FURTHER CHARACTERIZATION OF THE ACIDIC GPI-HYDROLYZING PHOSPHOLIPASE PRESENT IN HUMAN SERA

A phospholipase from human serum capable of hydrolyzing glycosylphosph atidylinositol membrane anchors was described and partially characteri zed by our group some years ago. This activity presented a pH optimum between 5.0 and 6.0 and was inhibited by EDTA, EGTA and 1,10-phenanthr oline. Partial purification showed that the enzyme was a glycoprotein with an apparent molecular weight of 140 kDa as judged by gel filtrati on. Other investigators characterized at the same time a phospholipase D with similar properties but with a pH optimum near 7.5. We now conf irm that the human serum enzyme is indeed a phospholipase D capable of hydrolyzing mfVSG and glycolipids A and C from T. brucei. Isoelectric focusing of whole sera suggests the presence of two isoforms, one wit h a pi of 4.7 which was the form previously purified by our group, and others with pI from 6.2 to 7.4.