ADSORPTION OF STREPTOCOCCUS-SOBRINUS DEXTRANASE INHIBITOR TO WATER-INSOLUBLE ALPHA-D-GLUCANS OF ORAL STREPTOCOCCI

A low molecular weight dextranase inhibitor from Streptococcus sobrinu s 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 chemo stat. These studies have revealed that when dextranase production exce eds that of the inhibitor, all the inhibitor is tightly bound into enz yme-inhibitor complexes, and the excess enzyme remains active. Another factor that influences the activity of dextranase inhibitor has now b een identified, namely the ability of the inhibitor to bind to water-i nsoluble glucans. Adsorption to water-insoluble alpha-D-glucans, produ ced by oral streptococci that were grown in batch culture, increased w ith their proportion of alpha-1,3-linked sequences of glucose residues . Studies with water-insoluble dextrans of Leuconostoc mesenteroides s trains showed that alpha-1,6-linked sequences were also important for binding. The inhibitor was not active when adsorbed to glucan, but act ive inhibitor was released by incubation with soluble dextran. The int eractions of sucrose, alpha-D-glucosyltransferases, alpha-D-glucans, d extranase and dextranase inhibitor are discussed in relation to the gr owth rate of S. sobrinus. At low growth rate in the chemo stat the pre dominant alpha-D-glucosyltransferase (GTF) is a GTF-S that converts su crose into soluble dextran, and the activity of free dextranase inhibi tor 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. T hus the activity of free, extracellular dextranase inhibitor is contro lled by (i) the extent of binding to dextranase and (ii) the extent of adsorption to water-insoluble alpha-D-glucan.