Control of glutamate dehydrogenase activity during glutamate oxidation in isolated rat-liver mitochondria

Abstract

1. The kinetics of the reaction of glutamate dehydrogenase with the intramitochondrial nicotinamide nucleotides has been followed in isolated rat-liver mitochondria preincubated with phosphate and phosphate acceptor in order largely to oxidize intramitochondrial nicotinamide nucleotides. In the presence of rotenone and arsenite, the oxidation of glutamate to α-oxoglutarate is accompanied by extensive reduction of NADP + and very little reduction of NAD +, and ceases when NADP is maximally reduced, even though the level of NAD + is still quite high. 2. In mitochondria preincubated in the presence of ATP, oligomycin or rotenone in order largely to reduce intramitochondrial nicotinamide nucleotides, NADPH was oxidized much more rapidly than NADH on the addition of ammonia (α-oxoglutarate was already present). 3. On addition of glutamate to mitochondria preincubated as in 1, a small amount of ammonia is formed and some α-oxoglutarate accumulates in the first few minutes of incubation, during which NADP + is rapidly reduced and NAD + slowly reduced. Aspartate is not formed in the first few seconds. α-Oxoglutarate formation, on the other hand, is greater during the first few seconds than in the subsequent 2 min. 4. When malonate is added together with glutamate, there is a marked deamination of glutamate and only a partial reduction of NADP +. 5. 2-Methyl-1,4-naphthoquinone stimulates the deamination of glutamate markedly both in the absence and presence of malonate, and brings about an oxidation of NADPH. 6. It is concluded that the most important factor controlling the activity of glutamate dehydrogenase during glutamate oxidation in isolated rat-liver mitochondria is the oxidoreduction state of NADP.