Kinetic mechanism of Na+ -glucose cotransport through the rabbit intestinal SGLT1 protein.
No consensus has yet been reached regarding the order of substrate addition to the high-affinity Na+ -D-glucose cotransporter (SGLT1). This problem was addressed by computer-assisted derivation of the steady-state velocity equations characterizing the eight-state Na+:Na+:substrate (NNS) and Na+:substrate:Na+ (NSN) mechanisms of cotransport. A notable difference was found in their denominator expressions and used to device a new strategy aimed at model discrimination in which the initial rate data are recorded at fixed S and analyzed relative to the N dependence of transport using a Hill equation. According to this protocol, the values of the Hill coefficient (n(H)) should be finite at all S (1.0 < n(H) < or =2.0) or decrease down to a limit value of 1.0 at high S in the case of the NNS and NSN models, respectively. These key experiments were performed in rabbit intestinal brush border membrane vesicles and demonstrated that a Hill equation with n(H) = 2.0 best describes the steady-state kinetics of Na+ -glucose cotransport at all S. We therefore propose a kinetic mechanism whereby Na+ binding should occur with very strong cooperativity within a rapid equilibrium segment of the transport cycle and be followed by a slow isomerization step before glucose addition.
Berteloot
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Groupe de Recherche en Transport Membranaire, Département de Physiologie, Faculté de Médecine, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Quebec H3C 3J7, Canada. Alfred.Berteloot@umontreal.ca
D000818:Animals; D002478:Cells, Cultured; D003198:Computer Simulation; D005947:Glucose / Q000493:pharmacokinetics*; D017136:Ion Transport / Q000502:physiology; D007583:Jejunum / Q000378:metabolism; D007700:Kinetics; D008297:Male; D008562:Membrane Glycoproteins / Q000378:metabolism / Q000502:physiology*; D008871:Microvilli / Q000378:metabolism*; D008954:Models, Biological; D009004:Monosaccharide Transport Proteins / Q000378:metabolism / Q000502:physiology*; D011817:Rabbits; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012964:Sodium / Q000493:pharmacokinetics*; D051269:Sodium-Glucose Transporter 1
D008562:Membrane Glycoproteins; D009004:Monosaccharide Transport Proteins; D051269:Sodium-Glucose Transporter 1; D012964:Sodium; D005947:Glucose