Britain Studies on the Control of 4 - Aminobutyrate Meftbolism in ' Synaptosomal ' and Free Rat Brain Mitochondria

1. The specific activities of4aminobutyrate aminotransferase (EC 2.6.1.19) and succinate semialdehyde dehydrogenase (EC 12.1.16) were significantly higher in brain mitochondria of non-synaptic origin (fraction M) than those derived from the lysis ofsynaptosomes (fraction SM2). 2. The metabolism of4-aminobutyrate in both 'free' (non-synaptic, fraction M) and 'synaptic' (fraction SM2) rat brain mitochondria was studied under various conditions. 3. It is proposed that 4-aminobutyrate enters both types of brain mitochondria by a non-carrier-mediated process. 4. 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. 5. The initramitochondrial concentration of 2-oxoglutarate appears to be an important controlling parameter in the rate of 4-amninobutyrate metabolism, since, although 2-oxoglutarate is required, high concentrations (2.5mM) of extramitochondrial 2-oxoglutarate inhibit the formation of aspartate via the glutamate-oxaloacetate transaminase. 6. The redox state ofthe intramitochondrial NAD pool is also important in the control of 4-aminobutyrate metabolism; NADH exhibits competitive inhibition of 4-aminobutytate metabolism by both mitochondrial populations with an apparent K1 of 102 gM. 7. Increased potassium concentrations stimnulate 4-aminobutyrate metabolism in the synaptic mitochondria but not in 'free' brain mitochondria. This is discussed with respect to the putative transmitterrole of4-aminobutyrate.