γ-Glutamyl Transpeptidase Inhibition Suppresses Milk Protein Synthesis in Isolated Ovine Mammary Cells

Abstract

The membrane spanning enzyme gamma-glutamyl transpeptidase (gamma-GT; EC 2.3.2.2) catalyses the breakdown of the tripeptide glutathione and uses free amino acids (AA) to form gamma-glutamyl transpeptidase (GT) AA that become transported into cells and converted back into free AA. gamma-Glutamyl transpeptidase activity has been shown to be important for mammary AA uptake in rodent systems, and while gamma-GT activity is high in lactating bovine mammary tissue, the role of this enzyme in milk protein synthesis of the ruminant has not been defined. The present study shows that gamma-GT activity in the ovine mammary gland, like that of rodents, increases during pregnancy and peaks early in lactation. Acivicin, a well-known inhibitor of gamma-GT, decreased gamma-GT activity in acini isolated from the ovine mammary gland and did not have secondary toxicity effects on cell viability or the uptake of radiolabeled amino-isobutyric acid. Isolated ovine acini were incubated in the presence of radiolabeled leucine, and incorporation of label into secreted protein increased during incubation. Incubation of acini with acivicin decreased milk protein secretion by 75%, indicating that gamma-GT plays an important role in milk protein production in the ruminant. Acivicin did not inhibit secretion of specific caseins but caused a global decrease in individual proteins suggesting that gamma-GT may be responsible for providing a complement of AA for milk protein synthesis.

Gammaglutamyltransferase activity in buffalo mammary tissue during lactation

“…High levels of GGT activity have been detected in lactating mammary tissue of the cow (Baumrucker and Pocius, 1978). In addition, in the sheep mammary gland, the activity of GGT increases towards parturition and during early lactation it was six times higher than levels in non-pregnant, non-lactating sheep (Johnston et al, 2004). Similarly, in rodents, GGT activity has been clearly shown to be related to lactation.…”

“…Inhibition of GGT activity in ovine isolated mammary cells induces a significant decrease in milk protein synthesis (Johnston et al, 2004). In the rat lactating mammary gland oxoproline, an intermediate of the g-glutamyl cycle, is able to stimulate AA transport (Viñ a et al, 1985(Viñ a et al, , 1989(Viñ a et al, and 2001.…”

“…In particular, GGT activity has been shown to be important for mammary AA uptake, thus suggesting a specific rô le in milk protein synthesis (Johnston et al, 2004). From a practical point of view, the enzyme is easy to measure and, because of the relative hepatic specificity, it has been widely used as a marker of hepatic damage.…”

Relationship between activity of some fat globule membrane enzymes and the lipidic fraction in ewes' milk: Preliminary studies

“…The g-GT activity in the ovine mammary gland increases during pregnancy and reaches a maximum early in lactation (Johnston, Kitson, Tweedie, Davis, & Lee, 2004). Alkaline phosphatase (AP) is another MFGM enzyme, but its physiological role in milk is unknown.…”

Protein synthesis in mammary epithelial cells harvested from cows treated with growth hormone or atropine

“…Milk samples were collected prior to slaughter and mammary tissue was removed immediately post mortem, and kept at 37 o C until isolation of epithelial cells. Epithelial cells were isolated from bovine mammary tissue using collagenase digestion, and all cells were cultured as previously described (Johnston et al, 2004)…”