Purification and Identification of γ-Glutamyl Transpeptidase of Milk Membranes

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Abstract
Recent evidence suggests that .gamma.-glutamyl transpeptidase may be involved in the transport of amino acids into the lactating mammary gland. The enzyme also is secreted in milk and is associated mainly with milk membranes. The objective of this study was to purify and characterize .gamma.-glutamyl transpeptidase from milk membranes. The enzyme was purified from milk membranes by solubilization with Lubrol WX, treatment with acetone, deoxylcholate and bromelain, and chromatography on ion exchange and molecular-sieving resins. .gamma.-Glutamyl transpeptidase was purified over 11,000-fold from milk. Electrophoresis on sodium dodecyl sulfate polyacrylamide gels [SDS/PAGE] indicates that the enzyme is composed of 2 subunits with MW of 57,000 and 25,500. Both subunits are glycoproteins and were identified in the SDS/PAGE patterns of whole milk membrane. Kinetic characteristics of the purified enzyme are similar to those determined for intact milk membranes and lactating mammary tissue, indicating that the purified enzyme was not modified functionally by the purification procedure.
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