pH-dependent inhibitory effects of angiotensin-converting enzyme inhibitors on cefroxadine uptake by rabbit small intestinal brush-border membrane vesicles and their relationship with hydrophobicity and the ratio of zwitterionic species.

The inhibitory effects of five angiotensin-converting enzyme (ACE) inhibitors on the uptake of an aminocephalosporin antibiotic, cefroxadine, by rabbit small intestinal brush-border membrane vesicles were examined in the presence of an inward H+ gradient. Dixon plot analysis showed that all these ACE inhibitors inhibited the uptake of cefroxadine, which is transported by a H+/oligopeptide transporter in the membrane, in the order of enalapril<quinapril, benazepril<temocapril, trandolapril at extravesicular pH 6.0. These drugs, except for enalapril, which are relatively hydrophobic, exhibited a mix of competitive and noncompetitive inhibition. We also examined the inhibitory effects of quinapril and temocapril at extravesicular pH 5.5, at which the ratio of the zwitterionic species of the drugs increased. The inhibition occurred in a nearly competitive manner at this pH and the inhibitory effects were stronger than at pH 6.0. Regression analysis of the inhibitory effects suggested that the affinity of these ACE inhibitors for the transporter was regulated by the hydrophobicity of these ACE inhibitors and the ratio of the zwitterionic species of the drugs.