Abstract: | Fructose 1,6-bisphosphatase (α-D-fructose 1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11; FBPase) from immature wheat endosperm has been resolved into two forms, FBPase-I and FBPase-II. Their specific activities over crude homogenate increased 47- and 77-fold, respectively, by using ammonium sulfate fractionation, DEAE-cellulose chromatography and gel filtration through Sephadex G-200. The pH optimum was 7.6 for FBPase-I and 8.4 for FBPase-II. The two forms were highly specific for the substrate FBP with Km values of 0.17 and 0.08 m M , respectively, for FBPase-I and FBPase-II at their respective pH optimum and saturating Mg2+ concentration. pH had no effect on the Km value for FBPase-I, but that for FBPase-II increased below optimum pH. Neither of the forms had an absolute requirement for Mg2+, although it was essential for maximum activity. Mg2+ could not be replaced by Cu2+, Ca2+, Ba2+, Co2+ or Ni2+. Sulfhydryl reagents inactivated both FBPase-I and FBPase-II. Of the metabolites, only 6-phosphogluconate was inhibitory with 50% inhibition at 2 and 4 m M for FBPase-I and FBPase-II, respectively. |