Adsorption of Streptococcus sobrinus dextranase inhibitor to water-insoluble alpha-D-glucans of oral streptococci.

A low molecular weight dextranase inhibitor from Streptococcus sobrinus has previously been identified and purified. The range of conditions under which inhibition occurs, and the situations in which dextranase activity of S. sobrinus can reappear, have been examined in the chemostat. These studies have revealed that when dextranase production exceeds that of the inhibitor, all the inhibitor is tightly bound into enzyme-inhibitor complexes, and the excess enzyme remains active. Another factor that influences the activity of dextranase inhibitor has now been identified, namely the ability of the inhibitor to bind to water-insoluble glucans. Adsorption to water-insoluble alpha-D-glucans, produced by oral streptococci that were grown in batch culture, increased with their proportion of alpha-1,3-linked sequences of glucose residues. Studies with water-insoluble dextrans of Leuconostoc mesenteroides strains showed that alpha-1,6-linked sequences were also important for binding. The inhibitor was not active when adsorbed to glucan, but active inhibitor was released by incubation with soluble dextran. The interactions of sucrose, alpha-D-glucosyltransferases, alpha-D-glucans, dextranase and dextranase inhibitor are discussed in relation to the growth rate of S. sobrinus. At low growth rate in the chemostat the predominant alpha-D-glucosyltransferase (GTF) is a GTF-S that converts sucrose into soluble dextran, and the activity of free dextranase inhibitor in the culture filtrate is high. By contrast, at high growth rate the streptococci produce GTFs capable of synthesizing water-insoluble alpha-D-glucans, and no free inhibitor is found in culture filtrate. Thus the activity of free, extracellular dextranase inhibitor is controlled by (i) the extent of binding to dextranase and (ii) the extent of adsorption to water-insoluble alpha-D-glucan.