Transport of angiotensin peptides across the Caco-2 monolayer

The bidirectional transport of the angiotensin peptides--des-Asp-angiotensin I (DAAI), angiotensins III and IV--were studied using human intestinal Caco-2 monolayers. The peptides had low permeability rates but were relatively stable to enzymatic hydrolysis. DAAI was transported by diffusion while angiotensins III and IV were transported by an energy requiring, carrier-mediated process. The physicochemical properties and solution conformations of the peptides were investigated in an attempt to establish structure-transport correlations. Among the three peptides, DAAI was the most hydrophobic, had the highest hydrogen bonding potential and was the only peptide to have a random solution conformation, as determined from circular dichroism, two-dimensional (1)H NMR and molecular modelling. On the other hand, the more hydrophilic angiotensin IV had less hydrogen bonding potential and a solution conformation characterized by a beta turn. These factors may influence the transport characteristics of DAAI and angiotensin IV.