Genes and Physiological Role of the Eukaryotic Nicotinamide Nucleotide Transhydrogenase
Nicotinamide nucleotide transhydrogenase (NNT, EC 18.104.22.168) is a proton pump which catalyses the reversible reduction of NADP+ by NADH linked to proton translocation across the membrane, according to the reaction:
H+out + NADP+ + NADH <-> H+in + NAD+ + NADPH
In higher eukaryotes the enzyme is located in the inner mitochondrial membrane and is thus a possible provider of mitochondrial NADPH. NADPH is, among other things, important in the mitochondrial defence against oxidative damage. Reactive oxygen species and the damage they do to biological systems have been implicated in neurodegenerative diseases and ageing.
In the present work the NNT genes from mouse, man and a green algae have been cloned and characterised, showing that the mitochondrial NNT genes have the same structure. The expression pattern in mouse tissues and subsections of human brain has been studied, as well as the expression pattern of Green Fluorescent Protein under control of the NNT promoter in the nematode Caenorhabditis elegans. The results show a tissue and cell-specific expression and, interestingly, a high expression in certain nematode neurons. A C. elegans mutant, lacking functional NNT enzyme, has been isolated and shown to grow slower than wild-type worms in the presence of the superoxide anion generator methyl viologen, providing evidence for the importance of the NNT enzyme as a supplier of mitochondrial NADPH for detoxification, e.g. of peroxides.