PLANT PHYSIOLOGY , Vol 115, Issue 2 727-736, Copyright © 1997 by American Society of Plant Biologists
|
BIOCHEMISTRY AND ENZYMOLOGY |
L-myo-lnositol 1-Phosphate Synthase from Plant Sources (Characteristics of the Chloroplastic and Cytosolic Enzymes)
A. RayChaudhuri, N. C. Hait, S. Dasgupta, T. J. Bhaduri, R. Deb and A. L. Majumder
Biochemistry Laboratory, Department of Botany, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Calcutta 700009, India
L-myo-inositol 1-phosphate synthase (EC 5.5.1.4) from cyanobacterial
(Spirulina platensis), algal (Euglena gracilis), and higher plant (Oryza
sativa, Vigna radiata) sources was purified to electrophoretic homogeneity,
biochemically characterized, and compared. Both chloroplastic and cytosolic
forms of the enzyme were detected in E. gracilis, O. sativa, and V.
radiata, whereas only the cytosolic form was detected in
streptomycin-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 chromatographic
protocol. L-myo-inositol 1-phosphate synthases purified from these
different sources do not differ significantly with respect to biochemical
and kinetic parameters except for the molecular mass of the chloroplastic
and cytosolic native holoenzymes, which appear to be homotetrameric and
homotrimeric associations of their constituent subunits, respectively.
Monovalent and divalent cations, sugar alcohols, and sugar phosphates are
inhibitory to the enzyme activity. N-ethylmaleimide inhibition of synthase
activity could be protected by the combined presence of the substrate
glucose-6-phosphate and cofactor NAD+. Antibody raised against the
cytosolic enzyme from E. gracilis immunoprecipitates and cross-reacts with
both chloroplastic and cytosolic forms from the other sources studied.