Nutritional and Physiological Regulation of Serine and Threonine Metabolism in Rat Liver

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Serine dehydratase (SDH) was induced in liver when rats were fed on a complete synthetic diet containing serine as the sole source of non-essential amino nitrogen (serine-diet). The induction was specific for serine among non-essential amino acids, and the rate of increase in activity was parallel with the content of serine in the diet when a part of the amino acid was replaced by isonitrogenous ammonium citrate. The induction was inhibited by intraperitoneal injection of cycloheximide. By means of pulse-labeling technique with 14C-amino acids and specific precipitation of the enzyme protein with anti-serine dehydratase serum, it was confirmed that the induction of serine dehydratase was due to net synthesis of the enzyme protein. Adrenalectomy did not affect the substrate induction of serine dehydratase. Urea concentrations in liver and plasma were lower in normal rats fed on serine-diet in comparison with those in alloxan-diabetic rats fed on the same diet, whereas serine concentrations were higher in the normal rats in comparison with those in the alloxan-diabetic rats. The serine content was especially high in liver (10-2M) when serine diet was given. This reciprocal relationship between urea and serine concentrations suggests that the induction of serine dehydratase by the substrate is based on the requirement of amino nitrogen for growth of rats, whereas the induction of the enzyme in an alloxan-diabetic state is based on the requirement of the carbon moiety of the amino acid for gluconeogenesis, resulting in excretion of the nitrogen in the form of urea. Greenberg et al. reported that L-threonine aldolase (TA) and allothreonine aldolase (allo-TA) represent one and the same enzyme in rat liver. It is suggested, however, from the following findings that both enzymes are different eachother: 1) The TA activity was remarkably increased in the state of the increased gluconeogensis such as feeding of a high protein diet or alloxan-diabetes, whereas the allo-TA activity was not changed under the same condition. 2) The TA activity was increased in parallel with the age, whereas the allo-TA was not changed. 3) The highest activity was found in 33-45% saturated fraction in case of TA, but in 45-55% saturated fraction in case of allo-TA, when both of the enzymes were fractionated with ammonium sulfate. They were partially purified by the procedures including heat treatment, ammonium sulfate fractionation and DEAE-cellulose column chromatography. They contained no SDH (Threonine dehydratase was shown to be the same enzyme as SDH.). Serine was shown to be a strong competitive inhibitor for both of the enzymes. The data suggest that TA does not work in the presence of 10-2M serine in liver, otherwise under the condition that the substrate induction of SDH takes place. When the rats were maintained at 5±1°C, gluconeogenetic key enzymes such as SDH and phosphoenolpyruvate carboxykinase were markedly enhanced. Fructose 1, 6-diphosphatase and glucose 6-phosphatase were also approximately 2-fold activated. Reciprocally, glycolytic key enzyme such as pyruvate kinase was repressed to half the controllevel (25±1°C). This enzyme pattern suggests that the enhanced gluconeogenesis in liver meets the thermogenesis responding to cold exposure. This phenomenon was seen only in the rats fed on serineand threonine-diet but not in the animals fed on the synthetic diet containing ammonium citrate as the sole source of non-essential amino nitrogen. The problem of whether serine or threonine plays a specific role in the gluconeogenesis in case of cold exposure is now under investigation.
Japan Society of Clinical Chemistryの論文

Nutritional and Physiological Regulation of Serine and Threonine Metabolism in Rat Liver

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