The purified and reconstituted ornithine/citrulline carrier from rat livermitochondria catalyses a second transport mode: ornithine(+)/H+ exchange

The mechanism of unidirectional transport of ornithine (i.e. in the absence of a counter-metabolite) has been investigated in proteoliposomes reconsti tuted with the ornithine carrier purified from rat liver mitochondria. The efflux of [H-3]ornithine from proteoliposomes was stimulated by the additio n of H+ (but not of other cations) to the incubation medium. On keeping the pH in the compartment containing ornithine constant at 8.0, the flux of or nithine into or out of the proteoliposomes increased on decreasing the pH i n the opposite compartment from 8.0 to 6.0, Ornithine influx was also stimu lated when a higher H+ concentration was generated inside the vesicles rela tive to the outside by the K+/H+ exchanger nigericin in the presence of an outwardly directed K+ gradient. A valinomycin-induced electrogenic flux of K+ did not affect ornithine transport in the absence of a counter-metabolit e. Furthermore, changes in fluorescence of the pH indicator pyranine, inclu ded inside the proteoliposomes, showed that the flux of ornithine is accomp anied by translocation of H+ in the opposite direction. It is concluded tha t the mitochondrial ornithine carrier catalyses an electroneutral exchange of ornithine(+) for H+, in addition to the well-known 1:1 exchange of metab olites. Lysine(+), but not citrulline, can also be exchanged for H+ by the ornithine carrier. The ornithine(+)/H+ transport mode of the exchanger is a n essential step in the catabolism of excess arginine.