Identification, partial purification and biochemical characterization of γ-glutamyltranspeptidase present as a membrane component in skimmed milk and milk fat-globule membranes, and in mammary-tumour virus from the milk of infected mice

Abstract

The enzyme γ-glutamyltranspeptidase was reproducibly found to be associated with mouse milk particles; it is present in milk fat-glouble membranes and mouse mammary-tumour virus of infected Swiss mice, also in particles from the milk of uninfected mice. The enzymatic activities observed range among the highest reported for mammalian tissues. The enzyme was partially purified from mouse mammary-tumour virus, and from milk fat-globule membranes. The molecule requires the presence of detergents to remain soluble, behaves as a high molecular weight component, properties characterizing integral membrane proteins. Kinetics, and the effect of competitors as well as of specific inhibitors show this enzyme to be identical to the well-known kidney γ-glutamyltranspeptidase ((γ-glutamyl)-peptide:amino-acid-γ-glutamyltransferase, EC 2.3.2.2.). Other oncornaviruses budding from cultured cells originally expressing the enzyme in their plasma membrane also incorporate the enzyme in their structure.