Phosphatidylinositol‐glycan‐specific phospholipase D is an amphiphilic glycoprotein that in serum is associated with high‐density lipoproteins

Phosphatidylinositol (PtdIns)‐glycan‐specific phospholipase D was purified from bovine and human serum by phase separation in Triton X‐114 and by chromatography on DEAE‐cellulose, octyl‐Sepharose, concanavalin‐A–Sepharose, and hydroxyapatite. The purification of the two enzymes was approximately 1200‐fold with a recovery of 3–5%. Bovine serum contained about 40 μ/ml of PtdIns‐glycan‐specific phospholipase D, about 10 times more than the amount determined in human serum. PtdIns‐glycan‐specific phospholipase D is also present in mammalian cerebrospinal fluid and in mammalian milk but to a much lesser extent than in serum. Enzyme from bovine and human serum displayed amphiphilic properties as revealed by sucrose density gradient centrifugation and gel filtration in the absence and presence of detergent. On density gradient centrifugation, both enzymes sedimented with an apparent sedimentation coefficient of about 6.0 S in the presence of 0.1% Triton X‐100, and formed aggregates up to 14.5 S in the absence of detergent. Upon gel filtration, the bovine and human enzymes migrated with a Stokes' radius of 6.5 nm and 6.6 nm, respectively, in the presence of Triton X‐100. In the absence of Triton X‐100, both enzymes gave a Stokes' radius of 8.8 nm. Serial centrifugation of serum at increasing NaBr concentrations revealed that the majority of the enzyme is contained in the high‐density lipoprotein fraction. PtdIns‐glycan‐specific phospholipase D from bovine and human serum contained 27 and 28 N‐acetylglucosamine residues, respectively. Treatment with N‐glycosidase F decreased the apparent molecular mass of the bovine and human enzyme from 115 and 123 kDa to 91 and 87 kDa, respectively. Sequence analysis of peptides derived from PtdIns‐glycan‐specific phospholipase D of bovine serum by CNBr cleavage gave 100% identity to the sequence published for the bovine liver enzyme while there was 83% similarity and 74% identity to the sequence of peptides obtained from the human serum enzyme.