Multiple forms of serum gamma-glutamyltransferase. Association of the enzyme with lipoproteins.

The association of gamma-glutamyltransferase activity with lipoproteins was investigated in serum from hepatobiliary diseased patients. From 60 to 80% of the total activity in such sera was bound in lipoprotein complexes. These complexes or multiple forms of gamma-glutamyltransferase could be separated into two main fractions by gel filtration or agarose gel electrophoresis. One fraction was characterized by gamma-glutamyltransferase of high molecular mass (Mr greater than 600,000) and beta-mobility and was detected in increased amounts in serum from patients with cholestasis. The main part of this activity was associated with lipoprotein-X and this complex could amount to 50% in some sera. The other fraction contained gamma-glutamyltransferase eluting in the molecular mass range of 250,000 to 450,000 and migrating with alpha 1 alpha 2-mobility. In this fraction complexes of gamma-glutamyltransferase and high-density lipoprotein were detected which could amount to 70% in non-icteric sera. However, heterogeneity in size, charge and density could be demonstrated in both fractions.