Comparative studies on the absorption mechanism of p-aminobenzoic acid and p-acetamidobenzoic acid from the rat intestine

The absorption characteristics of p-aminobenzoic acid (PABA) from the rat intestine was investigated comparing with that of p-acetamidobenzoic acid (Ac-PABA). The ionized form of PABA was well absorbed from the small intestine and the pH-profile of the absorption rate of PABA was inconsistent with the pH-partition theory. This phenomenon was not observed in the large intestine. In contrast, the absorption of Ac-PABA was consistent with the pH-partition theory in both small and large intestine. The transfer rate of PABA from mucosal side to serosal side was markedly faster than that in the opposite direction. This directional difference in the transfer rate was not observed with Ac-PABA. The uptake against a concentration gradient was found during the absorption process of PABA. This phenomenon disappeared in the presence of 2, 4-dinitrophenol, p-chloromercuribenzene sulfonate or by the pretreatment of the intestine with HgCl₂. These findings indicate the anomalous aspects of the absorption of PABA from the rat small intestine at pH 6.5. Furthermore, the absorption of PABA was inhibited by the mercurial modification of the brush border membrane. This suggests that protein and sulfhydryl groups within the brush border membrane are involved in the absorption process of PABA.