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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: 1. 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 sucrose, Therefore, it is possible to compare kinetic parameters of solubilized sucrase with kinetics parameters of the intestinal sugar uptake system. 2. 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.3. K+ and Li+ have effects similar to those of Na+. 4. The Ksapp = f (Na+]) functions indicate a non-compulsory reaction sequence in Na+-activation of sucrase. 5. Hamster and rat sucrases show a cooperative interaction between homologous sites. Papain solubilization does not make it disappear. 6. The pH-activity curves are different in each species. In some species the percent activation by Na+ depends on the pH. 7. The Kochsaft of small intestine contains low molecular weight substance(s) which decrease the color yield in the glucose-oxidase/peroxidase assay for glucose. [ABSTRACT FROM AUTHOR]

In contrast to MN receptors, Pr1 and Pr2 antigens of isolated glycoprotein of erythrocytes are not inactivated by acylation of free amino groups. By introduction of easily leaving groups, e.g. trifluoroacetic residues, inactivation of the MN activity can be made reversible. No differences between Pr1 and Pr2 antigens could be found with the reactions described in this paper. Acylation m aqueous phase (buffer/anhydrous acetic acid) and m organic phase (pyridine/ anhydrous acetic acid) gave identical results qualitatively and quantitatively. It was not possible to split the N-acetic binding neither under mild alkaline conditions nor with aminoacylase. 4-5 amino groups out of 11 could be acylated, whereas succinylation in aquous phase (buffer/ anhydrous succinic acid) resulted in modification of only one free amino group, the calculations being based on a molecular weight of 30 000. Following succinylation MN activity was also lost, whereas the antigenic activity of Pr1 and Pr2 remained unchanged. The appearance of a negative charge instead of the positive charge of the free amino group on the glycoprotein did not affect Pr1 or Pr2 activity m any way. If glycoprotein was esterified under conditions which cause esterification of free sialic acid to a wide extent neither MN activity nor Pr1/Pr2 activity was altered. [ABSTRACT FROM AUTHOR]