Nonessential amino acids NEAAs

That some of the EAAs (methionine, phenylalanine, tryptophan, histidine and tyrosine) appear to be transferred stoichiometrically from blood to milk, has been taken as circumstantial evidence that these acids are likely to limit the synthesis and secretion of milk protein (Mepham, 1982; Rook and Thomas, 1983). However, we might equally well argue that limiting amino acids could be taken up in amounts well below their output in milk or, indeed, taken up in great excess relative to their output in milk. This alternative view has focused on glutamate (Rook and Thomas, 1983), glutamine (Meijer et ai, 1993) and proline (Bruckental et ai, 1991). Intravascular infusion of glutamate did not affect the yield of milk protein in goats (Mepham and Linzell, 1974) or cows (Oldham and Bines, 1983) so lending no support to the suggestion that glutamate might be limiting. Although abomasal infusion of glutamine increased the output of milk protein in one of two experiments (Meijer et ai, 1996), the effect was put down to stimulated gluconeogenesis because abomasal infusion of propionate had a similar result. The idea of proline as limiting comes from the low mammary uptake of proline itself coupled with the high uptake of arginine, a precursor of proline. This prompted Bruckental et ai (1991) to examine the effect of infusing 80 g day-1 of proline into the duo denum in cows in early lactation and in mid-lactation. In mid-lactation cows, but not in cows in early lactation, infusion of proline led to an increased yield of milk protein. Further investigation of the proline response is long overdue; in the meantime, the basis of the effect remains unknown.

Indirect effects of NEAAs on the synthesis of milk protein, by way of their role in gluconeogenesis and subsequent sparing of EAAs for protein synthesis, have been investigated in experiments using mixtures of amino acids infused intravenously. Metcalf et ai (1996b) infused cows with 400 g day-1 of an amino acid mixture simulating the composition of milk protein. They found that the milk response to the infusion of the total amino acid mixture could be obtained with the infusion of the EAA component alone (208 g day-1), suggesting that the NEAAs made no contribution to the milk response. Kim et ai (2000b) examined the question in more detail in two experiments in which dairy cows were given intravenous infusions of mixtures of amino acids simulating casein. In the first experiment, the treatments were: 182 g day-1 of the total amino acid mixture (TAA); the EAA component only (101 g day-1); and EAA plus 50 g day-1 of glucose, the glucose equivalent of the NEAA component. Only the TAA infusion led to a statistically significant increase in the yield of milk protein; increases with the EAA and EAA plus glucose failed to reach significance (Fig. 20.3). In the second experiment, equal amounts (182 g day-1) of TAA and NEAA were infused and compared with 100 g day-1 of glucose as the equivalent of the NEAA treatment. Again, only the TAA increased the output of protein in milk; neither NEAA nor glucose affected milk protein yield (Fig. 20.3). All three experiments offer no support for the suggestion that increasing the supply of NEAAs, or their glucose equivalent, can affect the yield of milk protein. This conclusion is in keeping with results from experiments in which glucose was infused into the postruminal gut, and in which metaboliz-able energy (ME) intake was kept constant (an essential requirement if interpretation is not to be confounded) (Hurtaud et ai, 1998a). Although it has been claimed that cows eating a basal diet of grass silage increase the yield

Fig. 20.3. E ffscts on the yield of milk protein of intravenous infusion of mixtures of amino acids. In Experiment 1, the infusions were: 182 g day-1 of total amino acids (TAA); the essential amino acid component of TAA, 101 g day1 (EAA); and EAA plus 50 g day 1 of glucose, the glucose equivalent of the non-essential amino acid component of TAA. In Experiment 2, the infusions were: 182 g day 1 of TAA; 182 g day 1 of non-essential component of TAA (NEAA); and 100 g day 1 of glucose (G), the glucose equivalent of NEAA. (Data of Kim et al., 2000b.)

Fig. 20.3. E ffscts on the yield of milk protein of intravenous infusion of mixtures of amino acids. In Experiment 1, the infusions were: 182 g day-1 of total amino acids (TAA); the essential amino acid component of TAA, 101 g day1 (EAA); and EAA plus 50 g day 1 of glucose, the glucose equivalent of the non-essential amino acid component of TAA. In Experiment 2, the infusions were: 182 g day 1 of TAA; 182 g day 1 of non-essential component of TAA (NEAA); and 100 g day 1 of glucose (G), the glucose equivalent of NEAA. (Data of Kim et al., 2000b.)

of milk protein in response to duodenal infusion of glucose (Hurtaud et al., 2000), the effect was not evident until 500 g day 1 were infused (250 g day 1 were without effect). And, even then, the response was very small: an increase of <40 g day-1 of milk protein in return for 500 g day-1 of glucose. If the use of amino acids for milk protein synthesis were severely limited by the competing demands of gluconeogenesis, we should expect clear responses of milk protein yield to small amounts of glucose because even 50 g of glucose would release around 90 g amino acid for synthesis of milk protein (Krebs, 1964).

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