphate, and the appropriate free alcohol. The rate of glycerophosphorylcholine hydrolysis is 70% greater than the rate of hydrolysis of the other glycerophosphodiesters. Apparent K(m) values for glycerophosphorylcholine, glycerophosphorylethanolamine, and glycerophosphoryl-beta-methylcholine are 2-4 x 10(-4) m, and for glycerophosphorylinositol, 2 x 10(-3) m. Competitive studies using various pairs of substrates, as well as the exchange of free choline into both glycerophosphorylcholine and glycerophosphorylinositol, suggest that a single enzyme cleaves all substrates. Product inhibition and reversal of the reaction were not detected. Choline, but not l-alpha-glycerophosphate, exchanges into glycerophosphorylcholine and glycerophosphorylinositol.