TITLE	Biochemical characterization of rice sucrose phosphate synthase under illumination and osmotic stress
AUTHOR	Mu-Ho Lee Institute of Microbiology and Biochemistry Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei 106, TAIWAN Chien-Chi Yang Institute of Microbiology and Biochemistry Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei 106, TAIWAN Jong-Ching Su Institute of Microbiology and Biochemistry Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei 106, TAIWAN Ping-Du Lee* Institute of Microbiology and Biochemistry Department of Biochemical Science and Technology, College of Life Science, National Taiwan University, Taipei 106, TAIWAN
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ABSTRACT	Sucrose phosphate synthase (SPS) is one of a number of sucrose-metabolizing enzymes that regulates the sucrose synthesis pathway. SPSs were purified from etiolated rice seedlings (ERS), green rice seedlings (GRS), rice grain suspension cells under osmotic stress (RGSO), and rice grain suspension cells under illumination (RGSI). A native molecular mass of ca. 420 and 520 kDa was found using native-PAGE. The SDS-PAGE analyses revealed SPSs to be homotetramers composed of subunits with a mass of 116-120 kDa. The maximum activity for SPSs was observed on the third day. As far as their biochemical characterization was concerned, the optimum pH of the enzyme reactions lay generally between 6-8, the optimum temperatures between 35-40°C. The ERS and RGSO SPS Km values for Fru 6-P and UDPG were 1.8 and 35 mM, respectively. However, the GRS and RGSI SPS had similar Km values for Fru 6-P and UDPG of 1.5 and 28 mM, respectively. GRS and RGSI SPS activities were allosterically regulated by Glc 6-P (activator) or Pi (inhibitor), but ERS and RGSO SPS had no effect. From their regulations and Km values two enzyme forms (SPS-I and SPS-II) could be discriminated in the rice. SPS-II was induced by illumination, but SPS-I by osmotic stress. All SPSs were activated by Mg2+. The nucleotides AMP, ADP, ATP, UMP, UDP, GDP and UTP inhibited enzyme activity by about 25-50%. Thiol reagents became sensitized to the enzyme activity, but could be restored with DTT or b-ME. Glucose, galactose, glucosamine, maltose, and lactose activated the enzymes and were inhibited by d-gluconolactone and mannose. SPSs were also inhibited by PCMBS, cibacron blue F3G-A, and DEP.
KEYWORD	Etiolated rice seedlings; Green rice seedlings; Illumination; Isoforms; Osmotic stress; Rice grain suspension cells; Sucrose phosphate synthase;
ARTICLE INFO	Botanical Bulletin of Academia Sinica, Volume 46 Number 1 January 2005, page 43-52, 10 pages
PUBLISHER	Institute of Plant and Microbial Biology, Academia Sinica, Taipei, Taiwan, Republic of China