Alkali-metal-ion- and H+-dependent activation and/or inhibition of intestinal brush-border sucrase. A model involving three functionally distinct key prototropic groups

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Abstract
For rabbit intestinal brush-border sucrase, a model based on classical Michaelis-Dixon theory cannot fully explain the peculiar antagonistic relationship existing between the substrate and one key proton, Hx, which at acid pH values behaves as a fully competitive inhibitor. In the same pH range, a second proton, Hy, is responsible for changes in catalytic activity and behaves as a mixed-type partially non-competitive inhibitor [Vasseur, Tellier and Alvarado (1982) Arch. Biochem. Biophys. 218, 263-274]. Although involved in the same ionization reaction, these two protons have different kinetic functions, since they are responsible for affinity-type and capacity-type effects respectively. Depending on whether Hx is bound or not, we postulate the enzyme to alternate between two distinct forms differing in their binding properties. The alkali-metal ions Na+ and Li+ have a concentration-dependent biphasic effect on this equilibrium. At low concentrations they facilitate the release of Hx, resulting in K-type activation. At higher concentrations they favour enzyme reprotonation, causing K-type inhibition. On the basic side of the pH spectrum, our results confirm the existence of separate non-competitive effects of the alkali-metal ions, particularly Li+ [Alvarado and Mahmood (1979) J. Biol. Chem. 254, 9534-9541]. To explain the molecular mechanisms underlying the alkali-metal-ion-and H+-dependent effects, we formulate a sucrase model, the three-protons model, in which the acid and basic ionization constants involve respectively two and one key prototropic groups that are functionally distinguishable. A global iterative fit of the relevant general equation to our whole set of data has permitted us to estimate the numerical value of each of the constants constituting the model.

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