Free and protein-bound amino acids in sow's colostrum and milk.
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The changes in the concentrations of free and protein-bound amino acids in sow's colostrum and milk were quantified in this study. In sow's colostrum, taurine and histidine were the most abundant free amino acids (1 mmol/L of defatted milk). The concentrations of free histidine decreased, whereas those of other free amino acids increased, as lactation progressed. Milk free glutamine concentrations increased to the greatest extent among all free amino acids during the 29-d lactation period, and became the most abundant free amino acid in the milk (1.9 and 3.4 mmol/L of defatted milk) on d 22 and 29 of lactation. Sow's mature milk was rich in free taurine, glycine and glutamate (1-1.4 mmol/L of defatted milk). In contrast to free amino acids, the concentrations of milk protein and protein-bound amino acids decreased during the first 8 d of lactation and then leveled off. In colostrum and milk, glutamine plus glutamate were the most abundant protein-bound amino acids, whereas histidine and cystine were the least abundant. Leucine was the second and fourth most abundant protein-bound amino acid in colostrum and milk, respectively, with proline and lysine being the second most abundant ones on d 8 to 29 of lactation. Both colostrum and milk contained large amounts of urea (4.8-7.7 mmol/L of defatted milk) and ammonia (1.1-2 mmol/L of defatted milk), with ammonia concentration decreasing with advancing lactation stages in a pattern similar to that of milk protein. Thus, this study demonstrates the abundance of free glutamine, taurine, glycine and histidine in sow's milk relative to other free amino acids, as well as the changes in free and protein-bound amino acids in sow's milk during the 29-d lactation period. In light of the present findings, the nutritional importance of amino acids in milk for the growth and development of suckling piglets merits further studies.