L-MYO-INOSITOL 1-PHOSPHATE SYNTHASE FROM PLANT SOURCES - CHARACTERISTICS OF THE CHLOROPLASTIC AND CYTOSOLIC ENZYMES

L-myo-inositol 1-phosphate synthase (EC 5.5.1.4) from cyanobacterial ( Spirulina platensis), algal (Euglena gracilis), and higher plant (Oryz a sativa, Vigna radiata) sources was purified to electrophoretic homog eneity, biochemically characterized, and compared. Both chloroplastic and cytosolic forms of the enzyme were detected in E. gracilis, O. sat iva, and V. radiata, whereas only the cytosolic form was detected in s treptomycin-bleached or chloroplastic mutants of E. gracilis and in S. platensis. Both the chloroplastic and cytosolic forms from different sources could be purified following the same three-step chromatographi c protocol. L-myo-inositol 1-phosphate synthases purified from these d ifferent sources do not differ significantly with respect to biochemic al and kinetic parameters except for the molecular mass of the chlorop lastic and cytosolic native holoenzymes, which appear to be homotetram eric and homotrimeric associations of their constituent subunits, resp ectively. Monovalent and divalent cations, sugar alcohols, and sugar p hosphates are inhibitory to the enzyme activity. N-ethylmaleimide inhi bition of synthase activity could be protected by the combined presenc e of the substrate glucose-6-phosphate and cofactor NAD(+). Antibody r aised against the cytosolic enzyme from E. gracilis immunoprecipitates and cross-reacts with both chloroplastic and cytosolic forms from the other sources studied.