Fabregat I, Satrústegui J, Machado A
Arch Biochem Biophys. 1983 Feb 1;220(2):354-60. doi: 10.1016/0003-9861(83)90424-1.
Citrate synthase (EC 4.1.3.7) from Tetrahymena pyriformis has been purified 185-fold. The molecular weight of the native enzyme was determined to be 120,000. The enzyme is labile at low ionic strength, but can be stabilized by KCl and glycerol. It is activated by KCl at low (below 60 mM) or high concentrations, and inhibited by divalent cations (Mn2+, Mg2+, Ca2+). The Michaelis constants are 0.1 mM for oxalacetate and 0.01 mM for acetyl-CoA. The kinetics with oxalacetate exhibit negative cooperativity, with a nH = 0.66. Among the metabolites tested, only ATP and GTP can inhibit the enzyme but Mg2+ relieves the ATP inhibition. Incubation with sulfhydryl reagents (DTNB) in the absence of its substrates results in a rapid inactivation of the enzyme. It is concluded that Tetrahymena citrate synthase is closer to the enzyme from Gram-positive bacteria than to those of eucaryotes.
嗜热四膜虫的柠檬酸合酶（EC 4.1.3.7）已被纯化了185倍。天然酶的分子量测定为120,000。该酶在低离子强度下不稳定，但可被氯化钾和甘油稳定。它在低浓度（低于60 mM）或高浓度的氯化钾作用下被激活，并受到二价阳离子（锰离子、镁离子、钙离子）的抑制。草酰乙酸的米氏常数为0.1 mM，乙酰辅酶A的米氏常数为0.01 mM。草酰乙酸的动力学表现出负协同性，nH = 0.66。在所测试的代谢物中，只有ATP和GTP能抑制该酶，但镁离子可解除ATP的抑制作用。在没有底物的情况下与巯基试剂（DTNB）一起温育会导致酶迅速失活。得出的结论是，嗜热四膜虫柠檬酸合酶与革兰氏阳性菌的酶更接近，而与真核生物的酶不同。
