Concentration-dependent atypical intestinal absorption of cyclic phenylalanylserine: small intestine acts as an interface between the body and ingested compounds.

Intestinal absorption of peptides in linear form has been studied extensively, but there is little knowledge of peptides in a cyclic form. In this report, intestinal absorption of cyclic phenylalanylserine (cyclo(Phe-Ser)), a precursor of gliotoxin, was studied in isolated rat small intestine as a model cyclic dipeptide. Absorption clearance (CLabs) decreased in the presence of glycylsarcosine, cephalexin or cephradine, substrates for H+/oligopeptide cotransporter (PEPT1). CLabs of cyclo(Phe-Ser) also decreased at 4 degrees C, thus indicating that cyclo(Phe-Ser) is in part transported by PEPT1. However, the Eadie-Hofstee plot of absorption revealed an atypical profile at lower concentrations of cyclo(Phe-Ser) (around 0.1 mM). Moreover, comparative experiments of absorptive and excretive transport showed that excretive transport from the serosal to mucosal side of isolated intestinal tissue at a 0.1 mM cyclo(Phe-Ser) was superior to absorptive transport from the mucosal side to the serosal side, and vice versa at a 1 mM cyclo(Phe-Ser). A kinetic model was constructed, in which cyclo(Phe-Ser) concentration for excretive transport was assumed to be at the binding site of excretive transporter, but not the unbound cytoplasmic concentration. These results as well as the results of kinetic analysis indicate that intestinal absorption consists of passive transport, carrier-mediated absorptive transport by PEPT1 and carrier-mediated excretive transport, resulting in atypical absorption. Although cyclic dipeptides have potentials as drugs, their intestinal absorption may be complex. The results of this study lead us to conclude that absorptive and excretive transport by the small intestine acts as an interface between the body and ingested compounds.