Glycosyl phosphatidyl inositol phospholipase D activity in human serum

We measured serum glycosyl phosphatidyl inositol phospholipase D (GPI-PLD) by its alkaline phosphatase releasing activity in healthy and diseased individuals. Linearity with respect to serum concentration was obtained only with very low serum volumes (below about 0.2 microL) necessitating a large predilution of serum to avoid potential artefacts. The assay was sufficiently precise for routine use. Patients with liver disease had lower activities and those with renal disease had higher activities than healthy controls. Following liver transplantation there was no correlation between GPI-PLD and conventional markers of liver function but there was a marked correlation with cholesterol concentration. These observations suggest that liver is a major source of GPI-PLD in serum. Its function remains unknown.