Flavin adenine dinucleotide Subject

The catabolic pathway of vitamin Bg (1) is probably the best studied. In animals (including humans) 4-pyridoxic acid (4) is the primary catabolic product of vitamin Bg, found in urine. It is formed by the oxidation of pyridoxal (2) by a nonspecific flavin adenine dinucleotide (FAD)-dependent aldehyde oxidase. The catalytically active form of vitamin Bg, pyridoxal-5 -phosphate, PLP (17), does not undergo similar oxidation to form 4. The various forms of vitamin Bg (1, 2, 15) and their respective phosphate esters (16, 17) are readily enzymatically interconvertible. Further degradation of 4 is unlikely in humans as the subjects administered with doses of 4 were found to excrete it quantitatively. Estimation of 4 in urine serves as a nutritional marker, as lower than normal amounts of 4 is indicative of vitamin Bg deficiency. ... 

A more specific type of chemical assay is based on enzymatic measurement of vitamin co-enzyme activity. This approach is designed to detect a vitamin deficiency in tissues, and is only feasible for those vitamins that serve as co-enzymes. For instance, thiamin depletion in a subject can be diagnosed by measuring the transketolase activity in red blood cells with and without the addition of thiamin pyrophosphate (TPP) in vitro. If TPP increases the activity by more than a given amount, thiamin deficiency is indicated. Similarly, a subnormal level of riboflavin is indicated in tissues if the activity of erythrocyte glutathione reductase is increased after the addition of flavin adenine dinucleotide (FAD). Erythrocyte transaminase activation by pyridoxal-5 -phosphate (PLP) can be measured to establish a deficiency of vitamin B . 

Figure 1 The mitochondrial respiratory chain. Electron transfer (brown arrows) between the three major membrane-bound complexes (I, III, and IV) is mediated by ubiquinone (Q/QH2) and the peripheral protein c)dochrome c (c). Transfer of protons hnked to the redox chemistry is shown by blue arrows red arrows denote proton translocation. NAD+ nicotinamide adenine dinucleotide, FMN flavin mononucleotide, Fe/S iron-sulfur center bH,bi, and c are the heme centers in the cytochrome bc complex (Complex III). Note the bifurcation of the electron transfer path on oxidation of QH2 by the heme bL - Fe/S center. Complex IV is the subject of this review. N and P denote the negatively and positively charged sides of the membrane, respectively...

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