Involvement of thiol groups in the function of the dipeptide/proton cotransport system in rabbit renal brush-border...

The role of thiol groups in H+-gradient-dependent dipeptide transport in rabbit renal brush-border membrane vesicles was investigated using glycylsarcosine as the substrate. Treatment of the membrane vesicles with a thiol-group-reducing agent, dimercaptopropanol, stimulated Gly-Sar transport. On the other hand, treatment with thiol group oxidants such as 5,5'-dithiobis(2-nitrobenzoic acid), plumbagin and phenazine methosulfate inhibited Gly-Sar transport. These effects were irreversible, because washing the membranes after treatment failed to reverse the effects. Incubation of the membrane vesicles with phenylarsine oxide, a reagent which interacts specifically with vicinal dithiols, significantly inhibited Gly-Sar transport. In all cases, the stimulation or the inhibition of the dipeptide transport was primarily due to changes in the maximal velocity of the transport system, the apparent affinity constant remaining unaltered. These results demonstrate the involvement of one or more vicinal dithiol groups in the function of the renal dipeptide transport system and that these thiol groups must exist in reduced form to maintain maximal transport activity. In addition, these data indirectly suggest that a dithiol-disulfide interchange may play a role in the function of the renal dipeptide transport system.