Veech R L, Lawson J W, Cornell N W, Krebs H A
J Biol Chem. 1979 Jul 25;254(14):6538-47.
The tissue contents of the reactants of the myokinase (EC 2.7.4.3) and the combined glyceraldehyde-3-phophate dehydrogenase (EC 1.1.1.29)-3-phosphoglycerate kinase (EC 2.7.2.3) reactions were measured in rapidly inactivated samples of human blood and rat brain, muscle, and liver. The tissue contents of the reactants of the creatine kinase (EC 2.7.3.2) reaction were measured in rat brain and muscle. In vitro the value of the expression: KG+G = [sigma3PG] . [sigmaATP] . [sigmalactate] KLDH = [sigmaHAP]/22] . [sigmaADP][sigmaPi] . [sigmaRUVATE] (1) was found to be 0.725 x 10(7) M-1 at I = 0.25, T = 38 degrees C, and free [Mg2+] = 0.15 mM and the value measured in vivo in red cell was 0.699 x 10(7) M-1. The value of the expression KMYK = ([sigma ATP] [sigma AMP]/[ADP2]) measured under the above conditions and at pH 7.2 was found to be 0.744 while the value found in red cell was 0.784 +/- 0.037. These reactions, therefore, appear to be in a state of near-equilibrium in the red cell and the measured tissue contents of ATP and ADP, which are common reactants in both reactions, approximate closely the activity of these reactants in vivo. In brain and muscle, the value of KG + G/KLDH calculated from the measured tissue contents of the reactants was a factor of 20 or more lower than that expected at equilibrium as was the measured value of the expression: KCK = [sigma ATP] [sigma creatine] divided by [sigma ADP] [sigma creatine-P] [H+] (2) Substitution of calculated free [sigma ADP] values in the expression of KG + G/KLDH gave values of 0.83 +/- 0.19 x 10(7) M-1 for brain and muscle, respectively, which agreed well with the value of 1.65 x 10(7) M-1 measured in vitro at I = 0.25, free [Mg2+] = 1 mM, T = 38 degrees C. This agreement between two highly active enzyme systems in the same compartment is taken as evidence of the existence of near-equilibrium in both these systems and suggests that free cytosolic [sigma ADP] is probably 20-fold lower than measured cell ADP content in mitochondrial-containing tissues.
在人血以及大鼠的脑、肌肉和肝脏的快速灭活样本中，测定了肌激酶（EC 2.7.4.3）以及联合的甘油醛-3-磷酸脱氢酶（EC 1.1.1.29）-3-磷酸甘油酸激酶（EC 2.7.2.3）反应的反应物的组织含量。在大鼠的脑和肌肉中，测定了肌酸激酶（EC 2.7.3.2）反应的反应物的组织含量。在体外，发现在离子强度I = 0.25、温度T = 38℃且游离[Mg2+] = 0.15 mM的条件下，表达式KG+G = [sigma3PG]·[sigmaATP]·[sigmalactate] / KLDH = [sigmaHAP]/22]·[sigmaADP][sigmaPi]·[sigmaRUVATE]（1）的值为0.725×10⁷ M⁻¹，而在红细胞中体内测得的值为0.699×10⁷ M⁻¹。在上述条件以及pH 7.2下测得的表达式KMYK = ([sigma ATP] [sigma AMP]/[ADP²])的值为0.744，而在红细胞中测得的值为0.784±0.037。因此这些反应在红细胞中似乎处于接近平衡的状态，并且在这两个反应中作为共同反应物的ATP和ADP的测得组织含量，与这些反应物在体内的活性非常接近。在脑和肌肉中，根据测得的反应物组织含量计算得到的KG + G/KLDH值比平衡时预期的值低20倍或更多，表达式KCK = [sigma ATP] [sigma肌酸] / [sigma ADP] [sigma磷酸肌酸] [H⁺]（2）的测得值也是如此。将计算得到的游离[sigma ADP]值代入KG + G/KLDH表达式中，分别得到脑和肌肉的值为0.83±0.19×10⁷ M⁻¹，这与在离子强度I = 0.25、游离[Mg2+] = 1 mM、温度T = 38℃的体外测得值1.65×10⁷ M⁻¹非常吻合。同一区室中两个高活性酶系统之间的这种吻合被视为这两个系统中存在接近平衡的证据，并且表明在含有线粒体的组织中，游离胞质[sigma ADP]可能比测得的细胞ADP含量低20倍。
