The role of the γ-glutamyl cycle in milk protein synthesis in the ruminant : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biochemistry at Massey University, Palmerston North, New Zealand
Dairy products are New Zealand's primary export commodity. The manufacturing efficiency for dairy products would be maximised if the Dairy Industry had the ability to control milk protein production to suit the manufacture of specific products. Understanding the role of amino acid transport in regulating milk protein synthesis may allow manipulation of proteins in milk. γ-Glutamyl transpeptidase (γ-GT), an enzyme thought to play a key role in mediating amino acid transport, has been demonstrated in mammary tissue, but the role of this enzyme and its associated biochemical pathway, the γ-glutamyl cycle, has not been fully elucidated. The γ-glutamyl cycle consists of synthetic and degradative enzymes for the cysteine-containing tripeptide glutathione. γ-GT transfers the γ-glutamyl moiety from glutathione to amino acids, and has a high affinity for cyst(e)ine. The vascular supply of cysteine is thought to be insufficient to maintain milk protein synthesis. In this study, the role of the γ-glutamyl cycle in amino acid transport for milk protein synthesis was investigated using two systems, firstly, in acini isolated from the udder of lactating sheep, and secondly in lactating goats. Milk protein secretion from isolated acini significantly decreased (70%) as a result of γ-GT inhibition with acivicin, and significantly increased (250%) when supplied with cysteine as N-acetylcysteine (NAC). In lactating goats, acivicin did not affect milk yield or total protein concentration or yield, but significantly increased αs2- and κ-casein concentration in milk. This may have resulted from increased uptake of some amino acids by the mammary gland and suggests that γ-GT negatively regulates uptake of some amino acids for milk protein synthesis. NAC significantly increased milk yield, protein concentration and protein yield as a result of increased uptake of some amino acids, which may have been due to increased mammary blood flow. This increase was prevented by acivicin, however, suggesting that γ-GT plays an important role in amino acid supply. Inhibition of γ-GT may up-regulate sub-saturated transport systems leading to increased uptake of amino acids required for milk protein synthesis. Further testing of NAC and a greater understanding of the function and regulation of γ-GT may allow increased, and targeted, milk protein production as required by the Dairy Industry.
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Johnston, S. L., Roy, N.C., Kitson, K. E., Tweedie, J. W., Davis, S. R., & Lee, J. (2001). ?-Glutamyl transpeptidase and amino acid transport for milk protein production in vivo. Proceedings of the New Zealand Society of Animal Production, 61, 244-247