Different pathways for lysine transport across neonatal pig intestine.

1. Undirectional fluxes of lysine, presented free and in the form of a dipeptide, have been measured in vitro using the small intestines of pigs taken during the first 2 weeks of post-natal life. 2. The concentration dependence of free lysine transfer to the serosal surface of new-born pig intestine (Jms) can be described as the sum of a saturable (Km 0.2 mM; Jm 3 n- mole cm-2 in-1) and a non-saturable (1.0 n-mole cm-2 min-1 mM-1) component. 3. The dipeptide isoleucyl-lysine was hydrolysed completely during transfer to the serosal surface of intestines taken at all stages of development. The concentration dependence of peptide-derived lysine transfer can be described as if it took place through a single saturable process (Km 5 mM; Jm 27 n-mole cm-2 min-1). 4. The mucosa to serosa transfer of free lysine fell to a half and that of peptide-derived lysine fell to a quarter, during the first 8 days of post-natal life. The decline in peptide-derived lysine transport correlated with changes in peptide hydrolase activity, measured in homogenates of intestinal mucosa using isoleucyl-lysine as substrate. 5. The new-born pig intestine transported lysine derived from isoleucyl-lysine twice as quickly as from lysyl-isoleucine. Lysine and isoleucine did not inhibit the transfer of isoleucine and lysine derived from isoleucyl-lysine. The transport of lysine, but not isoleucine, from isoleucyl-lysine took place more quickly than from equimolar solutions of the free amino acids. 6. It is suggested from these results that lysine and isoleucyl-lysine use different pathways when crossing the intestinal mucosa. Attempts are made to assess the probable contributions made by these pathways to the total absorption of lysine taking place during neonatal development.