CMP-dependent incorporation of [14C]Glycerol 3-phosphate into phosphatidylglycerol and phosphatidylglycerol phosphate by rabbit lung microsomes.

Rabbit lung microsomes were found to catalyze CMP-dependent incorporation of [14C]glycerol 3-phosphate into a total lipid extract. The radioactively labeled products in the lipid extract were identified as phosphatidylglycerol and phosphatidylglycerol phosphate. CMP-dependent incorporation of [14C]glycerol 3-phosphate by lung microsomes proceeded optimally at pH 7.4 and required Mn2+. The apparent Km value for CMP in this reaction was calculated to be 0.19 mM. No other cytidine nucleotide could substitute completely for CMP in supporting [14C]glycerol 3-phosphate incorporation into lipid. Cytosine-beta-D-arabinofuranoside-5'-monophosphate-dependent incorporation of [14C]glycerol 3-phosphate was observed at pH 8.5 but not at pH 6.8 CMP-dependent incorporation of [14C]glycerol 3-phosphate by microsomes was inhibited by inositol. The optimal in vitro rates of CMP-dependent and CDP diacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate into lipid were similar (approximately 1 nmol . mg-1 protein . h-1) and were not additive. Both CMP -dependent and CDP diacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate by lung microsomes appeared to involve CDPdiacylglycerol:glycerol-3-phosphate phosphatidyltransferase. However, the specific activity of this enzyme in a particular subcellular fraction did not relate directly in the extent of CMP-dependent [14C]glycerol 3-phosphate incorporation in that fraction. Preincubation of lung microsomes with 5 mM CMP plus 3 mM phosphatidylinositol increased CMP-dependent incorporation of [14C]glycerol 3-phosphate. When lung microsomes were depleted specifically of phosphatidylinositol by incubating with a phosphatidylinositol-specific phospholipase C, CMP-dependent incorporation was diminished. The Mn2+ requirement for CMP-dependent incorporation of [14C] glycerol 3-phosphate, its phosphatidylinositol requirement and its inhibition by Triton X-100 (0.2%) were not features shared by CDPdiacylglycerol-dependent incorporation of [14C]glycerol 3-phosphate but were characteristics of the reverse reaction catalyzed by CDPdiacylglycerol: inositol phosphatidyltransferase. Together with the previous finding of a developmental increase in the CMP content of fetal rabbit lung, these observations are consistent with a role for CMP in the regulation of the phosphatidylinositol and phosphatidylglycerol content of lung surfactant during lung maturation.