The Role Of Glutamate Transport Systems In The Metabolism Of Glutamate By Rat - Liver Mitochondria

Abstract

INTRODUCTION The purpose of this project was to investigate the regulation of pathways of glutamate metabolism in isolated rat - liver mitochondria. Special emphasis was laid on the role of the two transport systems for glutamate In the inner - mitochondrial membrane namely: 1. + the glutamate - H symport system (alternatively ,described as glutamate - hydroxyl exchange diffusion carrier) which is linked to the deamination pathway of glutamate oxidation, leading to the formation of ammonia and 2. the electrogenic exchRnge of extramitochondrial glutamate (plus H+) for intramitochondrial aspartate which is linked to the transamination pathway of glutamate Qxidation, leading to the formation of aspartate (plus CO2), PART ONE Glutamate Metabolism A detailed study of the rate of formation of ammonla and aspartate from added glutamate under different metabolic conditions was done in order to confirm and extend the work of other investigators (Borst, 1962; De Haan, 1967). Effects of uncouplers, malate, Vitamin K3, inhibitors of transamination pathway (arsenite, malonate and aminooxyacetate) and inhibitor of glutamate dehydrogenase (glutarate) were investigated on the rate of formation of products of glutamate oxidation. The effect of different inhibitors of the + glutamate - H translocator (avenaciolide, bromocresolpurple and HEM) on the rate of glutamate oxidation was studied under different metabolic conditions, particularly in partially or fully uncoupled mitochondria or in the presence of vitamin K3. These experiments led to the following conclusions: 1. under many metabolic conditions the rate of ammonia formation from added glutamate (and thus presumably the rate of glutamate uptake V1a the glutamate - H+ translocator) 1S far in excess of the reported maximal activity of the glutamate - H+ translocator (Meyer and Vignais, 1973; Bradford and McGivan, 1973). 2. Except when the rate of ammonia formation was maximally stimulated~ the inhibitors of glutamate + - H symport system did not have any inhibitory - effect (unless very high concentrations were used) indicating that the capacity of the transport system exceeded the rate of glutamate oxidation. Further information was obtained from studies on the metabolism of intramitochondrially synthesized glutamate. from two systems.