Effect of Phosphorus Deficiency on RuBP Carboxylase-Oxygenase Activity in Rice Plants

[摘要] In the previous paper5), phosphorus deficiency in rice plant caused the specific decrease of photorespiratory activity both in gas exchange (P3/P21) and enzyme activities (ratio of RuBP oxygenase to carboxylase). To elucidate the mechanism of this phenomenon, RuBP carboxylase-oxygenase from control and phosphorus deficient plants were purified and investigated. Fresh leaves were ground in a mortar and the extracts were precipitated with ammonium sulfate (30 to 50% saturation). The precipitate was applied to a sepharose 6B column and the eluted peak fractions with RuBP carboxylase activity were stored at -20°C for the subsquent assay. RuBP carboxylase was assayed by measuring the rate of 14C incorporation into acid-stable products in the reaction mixture containing 100 mM BICINE (pH 8.2), 1 mM DTT, 20 mM MgCl2, 40 mM NaH14CO3(0.25 μCi/μmole), 1 mM RuBP and the enzyme. RuBP oxygenase was assayed by an oxygen electrode. Reaction mixture was as the same as carboxylase with the exception of NaH14CO3 depletion. Both carboxylase and oxygenase assay were initiated at 30°C by addition of the enzyme previously activated with MgCl2 and NaHCO3. The results obtained were as follows: 1. The frozen enzyme was fully activated by preincubation for more than 20 minutes at 30°C in the presence of MgCl2 and NaHCO3. High temperature (40 and 50°C) inactivated the enzyme (Fig. 1). 2. The mode of enzyme activation with MgCl2 and NaHCO3 was quite similar in control and phosphorus deficient plants (Fig. 3). 3. The pH curve of the enzyme reaction from phosphorus deficient plant was identical to that from control plant in both carboxylase and oxygenase assay. As the results, the ratio of oxygenase to carboxylase showed approximately the same value with each other at any pH (Fig. 4). 4. The Km(CO2) of carboxylase from both control and phosphorus deficient plants showed almost the same value in various O2 concentration of gas phase. Vmax was also identical with each other (Fig. 5). From these results, it was suspected that the decrease of the photo-respiratory activity observed in the phosphorus deficient rice leaves didn't reflect the varied nature of RuBP carboxylase-oxygenase. The environment surrounding the enzyme seemed to be responsible for the variation.

[学科分类] 农业科学（综合）