Nicotinamide nucleotide transhydrogenase mitochondrial

Nicotinamide nucleotide transhydrogenases may be divided into two classes. One class is present in certain bacteria, and possibly in some plants, is an easily extractable, water-soluble enzyme is not functionally linked to the energy-transfer system of the bacterial membrane is a fiavoprotein and is specific for the 4B-hydrogen atom of both NADH and NADPH. The other class is present in both certain bacteria and in mitochondria is a firmly membrane-bound water-insoluble enzyme is functionally linked to the energy-transfer system of the bacterial or mitochondrial membrane is not known to be a flavoprotein and is specific for the 4A-hydrogen atom of NADH and the 4B-hydrogen atom of NADPH. For the sake of convenience, the two classes of enzyme will be referred to below as BB-specific and AB-specific transhydrogenases, respectively. 

Mitochondrial nicotinamide nucleotide transhydrogenase can be estimated in the intact organelle either by removing aliquots and determining the individual oxidized and reduced nicotinamide nucleotides. (90) or by measuring changes of the intrinsic absorption (90, 37) or fluorescence... 

Comparison of methods for the purification of mitochondrial nicotinamide nucleotide transhydrogenase from beef heart... 

Cleland (160), steady-state kinetics of a Theorell-Chance mechanism can generally apply also to a rapid-equilibrium random mechanism with two dead-end complexes. However, in view of the data obtained with site-specific inhibitors this latter mechanism is unlikely in the case of the transhydrogenase (70, 71). The proposed mechanism is also consistent with the observation of Fisher and Kaplan (118) that the breakage of the C-H bonds of the reduced nicotinamide nucleotides is not a rate-limiting step in the mitochondrial transhydrogenase reaction. 

The majority of synthetic reactions in mammalian cells takes place in the cytosol. The intramitochondrial localization of transhydrogenase excludes a direct participation in these anabolic processes. Substrate shuttle mechanisms (176, 177) are required to allow for the interaction between intra- and extramitochondrial nicotinamide nucleotide-dependent reactions. In the first instance transhydrogenase can be regarded to be functionally related to intramitochondrial NADP-linked reactions. A number of studies on isolated mitochondria have elaborated these relationships in some detail, in particular with regard to mitochondrial monooxygenation reactions and to the metabolism of glutamate and isocitrate. 

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