Two forms of fructose 1,6‐bisphosphatase from immature wheat endosperm

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 mM, 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 mM for FBPase‐I and FBPase‐II, respectively.