The regulated catabolism of endogenous and exogenous phosphatidylinositol by Saccharomyces cerevisiae leading to extracellular glycerophosphorylinositol and inositol

Abstract

It was previously shown that phosphatidylinositol catabolism leads to the accumulation of glycerophosphorylinositol in the culture medium of Saccharomyces cerevisiae. We now find that lack of an energy source (glucose) reduces the formation of glycerophosphorylinositol and increases extra-cellular inositol. This situation is reversed by refeeding glucose. [3H]Phosphatidylinositol is the precursor of extra-cellular [3H]inositol with energy-starved cells. Extracellular glycerophosphorylcholine and glycerophosphorylethanolamine accumulate more slowly than glycerophosphorylinositol in the growth medium and do not appear to be a strongly affected by energy starvation. Phosphatidylinositol deacylation appears to occur at the cell surface in a regulated manner. Exogenously added phosphatidylinositol apparently does not mix randomly with the endogenous pool since it is not converted to either inositol-containing sphingolipid or to diphosphoinositide, both previously shown to be derived in part from cellular phosphatidylinositol. Labeled exogenous phosphatidylinositol is, however, quantitatively converted to glycerophosphorylinositol with the probable intermediat formation of monoacyl-glycerophosphorylinositol. Breakdown of exogenous phosphatidylinositol requires an energy source and does not lead to free inositol. Deacylation of exogenously added 1-acyl-glycerophosphorylinositol occurs much faster than deacylation of phosphatidylinositol and does not require an energy source. Glycerophosphorylethanolamine formation from exogenous phosphatidylethanolamine occurs about as fast as the breakdown of phosphatidylinositol and is also inhibited in the absence of energy source. The much slower deacylation of exogenous phosphatidylcholine was also affected by an energy source. Glycerophosphorylinosiyolaccumulates in the culture medium of Kloeckera apiculata, Saccharomyces carlsbergenis, and Neurospora crassa.