The activation of intestinal brush border sucrase by alkali metal ions: An allosteric mechanism similar to that for the Na +-activation of nonelectrolyte transport systems in intestine

Abstract

Activation of intestinal brush border sucrase by alkali-metal ions is described by an allosteric, noncompulsory mechanism involving two distinct sites: one for sucrose and another for the metal activator. Both Na + and K + activate guinea pig sucrase but K + has ten times more affinity for the metal site. Li + is inert. Values for the dissociation constants for the interaction of sucrase with either sucrose, Na + or K + have been calculated for the guinea pig, rat, and hamster. Qualitatively, the activation of sucrase by alkali metal ions is similar to that described for the Na +-activation of amino acid and sugar transport in intestine. However, the Na-binding site in the two systems is apparently quite different. In the guinea pig, the Na-dissociation constant is in the order of 10 −3 m for sucrase, about two orders of magnitude smaller than that for transport. Also, K + is a strong activator of guinea pig sucrase, but an inhibitor of intestinal sugar transport.