gamma-Glutamyltransferase (GGT) fractions: characteristics and biogenesis

Four GGT fractions (b-, m-, s-and fGGT) have been described in plasma. The aim of this study was to characterize their molecular nature in human plasma and bile. Plasma was obtained from healthy volunteers and primary bile was collected from patients undergoing liver transplant. For each GGT fraction we determined MW, density, sedimentation conditions in centrifugation assays, and the sensitivity to detergent [deoxycholic acid (DOC)] and protease (papain). A partial purification of b-GGT for immunogold analysis was obtained by ultracentrifugation. Plasma b-GGT showed a MW of 2000 kDa and a density between 1.063-1.210 g/mL. Treatment with 1% DOC converted b-GGT into s-GGT fraction, while b-GGT was not sensible to papain treatment. Plasma m-GGT and s-GGT showed a MW of 1000 and 200 kDa, and their densities were between 1.006-1.063 g/mL and 1.063-1.210 g/mL, respectively. Both fractions were unaffected by DOC treatment, while GGT activity was completely recovered in f-GGT peak after their incubation with papain. Plasma f-GGT showed a MW of 70 kDa and a density > 1.21 g/mL. In human hepatic bile we identified two peaks showing the same characteristics of plasma b-and f-GGT fractions. Collected data showed that b-GGT is constituted by membrane microvesicles both in bile and plasma, as confirmed by immunogold; m-GGT and s-GGT might be constituted by bile-acid micelles, while f-GGT represents the free-soluble form of the enzyme. The understanding of the nature and properties of plasma GGT fractions may allow a better clinical utilization of GGT as a clinical biomarker.