Reductases lipoamide NADH reductase

Relative to the dithiol DTT but also to other monothiols such as 2-mercaptoeth-anol, GSH is a poor stimulator of microsomal deiodinase activity even when tested in the presence of NADPH and glutathione reductase [52,60,61]. Deiodinase activity of isolated microsomes is supported to a limited extent by GSH if tested with low (nM) but not high (/zM) rT3 concentrations or with T4 as the substrate. This low potency of GSH has led investigators to explore other physiological cofactors. As mentioned above, the paucity of cytoplasmic dihydrolipoamide makes it an unlikely candidate despite its unsurpassed potency [52], This is supported by the finding that addition of NADH, the cofactor for lipoamide hydrogenase, does not stimulate deiodinase activity of kidney homogenates unless supplemented with lipoamide [52]. 

Thioredoxin was shown to reduce the two interchain disulfides of insulin very efficiently around neutral pH and in the presence of either thioredoxin reductase and NADPH or lipoamide, lipoamide dehydrogenase and NADH [277,278], This reduction may be important in hormone action since the reduction of insulin disulfides is a prerequisite of proteolytic degradation of insulin. Thioredoxin has also been identified as the endogenous activator of the rat glucocorticoid receptor to a steroid-binding state [279]. Finally, recent data suggest that thioredoxin is secreted by immunocompetent cells and then behaves as an autocrine growth factor [280]. 

Lipoamide dehydrogenase (15, 16) and NADH-cytochrome 65 reductase (17) were covered in detail in the second edition of The Enzymes. The material in these chapters has stood the test of time remarkably well and will only need to be summarized here. 

The three enzymes are quite specific for their respective pyridine nucleotide substrates. Under conditions normally used for assay, lipoamide dehydrogenase is less than % as active with NADPH as with NADH IS) and thioredoxin reductase is less than 1% as active with NADH as with NADPH 36, Sff). Lipoamide dehydrogenase can transfer electrons to a number of NAD analogs 37). Yeast glutathione reductase is quite specific for NADPH 60), but the erythrocyte enzyme is 20% as active with NADH as with NADPH under the conditions of the standard assay 30,40,61). 

The importance of EH2 in catalysis by lipoamide dehydrogenase and glutathione reductase has been demonstrated by rapid reaction spectrophotometry. It is produced upon reduction with NADH or NADPH, respectively, in the dead time of the instrument (ca. 3 msec) and is rapidly reoxidized by lipoamide or glutathione at rates commensurate with catalysis (24, 50, 54) ... 

The stability of EH2 is very species dependent. All of the above results refer to the pig heart enzyme and, where tested, to other mammalian species. It was initially reported that no long wavelength absorption was observed upon reduction of E. coli enzyme with NADH 109), but reduction by 1 equivalent of NADH or dihydrolipoamide leads to the formation of 25% of the maximal 2-electron-reduced species 108) and similar results are obtained with the Azotobacter enzyme 114)- That this species is the catalytically important one in the E. coli enzyme as well as in the mammalian enzyme has also been demonstrated 50). Reduction with dihydrolipoamide in the rapid reaction spectrophotometer at 2° results in the full formation of EH2 followed by the slow k = 13 min, 1 mAf dihydrolipoamide) four-electron reduction. The spectrum of EHa generated in this way is shown in Fig. 7 and is identical with that of the pig heart enzyme. The 2-electron-reduced form, EHj of lipoamide dehydrogenase of spinach 99) may be somewhat unstable however, spectrally it is difficult to distinguish between instability and formation of the EHa-NADH complex (see above) on the basis of available spectral data. Either phenomenon could lead to inhibition by excess NADH. In glutathione reductase it is possible that the complex can be rapidly reoxidized by glutathione 53). 

D-lactate dehydrogenase and, 271 lipoamide dehydrogenase and, 126 NADH dehydrogenase and, 206 succinate dehydrogenase and, 247 thioredoxin reductase and, 147, 148 Ethyl hydrogen peroxide, catalase and, 391-392, 395 iV-Ethylmaleimide c> tochrome b, reductase and, 163,... 

Ru(II)tris(bipyridine) [Ru(bpy)3 +] as a photosensitizer, triammonium ethylene-diaminetetraacetic acid [(NH4)3EDTA] as a sacrificial electron donor and the enzyme ferredoxin NADP+ reductase (FDR) [215, 216]. Oxidative electron-transfer quenching of the excited Ru(bpy)3 + yields the A,A -dimethyl-4,4 -bipyridinium radical cation (reduced methylviologen, MV+), which mediates the reduction of NADP+ in the presence of FDR as a biocatalyst (Figure 32A). The quantum efficiency for NADH production corresponds to = 1.9 x 10 . A related system that includes Zn(II)wc50-(A-tetramethylpyridinium)porphyrin (Zn-TMPyP +) as a photosensitizer, mercaptoethanol as a sacrificial donor and lipoamide dehydrogenase (LipDH) as a biocatalyst has been applied for the photochemical reduction of NAD+ to NADH (Figure 32B). 

Lipoamide reductase (NADH). E3 component of alpha-ketoacid dehydrogenase complexes. Lipoyl dehydrogenase. Dihydrolipoyl dehydrogenase. 

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