Role of intestinal disaccharidases in sugar transport

Summary

A number of observations show that intestinal sucrase is closely located, in the brush borders, to the sugar transport system.

• 1. Both intestinal sucrase (Semenza et al. 1963, 1964; cf. also Crane Kinter 1961) and sucrase (Miller and Crane 1961; Doell, Rosen and Kretchmer 1963) are located in the brush borders.

• 2. The glucose moiety of sucrose is better absorbed than free glucose (Fridhandler and Quastel 1955; Miller and Crane 1961).

• 3. The glucose moiety of sucrose is absorbed directly and is not previously released into the medium (as shown by the fact that the presence of glucose oxidase in the medium does not reduce the absorption of glucose from sucrose, Miller and Crane 1961).

To this notion we intend to add that the sodium site of intestinal sucrase is either identical with, or very similar to, the sodium site of sugar transport system:

• 1. Both intestinal sucrase (Semenza et al. 1963, 1964, cfr. also Crane 1962) and sugar absorption (Riklis and Quast el 1958, Bihler and Crane 1962) are activated by Na⁺.

• 2. Both sucrase (Semenza et al. 1963, 1964) and sugar transport (Bosačková and Crane 1965) are inhibited competitively (toward Na⁺) by ammonium and by other cations of the first group.

• 3. In the hamster the K_a (Na-activation constant in the absence of sucrose) as well as K′_a (Na-activation constant at infinite sucrose concentration) are very similar (Semenza 1966) to the corresponding constants of sugar absorption (Crane, Forstner and Eichholz 1965). The ratio between the two is large: therefore, Na⁺-activation (both of sucrase and of sugar absorption) results in a marked decrease of the apparent K_m, with little or no effect on the maximal velocity.

• 4. In the rabbit, both the Na⁺-activation of sucrase (Semenza 1966) and the Na⁺-activation of sugar transport (Schultz and Zalusky 1964) are characterized by a large increase in maximal velocity with little change in the apparent K_m-values. The K′_K+/K′_Na+ ratio is essentially the same for both systems.

The sugar site of sucrase is probably not the same as the sugar site of the transport system: in fact, we have seen that phlorizin, a well known substrate-competitive inhibitor of sugar transport, does not appreciably affect intestinal sucrase. Conversely, Tris, a substrate-competitive inhibitor of sucrase, does not affect sugar transport (Bosačková and Crane 1965).