γ-Glutamyl Transpeptidase of Bovine Milk Membranes: Distribution and Characterization

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Abstract
The distribution and characterization of .gamma.-glutamyl transpeptidase [EC 2.3.2.2] 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 2 membrane fractions showed they had identical amino acid specificities, pH and NaCl effects, and similar apparent Km. The main enzymatic difference between the membranes was a 2-3 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.