A Kinetic Investigation on the Allosteric Effects in Intestinal Sucrase

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Abstract
This paper is a part of a project on the mechanism of activation of intestinal sucrase and of intestinal sugar transport by Na+. It reports a formal kinetic analysis of the following aspects: Investigation on the apparent Km for sucrose hydrolysis in intestinal rings and homogenates does not indicate the existence of any gross diffusion barrier for sucrose between lumen and brush‐border bound sucrase. Therefore, it is possible to compare kinetic parameters of solu‐bilized sucrase with kinetics parameters of the intestinal sugar uptake system. Intestinal sucrases from hamster, rat, Greek turtle, man and rabbit (the latter two reported in previous papers) are all activated by Na+. The modes of activation and the apparent Michaelis constants for Na+ are different for each species considered. K+ and Li+ have effects similar to those of Na+. The Kapps= f ([Na+]) functions indicate a non‐compulsory reaction sequence in Na+‐activation of sucrase. Hamster and rat sucrases show a cooperative interaction between homologous sites. Papain solubilization does not make it disappear. The pH‐activity curves are different in each species. In some species the percent activation by Na+ depends on the pH. The Kochsaft of small intestine contains low molecular weight substance(s) which decrease the color yield in the glucose‐oxidase/peroxidase assay for glucose.