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Milk Protein Synthesis is regulated by Lysine and Branched Chain Amino Acid Deficiencies in Lactating Bovine Mammary Glands
The supply of specific essential amino acids is tightly regulated by the lactating dairy cow to maintain milk protein production. To determine the effect of essential amino acid (EAA) deficiencies and imbalances on milk protein synthesis and metabolic parameters, early lactation fistulated dairy cows (105 ± 12 DIM) were abomasally infused with either saline, EAA, EAA less lysine, EAA less leucine, or EAA less the branched-chain amino acids (BCAA; isoleucine, leucine, and valine) in a 5 x 5 Latin square design. Cows were fed a diet to provide an NEL of 6.9MJ/kg DM and 11.7% crude protein. Compared to EAA, a BCAA deficiency significantly decreased plasma concentration of Leu, Val and total BCAA by 72, 67, and 66%, respectively (P < 0.001). In response to a leucine deficiency, plasma concentration of Ile and Val increased 71 and 62%, respectively, while plasma leucine decreased 72% (P < 0.001). Omission of lysine from the abomasal infusate resulted in a 72 and 77% decline in plasma lysine and asparagine, respectively (P < 0.04). Plasma concentrations of beta-hydroxybutrytrate and non-esterified fatty acids were not significantly different between treatments. While no significant treatment differences were observed for daily milk production (30.1 kg/d), milk protein yield increased 18% by the EAA infusion over saline (P = 0.001), while the omissions of lysine, leucine and the BCAA decreased milk yield by 10.2, 21.1, and 12.2 %, respectively, compared to EAA (P < 0.03). In comparison to EAA, milk protein concentration was 0.23 (P = 0.057), 0.3 (P = 0.01), and 0.29 (P = 0.01), percentage points lower for lysine, leucine and BCAA deficiencies, respectively. The increase in plasma concentration of Ile and Val in response to Leu deficiency suggests that compensatory measures were initiated to maintain substrate supply for milk protein synthesis. These results indicate that protein synthesis in the mammary gland is sensitive to the supply of Lys, Leu and the BCAA.
Keywords: mammary gland, milk protein synthesis, essential amino acid