STUDIES ON SUCROSE-PHOSPHATE SYNTHASE FROM RICE LEAVES

Sucrose-phosphate synthase (SPS, EC 2.4.1.14) biochemical properties a nd peptide composition have been analyzed in rice leaf seedlings. SPS was purified using DEAE-Sephacel chromatography, gel filtration on Sep harose 6B and anion exhange chromatography on Mono Q. At this stage tw o enzyme forms (SPS-I and -II) were separated. SPS-II was purifed 90-f old; however, SPS-I presented a lower specific activity regarding the previous purification step and an unstable activity. Both enzyme forms had similar apparent K-m values for Fru-6P but the SPS-I K-m for UDP- Glc was ca. 10-fold higher than the SPS-II one. In addition, they diff erenciate in the capacity of being modulated by Glc-6-P and Pi: while SPS-II activity was inhibited by Pi and activated by Glc-6-P, SPS-I wa s not affected by either effecters. A native molecular mass of ca. 420 kDa was found by gel filtration. In SPS expression analysis using lea f rice and wheat germ SPS antibodies, a 116 kDa polypeptide was reveal ed in rice leaf extracts and no polypeptide was immunoactive in rice r oots.