Fritz S, Bohnensack R
Institut für Biochemie, Medizinische Akademie Magdeburg, GDR.
Biomed Biochim Acta. 1988;47(12):923-32.
The effect of ammonia on the alanine metabolism was investigated in perfused rat liver. Gluconeogenesis was found to be stimulated by physiological concentrations of ammonia, while being inhibited at higher concentrations (5-10 mM). The stimulating effect of 0.5 mM ammonia was studied in greater detail. In addition to glucose formation seen enhanced five times, increased rates were observed for ureogenesis as well as the formation of lactate and pyruvate, demonstrating also activation of the total alanine turnover. Furthermore, the mitochondrial and cytosolic NAD systems were increasingly oxidized as reflected by the beta-hydroxybutyrate/acetoacetate and lactate/pyruvate ratios. The shift of the beta-hydroxybutyrate/acetoacetate ratio was correlated to the ATP demand by gluconeogenesis and ureogenesis. The elevated concentration of pyruvate was found to have caused stimulation of gluconeogenesis since there existed a Michaelis-Menten type relation between pyruvate concentration and glucose formation irrespective of the presence or absence of ammonia. The flux through glutamate dehydrogenase was calculated from the total alanine turnover and urea formation, and noted to be diminished in the presence of ammonia despite the increased alanine turnover. It is concluded that glutamate dehydrogenase, at least in part, controls the total alanine turnover in the absence of ammonia.
在灌注大鼠肝脏中研究了氨对丙氨酸代谢的影响。发现生理浓度的氨刺激糖异生，而在较高浓度（5 - 10 mM）时则抑制糖异生。对0.5 mM氨的刺激作用进行了更详细的研究。除了葡萄糖生成增加了五倍外，还观察到尿素生成以及乳酸和丙酮酸生成的速率增加，这也表明丙氨酸总周转被激活。此外，线粒体和胞质NAD系统越来越多地被氧化，这通过β-羟基丁酸酯/乙酰乙酸酯和乳酸/丙酮酸比值反映出来。β-羟基丁酸酯/乙酰乙酸酯比值的变化与糖异生和尿素生成对ATP的需求相关。发现丙酮酸浓度升高导致了糖异生的刺激，因为无论有无氨存在，丙酮酸浓度与葡萄糖生成之间都存在米氏类型关系。通过丙氨酸总周转和尿素生成计算谷氨酸脱氢酶的通量，并且注意到尽管丙氨酸周转增加，但在有氨存在时通量减少。得出的结论是，谷氨酸脱氢酶至少在一定程度上在无氨情况下控制丙氨酸总周转。
