Evaluation of amino acid transport and protein metabolism in the mammary gland of dairy cattle
Yoder, Peter Samuel
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Improving our understanding of milk protein production regulation and AA transport is important for successfully formulating diets for AA and improving N efficiency. The objectives were to study protein synthesis regulation and AA transport using in vitro and in vivo models. In the first experiment, the objective was to evaluate the ability of five distinct AA profiles and balancing Lys to Met ratio to 3:1 to stimulate protein translation. No single AA profile uniquely stimulated phosphorylation of translational machinery related proteins suggesting identification of a single optimal AA profile as unlikely. In the second experiment, an in vitro method using three different AA isotopes was developed to trace AA movement. The method assesses bi-directional transport of multiple AA simultaneously enabling evaluation of unidirectional uptake kinetics. This method was used to evaluate AA concentrations representing 16, 100, 186, and 271% of cow plasma AA concentrations. Amino acid uptake was not saturable within the in vivo range for eleven AA. Arginine, Val, and Pro exhibited saturation with the Michaelis-Menten km being 95, 49, and 65% of in vivo concentrations. Results suggest that AA transport is generally non-saturable and that high bi-directional transport exists which enables a mechanism for mitigating AA shortages. In experiment 3, the objective was to evaluate milk protein production and regulation from infusing Met, Lys, and His (MKH) or Ile and Leu (IL). The two EAA groups independently and additively increased milk protein yield. This finding contradicts the single limiting AA theory that a single nutrient will limit milk protein yield. Changes in udder AA extraction and blood flow from supplemental EAA reveal flexible delivery mechanisms. The phosphorylation state of proteins associated with the mTOR pathway was impacted by both EAA treatments. Changes in the udder proteome suggest negative feedback on mTOR pathway activation when milk protein yield was increased by the EAA groups separately but when supplemented together, negative feedback was lessened. Results indicate that multiple EAA can stimulate milk protein production, the ability of AA transport to match intracellular needs, and that the single limiting AA theory or existence of a unique optimal AA profile is likely irrelevant in dairy cows.
- Doctoral Dissertations