THE MITOCHONDRIAL REDOX STATE OF RAT BRAIN

The use of the glutamate dehydrogenase (EC 1.4.1.3) and β‐hydroxybutyrate dehydrogenase (EC 1.1.1.30) reactions for the calculation of the mitochondrial redox state of brain has been examined. To prevent post‐mortem anoxic metabolism, brains were frozen in less than a second by using a new technique. Levels of ketone bodies in brain were so low relative to the contamination by blood and extracellular fluid that calculation of the mitochondrial redox state using the β‐hydroxybutyrate dehydrogenase reaction was not practical. The concentrations of the non‐nucleotide substrates of the glutamate dehydrogenase reaction could be accurately measured in brain and themitochondrial [NAD⁺]/[NADH] ratio calculated from the ratio [α‐oxoglutarate] [NH₄⁺]/[glutamate]. The calculation is valid if the ratio [α‐oxoglutarate] [NH₄⁺]/[glutamate] in mitochondria is the same as that measured in whole tissue. The evidence supporting this conclusion is the near‐equilibrium of the aspartate aminotransferase (EC 2.6.1.l) reaction in brain and the observation by others that the distribution of label between α‐oxoglutarate and glutamate in brain, after administration of labelled precursors, conforms to expectation. The alanine aminotransferase (EC 2.6.1.2) reaction was not near equilibrium in brain, probably because of the low in vivo activity of the enzyme.