AB-Specific Transhydrogenases

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. 

AB-Specific transhydrogenase was ori nally discovered in beef heart by Kaplan et al. SO) but was also found in kidney, liver, other muscle... 

Assay of AB-specific transhydrogenase is principally identical to that of BB-specific transhydrogenase (see Section II,C) i.e., with the natural nicotinamide nucleotides, an enzymic regenerating system is used to keep the concentration of one of the substrates constant (30, SB, 66-68, 66)... 

AB-Specific transhydrogenases are functionally coupled to the energy-transfer system of the membrane in which they are located. This coupling... 

Michaelis constants of the non-energy-linked beef heart transhydrogenase reaction are 9 ftM for NADH, 40 nM for NADP , 28 )tM for NAD , and 20 pM for NADPH (68) these values are similar to those found with other AB-specific transhydrogenases (66, 89 see also 8). Dissociation constants for the E-NADH and E-NAD complexes, derived from... 

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