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y-Glutamyl Transpeptidase in Rat Mammary Tissue

Meister (1973) first suggested this enzyme to be a component of a y-glutamyl cycle for the transport of amino acids and peptides across cell membranes. We have shown that lactating mammary tissue from cow and rat have relatively high y-glutamyl transpeptidase activity (Baumrucker & Pocius, 1978). If mammary gland y-glutamyl transpeptidase functions as an amino-acid-transport mechanism, then it is conceivable that the activity of this enzyme (like other specific enzymes involved in milk biosynthesis) would increase at the initiation of lactation. Our first objective was to characterize the activity of rat mammary y-glutamyl transpeptidase during pregnancy, lactation, and involution. While this manuscript was in preparation, Puente et al. (1979) reported results from a similar study. Because prolactin is a component of the lactogenic complex necessary for the onset of lactation and milk biosynthesis in the rat (Tucker, 1974; Kuhn, 1977), our second objective was to examine the role of prolactin in regulating the activity of mammary y-glutamyl transpeptidase during lactogenesis.