TRANSPORT OF N-ALPHA(OR N-EPSILON)-L-METHIONYL-L-LYSINE AND ACETYLATED DERIVATIVES ACROSS THE RABBIT INTESTINAL EPITHELIUM

Intestinal absorption constitutes an important step in the biological utilization of L-Methionine and N-Acetyl-L-methionine covalently bound to epsilon-NH2 groups of food protein lysyl residues. The transport of synthesized and purified N-alpha-(or N-epsilon-)L-Methionyl-L-lysine and N-alpha-(or N-epsilon-)Acetyl-L-methionyl-L-lysine was studied in vitro in rabbit ileum mounted in the Ussing chamber and compared to that of both free L-Methionine and L-Lysine or both free N-Acetyl-L-methionine and L-Lysine. Addition of all solutes to the mucosal reservoir, except N-epsilon-Acetyl-L-methionyl-L-lysine, led to an increase in the short-circuit current, the lowest response obtained by N-alpha-Acetyl-L-methionyl-L-lysine. In all cases, only the constitutive amino acids were recovered in the serosal chamber. When free L-Methionine and L-Lysine were added together to the mucosal reservoir, comparable fluxes were obtained; when any of the dipeptides were added, transport of lysine was the highest. Although the mucosal reservoir disappearance of N-alpha-L-Methionyl-L-lysine was faster than that of N-epsilon-L-Methionyl-L-lysine, the mucosal to serosal fluxes of their constitutive amino acids were not significantly different. However, the transepithelial flux of L-Methionine originating from the dipeptides was 40-50% less than that of the free amino acid, whereas L-Lysine flux was unchanged. Substantial tissue oxidation of L-Methionine unlike L-Lysine, which are slowly released from peptide hydrolysis by the brush border membrane aminopeptidases, likely altered the amino acid transport. Moreover, the observed higher transepitheal flux for L-Lysine relative to L-Methionine when both were derived from the dipeptides may have resulted from intracellular L-Methionine stimulation of L-Lysine absorption. As compared to the normal di- and isodipeptides, lower net fluxes of L-Methionine, and to a lesser extent of L-Lysine, were observed from the acetylated peptides, the former being zero even in the case of N-epsilon-Acetyl-L-methionyl-L-lysine. The results demonstrate the significant contribution of brush border aminopeptidases to the intestinal absorption of N-epsilon-L-Methionyl-L-lysine in the rabbit. Our findings also suggest that the availability of amino acids from acetylated peptides is limited by their transport rates and/or their hydrolysis from cytosolic N-acylpeptide hydrolase and N-acylase.