Glutamate oxidation in liver mitochondria of rats fed on pyridoxine-deficient diet.

Two pathways are known to exist for glutamate oxidation in liver mito chondria. One is by way of glutamate dehydrogenase [EC 1.4.1.3.] reaction and the other by aspartate transaminase [EC 2.6.1.1.] reaction. Although both enzyme activities are high in mitochondrial fraction, it is known that glutamate is pre ferentially oxidized through the transaminase pathway in isolated intact mito chondria (1-5). Thus glutamate added to the mitochondrial system is stoichio metrically converted to aspartate and the amino group of aspartate formed is excreted in urine as an amino nitrogen of urea molecule. On the other hand, glutamate dehydrogenase reaction is far inclined to the formation of glutamate as judged by its equilibrium constant. In the present work, we studied the path way of glutamate oxidation in the liver mitochondria of rats subjected to a severe pyridoxine deficiency. The mitochondrial respiration was set in the state 3 where the respiration is most active. This condition should be the most favourable con dition to oxidize glutamate by means of glutamate dehydrogenase (6, 7). Wistar strain rats weighing about 50g were fed on a 70% casein diet for about one month. Dietary compositions and other feeding conditions were described previously (8). The preparation of liver mitochondria and the composition of reaction medium followed the method of HAGIHARA et al. (9). Aspartate trans aminase and glutamate dehydrogenase activities were determined by the method of KATUNUMA et al. (10) and STRECKER (11), respectively. As shown in Table 1, aspartate transaminase activity was decreased in the pyridoxine deficient group to 42% of the normal group. On the other hand, glutamate dehydrogenase activity was somewhat higher in the deficient group in terms of specific activity. But the significance of this finding should be viewed with caution because the amount of mitochondria which reflects the total activity of glutamate dehydro genase is not quantitated in both groups. TAKAMI et al. (12) have reported that glutamate dehydrogenase activity was not affected by pyridoxine deficiency, although their feeding conditions for rats were different from ours. Glutamate oxidation in isolated mitochondria was studied in state 3 in the presence of glu cose and hexokinase. After deproteinization, glutamate and aspartate were