Phosphatidylinositol-inositol exchange in a rabbit lung.
A microsomal fraction prepared from rabbit lung tissue was found to catalyze CDPdiacylglycerol-independent incorporation of [3H]inositol into phosphatidylinositol. This incorporation resulted from CMP-dependent phosphatidylinositol-inositol exchange and did not constitute a net synthesis of phosphatidylinositol. The phosphatidylinositol-inositol exchange activity was distinct from the phospholipid-base exchange enzymes and was specific for inositol. Optimal in vitro phosphatidylinositol-inositol exchange activity was observed at pH 8.5--8.8 and either Mn2+ or Mg2+ was essential for activity. Mercaptoethanol stimulated phosphatidylinositol-inositol exchange and Hg2+ inhibited this activity. In the absence of CMP, no phosphatidylinositol-inositol exchange was observed. CDP (and to a smaller extent CTP) also supported phosphatidylinositol-inositol exchange and this appeared to occur via the generation of CMP during incubations. The apparent Km values of the phosphatidylinositol-inositol exchange enzyme for CMP and inositol were 0.4 mM and 11 microM, respectively. When CDPdiacylglycerol was present at a concentration optimal for CDPdiacylglycerol : inositol transferase activity, CMP-dependent phosphatidylinositol-inositol exchange activity was still observed. However, in the presence of Hg2+ CDPdiacylglycerol inhibited phosphatidylinositol-inositol exchange activity. Several properties of the phosphatidylinositol-inositol exchange enzyme resemble those of CDPdiacylglycerol : inositol transferase, but the two enzymes appear distinct on the basis of different degrees of inhibition by either Ca2+, Hg/+ or heat, and on the basis of different changes in activity during lung development.
D000818:Animals; D051607:CDP-Diacylglycerol-Inositol 3-Phosphatidyltransferase; D003567:Cytidine Diphosphate Diglycerides / Q000494:pharmacology; D003568:Cytidine Monophosphate / Q000494:pharmacology; D005260:Female; D006863:Hydrogen-Ion Concentration; D007294:Inositol / Q000378:metabolism*; D007700:Kinetics; D008168:Lung / Q000187:drug effects / Q000201:enzymology*; D008861:Microsomes / Q000201:enzymology*; D010716:Phosphatidylinositols / Q000378:metabolism*; D010770:Phosphotransferases / Q000378:metabolism*; D011817:Rabbits; D017855:Transferases (Other Substituted Phosphate Groups)
D003567:Cytidine Diphosphate Diglycerides; D010716:Phosphatidylinositols; D007294:Inositol; D010770:Phosphotransferases; D017855:Transferases (Other Substituted Phosphate Groups); C031077:phosphatidylinositol-inositol exchange enzyme; D051607:CDP-Diacylglycerol-Inositol 3-Phosphatidyltransferase; D003568:Cytidine Monophosphate