Catabolism of nutritionally essential amino acids in developing porcine enterocytes

This study was conducted using the piglet model to test the hypothesis that mucosal cells of the neonatal small intestine can degrade nutritionally essential amino acids (EAA). Enterocytes were isolated from the jejunum of 0-, 7-, 14-, and 21-day-old pigs, and incubated for 45 min in Krebs buffer containing plasma concentrations of amino acids and one of the following L-[1-¹⁴C]- or L-[U-¹⁴C]-amino acids plus unlabeled tracees at 0.5, 2, or 5 mM: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. In these cells, branched-chain amino acids (BCAA) were extensively transaminated and 15-50% of decarboxylated branched-chain α-ketoacids (BCKA) were oxidized to CO₂ depending on the age of piglets. BCAA transamination increased but their decarboxylation decreased between 0 and 14 days of age. Addition of 1 and 2 mM α-ketoglutarate to incubation medium dose-dependently stimulated BCAA transamination without affecting their decarboxylation. Western blot analysis revealed that the abundance of mitochondrial BCAA aminotransferase declined but cytosolic BCAA aminotransferase increased between 0 and 14 days of age, with the cytosolic protein being the major isoform in 7- to 21-day-old pigs. BCKA dehydrogenase protein existed primarily as the phosphorylated (inactive) form in enterocytes of newborn pigs and its levels were markedly reduced in older pigs. All measured parameters of BCAA metabolism did not differ between 14- and 21-day-old pigs. In contrast to BCAA, catabolism of methionine and phenylalanine was negligible and that of other EAA was absent in enterocytes from all ages of piglets due to the lack of key enzymes. These results indicate that enterocytes are an important site for substantial degradation of BCAA but not other EAA in the neonatal gut.