Guynn R W, Veloso D, Lawson J W, Veech R L
Biochem J. 1974 Jun;140(3):369-75. doi: 10.1042/bj1400369.
The concentration of cytoplasmic free pyrophosphate was calculated in freeze-clamped livers of rats from the measured concentration of reactants and K(eq.) of the UDP-glucose pyrophosphorylase reaction (UDP-alpha-d-glucose 1-phosphate uridylyltransferase, EC 2.7.7.9). The K(eq.) of the UDP-glucose pyrophosphorylase reaction was redetermined at 38 degrees C, pH7.0, I=0.25mol/l and free [Mg(2+)]=1mm, and was 4.55 in the direction of glucose 1-phosphate formation. The activity of UDP-glucose pyrophosphorylase in rat liver was between 46 and 58mumol of glucose 1-phosphate formed/min per g fresh wt. in the four dietary conditions studied. A fluorimetric assay with enzymic cycling was developed for the measurement of glucose 1-phosphate in HClO(4) extracts of rat liver. The calculated free cytoplasmic PP(i) concentration in nmol/g fresh wt. of liver was 2.3+/-0.3 in starved, 3.8+/-0.4 in fed, 4.9+/-0.6 in meal-fed and 5.2+/-0.4 in sucrose-re-fed animals. These values agree well with recently determined direct measurements of total PP(i) in rat liver and suggest that there is not a large amount of bound or metabolically inert PP(i) in rat liver. The cytoplasmic [ATP]/[AMP][PP(i)] ratio is 10(3) times the cytoplasmic [ATP]/[ADP][P(i)] ratio and varies differently with dietary state. The reaction PP(i)+H(2)O-->2P(i) catalysed by inorganic pyrophosphatase (EC 3.6.1.1) does not attain near-equilibrium in vivo. PP(i) should be considered as one of the group of small inorganic ions which is metabolically active and capable of exerting a controlling function in a number of important metabolic reactions.
根据所测反应物浓度和UDP-葡萄糖焦磷酸化酶反应（UDP-α-D-葡萄糖1-磷酸尿苷酰转移酶，EC 2.7.7.9）的平衡常数（K(eq.)），计算了大鼠冷冻钳夹肝脏中细胞质游离焦磷酸的浓度。在38℃、pH7.0、I = 0.25mol/L和游离[Mg(2+)] = 1mmol的条件下，重新测定了UDP-葡萄糖焦磷酸化酶反应的K(eq.)，在葡萄糖1-磷酸形成方向上为4.55。在所研究的四种饮食条件下，大鼠肝脏中UDP-葡萄糖焦磷酸化酶的活性为每克新鲜重量每分钟形成46至58μmol葡萄糖1-磷酸。开发了一种酶循环荧光测定法，用于测量大鼠肝脏高氯酸提取物中的葡萄糖1-磷酸。计算得出的肝脏每克新鲜重量中游离细胞质PP(i)的浓度（以nmol计）在饥饿动物中为2.3±0.3，在喂食动物中为3.8±0.4，在进食后动物中为4.9±0.6，在蔗糖再喂食动物中为5.2±0.4。这些值与最近对大鼠肝脏中总PP(i)的直接测量结果非常吻合，表明大鼠肝脏中不存在大量结合或代谢惰性的PP(i)。细胞质[ATP]/[AMP][PP(i)]比值是细胞质[ATP]/[ADP][P(i)]比值的10³倍，并且随饮食状态的变化而不同。由无机焦磷酸酶（EC 3.6.1.1）催化的反应PP(i)+H₂O→2P(i)在体内未达到接近平衡的状态。PP(i)应被视为一组具有代谢活性且能够在许多重要代谢反应中发挥控制作用的小无机离子之一。
