Site‐Specific Inhibitors of Mitochondrial Nicotinamide‐Nucleotide Transhydrogenase

1 The mitochondrial nicotinamide nucleotide transhydrogenase is reversibly inhibited in a site-specific manner by adenosine derivatives both in the presence and in the absence of an energy supply. 2′- or 3′-phosphoadenosine derivatives are competitive with respect to NADP(H), whereas adenosine derivatives with nonesterified 2′- or 3′-carbon atoms are competitive with respect to NAD(H). NADP(H)-specific inhibitors with an additional hydrophobia ligand, such as palmityl-CoA, are particularly potent inhibitors. Since an increased hydrophobicity of the NADP(H)-specific inhibitors, in contrast to the NAD(H)-specific inhibitors, is parallelled by an increased efficiency, it is suggested that the environment of the NADP(H) site of the enzyme is more hydrophobic than that of the NAD(H) site. 2 3′-NADP(H) does not react with the mitochondrial transhydrogenase in contrast to enzymes (including nicotinamide nucleotide-unspecific dehydrogenases as well as certain bacterial transhydrogenases) where NAD(H) and NADP(H) probably react with the same binding-site. 3 From kinetic data obtained with site-specific inhibitors of the mitochondrial nicotinamide nucleotide transhydrogenase it is concluded that NAD(H) rather than NADP(H) is the first substrate bound to the enzyme. 4 The effect of palmityl-CoA on the mitochondrial transhydrogenase reaction suggests a role of this reaction in the regulation of fatty acid and lipid metabolism. Recent findings concerning similar effects on other enzymes are discussed.