We have characterized the effects of phenylglyoxal and diethyl pyrocarbonate (DEPC) on the catalytic cycle of the amine transporter in chromaffin granule membrane vesicles. Both reagents inhibited transport in a dose-dependent reaction (with IC50values of 8 and 1 mM, respectively). The inhibition by DEPC was specific for histidyl groups since transport could be restored by treatment with hydroxylamine. Neither phenylglyoxal nor DEPC inhibited binding of either R1-or R2-type ligands, indicating that the inhibition of transport is not due to a direct interaction with either of the known binding sites. Interestingly, however, the acceleration of reserpine binding (an Rl ligand) by a transmembrane H+gradient is inhibited by both reagents at concentrations identical to those which inhibit transport. As previously demonstrated, transport of one proton across the transporter is required for this acceleration to take place [Rudnick, G., Steiner-Mordoch, S., Fishkes, H., Stern-Bach, Y., & Schuldiner, S. (1990) Biochemistry 29, 603-608]. Therefore, we suggest that either proton transport or a conformational change induced by proton transport is inhibited by both types of reagents.