Abstract
Pathways for EDTA-insensitive degradation of phosphatidylinositol (PI) were investigated in porcine platelet membranes and cytosol. The incubation of platelet membranes with [ 3H]glycerol-labeled PI in the presence of 2mM EDTA produced [ 3H]lysoPI and aqueous radioactive products, but not radioactive neutral lipids. The degradation in the membranes was optimal at pH8.0–9.0, while EDTA-insensitive hydrolysis was also observed in cytosol with optimal pH at pH7.0–9.0. The major water-soluble product was identified as glycerophosphoinositol. Under the conditions, [ 14C]arachidonate was released from 1-stearoy1-2-[ 14C]arachidonyl PI without accumulation of [ 14C]lysoPI. The deacylation activity preferred PI to phosphatidylcholine and phosphatidylethanolamine. Collectivity, these results suggest that PI can be converted to lysoPI by phospholipase A 2 in the absence of free Ca 2+, providing the substrates for lysoPI-specific phospholipase C characterized earlier in porcine platelet membranes (Murase and Okuyama (1985) J.Biol. Chem. 260, 262–265).
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