Involved in the efflux of purine ribonucleosides, such as inosine and guanosine (PubMed:16040204). Adenosine may also be a substrate (PubMed:16040204). Confers resistance to the hypoxanthine analog 6-mercaptopurine, however the level of resistance is rather low (PubMed:16040204). H(+)(out) + inosine(in) = H(+)(in) + inosine(out) guanosine(in) + H(+)(out) = guanosine(out) + H(+)(in) Reducing the electrochemical proton gradient across the plasma membrane by the addition of the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) leads to a drastic reduction in the rate of inosine excretion. In stationary phase. Inactivation of the gene leads to increased susceptibility to purine ribonucleosides. Belongs to the major facilitator superfamily. DHA1 family. NepI (TC 2.A.1.2.26) subfamily.