A NOVEL FORM OF GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHOR CONVERTING ACTIVITY WITH A SPECIFICITY OF A PHOSPHOLIPASE-D IN MAMMALIAN LIVER MEMBRANES

It has been reported that rat liver membranes contain a glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) which may be involved in generation of phosphoinositol-glycan, a putative insulin second messenger (Saltiel, A.R. and Cuatrecasas, P. (1988) Am. J. Physiol. 255, C1-C11). Using GPI-anchored acetylcholinesterase (AChE) from bovine erythrocytes as substrate, we attempted to isolate GPI-PLC from bovine and rat liver membranes. A major part of the GPI-anchor converting activity present in liver could be washed away from the tissue by extraction with detergent-free buffer. Solubilisation of the washed membranes with 0.25% (v/v) Nonidet P-40 and ultracentrifugation resulted in a considerable amount of detergent soluble GPI-anchor converting activity in the supernatant. Anion-exchange chromatography on a Fractogel TSK-DEAE column of detergent-soluble GPI-anchor converting activity revealed two distinct peaks eluting at 50-80 mM and 120-170 mM NaCl, respectively. Using [I-125]TID-labelled mf-AChE as substrate, radiolabelled diradylglycerol was obtained with both peak activities. However, when the phosphatase inhibitors NaF and sodium orthovanadate were included in the assay systems, phosphatidic acid was detected in additon to diradylglycerol. Both GPI-anchor converting activities were Ca2+-sensitive and inhibited by heavy metal chelating agents. These results suggested the presence of two isoenzymes of GPI-PLD and a phosphatase, rather than a GPI-PLC activity, in liver. Further, it could be shown that the activity in the second peak was identical to GPI-PLD, abundantly present in serum, while the activity contained in the first peak seems to be genuine for liver cells and, thus, apparently represents a novel form of a GPI-PLD which is membrane-associated and distinctly different from the serum enzyme.