Studies on the control of 4-aminobutyrate metabolism in ‘synaptosomal’ and free rat brain mitochondria

Abstract

The specific activities of 4-aminobutyrate aminotransferase (EC 2.6.1.19) and succinate semialdehyde dehydrogenase (EC 1.2.1.16) were significantly higher in brain mitochondria of non-synaptic origin (fraction M) than those dierived from the lysis of synaptosomes (fraction SM2). The metabolism of 4-aminobutyrate in both free (non-synaptic, fraction M) and synaptic (fraction SM2) rat brain mitochondria was studied under various conditions. 4-Aminobutyrate may enter both types of brain mitochondria by a non-carrier-mediated process. The rate of 4-aminobutyrate metabolism was in all cases higher in the free (fraction M) brain mitochondria than in the synaptic (fraction SM2) mitochondria, paralleling the differences in the specific activities of the 4-aminobutyrate-shunt enzymes. The intramitochondrial concentration of 2-oxoglutarate appears to be an important controlling parameter in the rate of 4-aminobutyrate metabolism, since, although 2-oxoglutarate was required, high concentrations (2.5 mM) of extramitochondrial 2-oxoglutarate inhibited the formation of aspartate via the glutamate-oxaloacetate transaminase. The redox state of the intramitochondrial NAD pool was also important in the control of 4-aminobutyrate metabolism; NADH exhibited competitive inhibition of 4-aminobutyrate metabolism by both mitochondrial populations with an apparent Ki of 102 .mu.M. Increased K+ concentrations stimulated 4-aminobutyrate metabolism in the synaptic mitochondria but not in free brain mitochondria. This was discussed with respect to the putative transmitter role of 4-aminobutyrate.