CDPdiacylglycerol pyrophosphatase (E.C. 3.6.1.26) activity has been examined in rat lung mitochondrial and microsomal fractions. While the mitochondrial hydrolase exhibited a broad pH optimum from pH 6-8, the microsomal activity decreased rapidly above pH 6.5. Apparent Km values of 36.2 and 23.6 microM and Vmax values of 311 and 197 pmol.min-1.mg protein-1 were observed for the mitochondrial and microsomal preparations, respectively. Addition of parachloromercuriphenylsulphonic acid led to a marked inhibition of the microsomal fraction but slightly stimulated the mitochondrial activity at low concentrations. Mercuric ions were inhibitory with both fractions. Although biosynthetic reactions utilizing CDPdiacylglycerol require divalent cations, addition of Mg2+, Mn2+, Ca2+, Zn2+, Co2+, and Cu2+ all inhibited the catabolic CDPdiacylglycerol hydrolase activity in both fractions. EDTA and EGTA also produced an inhibitory effect, especially with the mitochondrial fraction. Although addition of either adenine or cytidine nucleotides led to a decrease in activity with both fractions, the marked susceptibility to AMP previously reported for this enzyme in Escherichia coli membranes, guinea pig brain lysosomes, and pig liver mitochondria was not observed. These results indicate that rat lung mitochondria and microsomes contain specific CDPdiacylglycerol hydrolase activities, which could influence the rate of formation of phosphatidylinositol and phosphatidylglycerol for pulmonary surfactant.

Studies have been carried out on the incorporation of32P-labeled α-glycerophosphate (α-G32P) into the lipids of preparations obtained from rat brain cerebral hemispheres. The optimal incorporating system contains Tris and phosphate buffers (pH 7.4), MgCl2, CoA, EDTA, NaF and ATP. The incorporation of α-G32P into monophosphoinositide was stimulated by CDP-choline, CDP-ethanolamine, CDP-glycerol and CTP. These nucleotides depressed the incorporation of α-G32P into phosphatidic acid. Microsomal preparations supported a high incorporation of α-G32P into phosphatidic acid, but the incorporation into monophosphoinositide was low compared with the whole homogenate. Similar observations were made with mitochondrial preparations undergoing oxidative phosphorylation with CoA added to the medium.