gamma-Glutamyl transpeptidase of bovine milk membranes: distribution and characterization

The distribution and characterization of gamma-glutamyl transpeptidase from bovine milk was examined. The enzyme was associated mainly with components of milk membrane. Skim milk membrane had 41% of the total activity in whole milk whereas milk fat globule membrane showed 7%. Enzyme kinetic characterization of these two membrane fractions showed they had identical amino acid specificities, pH, and sodium chloride effects, and similar apparent Michaelis constants. The main enzymatic difference between the membranes was a 2 to 3 times higher specific activity of the skim milk membranes. Skim milk membrane from milk would be a good source of this enzyme for continued studies of the function of gamma-glutamyl transpeptidase in amino acid transport during lactation.