Protons drive sugar transport through the Na+/glucose cotransporter (SGLT1).
Na(+)-dependent transporters, such as the Na+/glucose cotransporter (SGLT1) have long been characterized as having an overwhelming preference for Na+ as the essential ion. We have applied electrophysiological measurements to analyze the ability of protons to transport glucose with a cloned transporter, SGLT1, expressed in Xenopus laevis oocytes. Our results show that protons can drive sugar transport through SGLT1 in the absence of Na+ with the following characteristics: 1) the affinity of SGLT1 for H+ is 3 orders of magnitude higher than its affinity for Na+ (3 microM versus 20 mM); 2) H+ supports a higher maximum transport than Na+, suggesting an alteration in rate-limiting processes; and 3) the cation determines the transporter's affinity for sugar (at Vm = -50 mV, the apparent affinity for alpha-methyl-D-glucoside is 0.2 mM in Na+ and 20 mM in H+). The similarity in the kinetics of H(+)- and Na(+)-dependent sugar transport suggests that the transport mechanism for SGLT1 does not depend on the driving ion.
Hirayama
B A
BA
Department of Physiology, UCLA School of Medicine 90024-1751.
Loo
D D
DD
Wright
E M
EM
D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D004553:Electric Conductivity; D005947:Glucose / Q000378:metabolism*; D008562:Membrane Glycoproteins; D008565:Membrane Proteins / Q000378:metabolism*; D009004:Monosaccharide Transport Proteins; D009865:Oocytes; D011522:Protons; D011817:Rabbits; D011994:Recombinant Proteins; D012964:Sodium / Q000378:metabolism; D051269:Sodium-Glucose Transporter 1; D014982:Xenopus laevis
D002352:Carrier Proteins; D002412:Cations; D008562:Membrane Glycoproteins; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D011522:Protons; D011994:Recombinant Proteins; D051269:Sodium-Glucose Transporter 1; D012964:Sodium; D005947:Glucose