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J. Dairy Sci. 87:321-329
© American Dairy Science Association, 2004.

{gamma}-Glutamyl Transpeptidase Inhibition Suppresses Milk Protein Synthesis in Isolated Ovine Mammary Cells

S. L. Johnston1,*, K. E. Kitson2, J. W. Tweedie3, S. R. Davis1,{dagger} and J. Lee1,{ddagger}

1 AgResearch Limited, New Zealand
2 Institute of Food, Nutrition and Human Health and
3 Institute of Molecular BioSciences, Massey University, Palmerston North, New Zealand

Corresponding author: J. Lee; e-mail: julian.lee{at}agresearch.co.nz.

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.

Key Words: acivicin • {gamma}-glutamyl transpeptidase • isolated ovine mammary acini • milk protein

Abbreviation key: 3H-AIB = {alpha}-amino-isobutyric acid [methyl-3H], DMEM = Dulbecco’s modified Eagle’s medium, DTT = dithiothreitol, FCS = fetal calf serum, IC50 = constant at 50% inhibition, MeAIB = methylated amino-isobutyric acid, {gamma}-GT = {gamma}-glutamyl transpeptidase, HUVE = human umbilical vein endothelial




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