菠菜叶片蔗糖磷酸合成酶的调节

电泳均一的菠菜叶片蔗糖磷酸合成酶的活力受G6P,Mg~(2 ),Mn~(2 )的调节;G6P对此酶的促进作用在F6P浓度较低时表现得比较明显;此酶对Mn~(2 )较对Mg~(2 )敏感,Mg~(2 ),Mn~(2 )对此酶的促进作用可被EDTA解除。底物F6P的饱和曲线为S型,底物UDPG的饱和曲线为双曲线型。NADP是此酶的负效应剂,NADP对F6P表现为混合型抑制,使V_m(F6P)降低和K_m(F6P)增大,3mmol/L NADP使F6P的K_m值从2.5mmol/L上升至3.8mmol/L,但不影响希尔系数,n=1.3。NADP对UDPG表现为K_m不变的非竞争性抑制,K_m(UDPG)=3.8mmol/L。

Regulation of Spinach Leaf Sucrose Phosphate Synthetase

Enzyme preparation homogenous as examined by polyacrylamide gel electrophoresis was used. The results confirmed that at subsaturation concentration of F6P, G6P had a stimulatory effect (Fig. 1) on sucrose phosphate synthetase (EC 2.4.1.14). Both Mg~(2 ) and Mn~(2 ) activated sucrose phosphate synthetase (Fig. 2), while EDTA antagonized this activation (Fig. 3). We found that NADP inhibited the activity of sucrose phosphate synthetase. The saturation curve of F6P was shown in Fig. 4A. In the presence of NADP, V_m (F6P) decreased and K_m (F6P) increased from 2.5 to 3.8 mmol/L (Fig. 4B), but the Hill coefficient remained unchanged (n=1.3) (Fig. 4A). The saturation curve of UDPG was hyperbolic in shape (Fig. 5A). Inhibition of NADP was noncompetitive with UDPG (Fig. 5B). Concentration of UDPG for 50% maximal activity was 3.8 mmol/L. The results suggested that the activity of spinach leaf SPS was modulated by the concentration of NADP and other metabolites.