Isolation and characterization of circulating low molecular weight peptides in steer, sheep and rat portal and peripheral blood.

1. Low mol. wt peptides in plasma were isolated by reverse-phase HPLC from steer and sheep carotid arterial and rat heart blood and portal blood from all three species. 2. Elution profiles for peptide fractions were similar but the concentration of peptide-bound amino acids (PBAA) in fractions corresponding to different mol. wt peptides was not constant across species. 3. PBAA contributed between 65 and 78% to the plasma amino acid pool in steer and sheep but only 52% in the rat (P less than 0.05). 4. The percentage of many individual amino acids present in either free amino acid (FAA) or PBAA pools was different for ruminant compared with rat plasma but it was similar for steer and sheep apart from branch-chain amino acids (P less than 0.05).