Massey Documents by Type

Permanent URI for this communityhttps://mro.massey.ac.nz/handle/10179/294

Browse

Filters

20022

Settings

Subject: search.filters.subject.Milk protein production

Search Results

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    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
    (Massey University, 2002) Johnston, Sarah Louise
    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.
  • Thumbnail Image
    Item
    The role of insulin in the regulation of milk protein synthesis in pasture-fed lactating ruminants : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
    (Massey University, 2002) Back, Penelope Jane
    The primary aim of this thesis was to determine the role of insulin in milk protein production in pasture-fed lactating ruminants (ewes and cows), using the hyperinsulinaemic euglycaemic clamp (HEC) technique. Three experiments were carried out. In the first 2 experiments, the response of pasture-fed ewes and dairy cows to the HEC were established and compared to concentrate-fed ruminants (dairy cows and goats). Use of the HEC technique in pasture-fed ruminants did not result in an increase in milk protein yield or concentration. However, a reduction in feed intake along with maintenance of milk protein yield resulted in a change in efficiency of utilisation of dietary crude protein for milk protein production. This indicated that changes in blood insulin could result in changes in nutrient partitioning to maintain milk protein production. In Experiment 3, mechanisms were examined that could maintain milk protein production despite a reduction in feed intake. The arterio-venous concentration difference technique and a leucine tracer infusion were used to measure amino acid (AA) uptake and subsequent metabolism for milk protein production under conditions imposed by the HEC. This experiment demonstrated that the HEC reduced AA supply to the mammary gland and there was a decrease in the uptake of some AA. There was no increase in mammary blood flow to compensate for this. The deficit in the ratio of AA uptake to their secretion in milk protein suggests the use of plasma free AA concentrations underestimates uptake of AA by the mammary gland and there are contributions by alternative sources such as peptide AA and erythrocytes. There was no decrease in leucine oxidation in the mammary gland, indicating that AA were not conserved for milk protein production through an alteration in this mechanism. These results support the theory that the mammary gland has the ability to respond to modified precursor supply to maintain milk protein output.