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8 Functional Role of Histidine in Diets of Young Pigs


 Histidine is a constituent amino acid of body proteins and, once incorporated in protein, histidine can be methylated post-translationally to methyl-histidine. Histidine is also a precursor of histamine, a neurotransmitter and involved in the immune response. Histidine and histamine are constituents of a number of dipeptides, which act as pH buffers, metal chelating agents, and anti-oxidants, especially in skeletal muscles and in the brain. A considerable fraction of whole-body histidine is present as carnosine, the dipeptide of histidine and β-alanine. In the longissimus muscle, about 40% of the total histidine content is present as carnosine. The histidine in carnosine can be methylated to anserine or balenine, and the pig is among the few species that synthesize both forms. Hydrolysis of body protein and of histidine-containing dipeptides results in the release of the constituent amino acids. However, only the histidine of protein and carnosine can be reused for protein synthesis. Methyl-histidine is either excreted in the urine or remains bound in the dipeptides and accumulates in the body. Because carnosine represents such a large histidine reservoir, a dietary histidine deficiency may not directly lead to a reduction in growth, especially if growth is given a higher priority for histidine utilization than maintaining or depleting the histidine-containing dipeptide reserves. Few histidine dose-response studies have been done in piglets and differences in the estimated requirements may be due to differences in diluting or depleting the dipeptide reserves. However, at low histidine intakes, both feed intake and growth are reduced and a reduction of the histidine-to-lysine supply by 1 percentage point results in a growth reduction of 4%. Histidine dose-response studies need to consider the role of histidine as a constituent amino acid of body protein as well as its role in dipeptides.