Enzymatic cleavage of thymopoietin oligopeptides by pancreatic and intestinal brush-border enzymes

The intestinal enzymatic degradation of the immunomodulating peptides thymotrinan (TP3), thymocartin (TP4), and thymopentin (TP5), three oligopeptides derived from the naturally occurring thymus hormone thymopoietin, was investigated to evaluate their potential for peroral drug delivery. In the presence of brush-border membrane vesicles, crude pancreas extract and everted rings from duodenum, jejunum, ileum, and colon, all peptides were shown to be degraded both by pancreatic enzymes and brush-border aminopeptidases. Degradation clearances (Cldeg) of TP3, TP4, and TP5 were calculated for a quantitative comparison of peptide stability. In the presence of crude pancreas extract, there was a rapid degradation of TP5 (Cldeg 17.9 ml/min) in comparison with TP3 and TP4 (Cldeg 0.95 and 0.56 ml/min, respectively, at 0.2 mM peptide concentration) caused by the cleavage of the C-terminal tyrosine by carboxypeptidase A, whereas TP3 and TP4 underwent hydrolysis by aminopeptidase N. In the presence of brush-border membrane vesicles, the degradation clearances were 3.9, 3.1, and 2.4 ml/min at 0.2 mM concentrations of TP4, TP5, and TP3, respectively. The clearance of all peptides was lowered with increasing peptide concentrations, indicating saturable degradation processes. The degradation of the thymopoietin oligopeptides in the presence of brush-border membrane enzymes was exclusively catalyzed by aminopeptidase N. The degradation of all peptides was highly dependent on the intestinal segment, with the lowest degradation clearance observed in the colon.