One aspect of this investigation involves attempts to delineate the biochemical reactions involved in the inactivation and proteolytic degradation of a cell-associated fructosyltransferase (FT) produced by Streptococcus salivarius. We are focusing on the FT inactivation step because this reaction appears to 'mark' the enzyme for subsequent proteolysis. In an in vitro system, it has been demonstrated that a partially purified NADH oxidase is responsible for catalyzing a copper-dependent oxidative inactivation of FT. The reaction products of the NADH oxidase are hydrogen peroxide and superoxide anion. A second NADH oxidase has also been isolated that produces only water and this enzyme is completely ineffective in the in vitro inactivation of FT. Both superoxide and hydrogen perioxide are required in addition to copper for FT inactivation. Our results suggest that FT inactivation by these two dioxygen reduction products may occur in a site-specific manner by a metal-catalyzed Haber-Weiss reaction. We suggest that FT first binds cupric ions at or near the catalytic site. The bound cupric ions could then be reduced by superoxide anions and subsequently reoxidized by hydrogen peroxide with the generation locally of hydroxyl radicals. Hydroxyl radicals are most likely involved in the inactivation, since neither hydrogen peroxide nor superoxide anions alone have any effect on FT activity. A second aspect of our current studies deals with the mechanism of activation of streptococcal lactate dehydrogenases (LDHs) by fructose 1,6-biophosphate (FBP). We showed previously that FBP mediates a conformational change in the enzyme which results in a marked increase in its affinity for substrate and coenzyme. We now find that FBP also affects the quaternary structure of these LDHs. A partially purified LDH from S. salivarius exists as a tetramer in the presence of FBP. When FBP was removed from the enzyme, it rapidly lost activity and was converted to a diamer. Addition of FBP back to the inactive diamer resulted in a restoration of activity that was accompanied by a return to the tetrameric form.
