The mechanism of unidirectional transport of ornithine (i.e. in the absence of a counter-metabolite) has been investigated in proteoliposomes reconstituted with the ornithine carrier purified from rat liver mitochondria. The efflux of [3H]ornithine from proteoliposomes was stimulated by the addition 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 ornithine into or out of the proteoliposomes increased on decreasing the pH in the opposite compartment from 8.0 to 6.0. Ornithine influx was also stimulated when a higher H+ concentration was generated inside the vesicles relative 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-metabolite. Furthermore, changes in fluorescence of the pH indicator pyranine, included inside the proteoliposomes, showed that the flux of ornithine is accompanied by translocation of H+ in the opposite direction. It is concluded that the mitochondrial ornithine carrier catalyses an electroneutral exchange of ornithine+ for H+, in addition to the well-known 1:1 exchange of metabolites. Lysine+, but not citrulline, can also be exchanged for H+ by the ornithine carrier. The ornithine+/H+ transport mode of the exchanger is an essential step in the catabolism of excess arginine.

The purified and reconstituted ornithine/citrulline carrier from rat liver mitochondria catalyses a second transport mode: Ornithine+/H+ exchange

Indiveri C.;
1999

Abstract

The mechanism of unidirectional transport of ornithine (i.e. in the absence of a counter-metabolite) has been investigated in proteoliposomes reconstituted with the ornithine carrier purified from rat liver mitochondria. The efflux of [3H]ornithine from proteoliposomes was stimulated by the addition 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 ornithine into or out of the proteoliposomes increased on decreasing the pH in the opposite compartment from 8.0 to 6.0. Ornithine influx was also stimulated when a higher H+ concentration was generated inside the vesicles relative 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-metabolite. Furthermore, changes in fluorescence of the pH indicator pyranine, included inside the proteoliposomes, showed that the flux of ornithine is accompanied by translocation of H+ in the opposite direction. It is concluded that the mitochondrial ornithine carrier catalyses an electroneutral exchange of ornithine+ for H+, in addition to the well-known 1:1 exchange of metabolites. Lysine+, but not citrulline, can also be exchanged for H+ by the ornithine carrier. The ornithine+/H+ transport mode of the exchanger is an essential step in the catabolism of excess arginine.
Arginine catabolism
Lysine transport
Membranes
Ornithine aminotransferase
Proteoliposomes
File in questo prodotto:
Non ci sono file associati a questo prodotto.

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11770/310421
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus 18
  • ???jsp.display-item.citation.isi??? 17
social impact