Grundy J E, Storey K B
Institute of Biochemistry, Carleton University, Ottawa, Ontario, Canada.
Mol Cell Biochem. 1994 Feb 9;131(1):9-17. doi: 10.1007/BF01075719.
The effects of urea, cations (K+,NH4,Na+,Cs+,Li+), and trimethylamines on the maximal activities and kinetic properties of pyruvate kinase (PK) and phosphofructokinase (PFK) from skeletal muscle were analyzed in two anuran amphibians, an estivating species, the spadefoot toad Scaphiopus couchii, and a semi-aquatic species, the leopard frog Rana pipiens. Urea, which accumulates naturally to levels of 200-300 mM during estivation in toads, had only minor effects on the Vmax, kinetic constants and pH curves of PK from either species and no effects on PFK Vmax or kinetic constants. Trimethylamine oxide neither affected enzyme activity directly or changed enzyme response to urea. By contrast, high KCl (200 mM) lowered the Vmax of toad PFK and of PK from both species and altered the Km values for both substrates of frog PFK. Other cations were even more inhibitory; for example, the Vmax of PK from either species was reduced by more than 80% by the addition of 200 mM NH4Cl, NaCl, CsCi, or LiCl. High KCl also significantly changed the Km values for substrates of toad lactate dehydrogenase and strongly reduced the Vmax of glutamate dehydrogenase and NAD-dependent isocitrate dehydrogenase in both species whereas 300 mM urea had relatively little effect on these enzymes. The perturbing effect of urea on enzymes and the counteracting effect of trimethylamines that has been reported for elasmobranch fishes (that maintain high concentrations of both solutes naturally) does not appear to apply to amphibian enzymes.(ABSTRACT TRUNCATED AT 250 WORDS)
在两种无尾两栖动物中，即处于夏眠状态的锄足蟾Scaphiopus couchii和半水生的豹蛙Rana pipiens，分析了尿素、阳离子（K⁺、NH₄⁺、Na⁺、Cs⁺、Li⁺）和三甲胺对骨骼肌丙酮酸激酶（PK）和磷酸果糖激酶（PFK）的最大活性及动力学特性的影响。尿素在蟾蜍夏眠期间自然积累至200 - 300 mM的水平，对这两种动物的PK的Vmax、动力学常数和pH曲线仅有轻微影响，对PFK的Vmax或动力学常数无影响。氧化三甲胺既不直接影响酶活性，也不改变酶对尿素的反应。相比之下，高浓度KCl（200 mM）降低了蟾蜍PFK以及两种动物PK的Vmax，并改变了青蛙PFK两种底物的Km值。其他阳离子的抑制作用更强；例如，添加200 mM NH₄Cl、NaCl、CsCl或LiCl可使两种动物PK的Vmax降低80%以上。高浓度KCl还显著改变了蟾蜍乳酸脱氢酶底物的Km值，并强烈降低了两种动物谷氨酸脱氢酶和NAD依赖的异柠檬酸脱氢酶的Vmax，而300 mM尿素对这些酶的影响相对较小。已报道的尿素对酶的干扰作用以及三甲胺的抵消作用（在板鳃鱼类中自然维持高浓度的这两种溶质）似乎不适用于两栖动物的酶。（摘要截选至250字）
