Dehydrogenases general

Nicotinamide adenine dinucleotide (NAD )-dependent dehydrogenases are enzymes that typically behave according to the kinetic pattern just described. A general reaction of these dehydrogenases is... 

Another quite general approach is to employ a coupled assay (Figure 7-10). Typically, a dehydrogenase whose substrate is the product of the enzyme of interest is added in catalytic excess. The rate of appearance or disappearance of NAD(P)H then depends on the rate of the enzyme reaction to which the dehydrogenase has been coupled. 

Generally, NAD-linked dehydrogenases catalyze ox-idoreduction reactions in the oxidative pathways of metabolism, particularly in glycolysis, in the citric acid cycle, and in the respiratory chain of mitochondria. NADP-linked dehydrogenases are found characteristically in reductive syntheses, as in the extramitochon-drial pathway of fatty acid synthesis and steroid synthesis—and also in the pentose phosphate pathway. 

Figure 1 General dehydrogenase mechanism. In this example, the A hydride of NAD(P)H is transferred to the carbonyl substrate, which is activated by interaction with a Lewis acid (LA). A proton is donated to the developing oxyanion by a general acid (HX).

Immunohistochemical studies carried out in our laboratories have demonstrated the presence of xanthine oxidase in synovial endothelial cells (Stevens etal., 1991). As expected, the activity of this enzyme per unit weight of tissue is generally higher in synovia taken from RA patients due to their increased vascularity (Allen et al., 1987). In addition, it has also been shown that rheumatoid synoviocytes contain increased levels of iron-saturated ferritin (Morris et d., 1986). Xanthine oxidase (but not dehydrogenase) is able to mobilize iron from ferritin, supplying the necessary transition metal catalyst for the Haber-Weiss reaction and promoting OH formation (Biemond eta/., 1986). 

So little is known about molybdenum enzymes other than milk xanthine oxidase that there is little to be said by way of general conclusions. In all cases where there is direct evidence (except possibly for xanthine dehydrogenase from Micrococcus lactilyticus) it seems that molybdenum in the enzymes does have a redox function in catalysis. For the xanthine oxidases and dehydrogenases and for aldehyde oxidase, the metal is concerned in interaction of the enzymes with reducing substrates. However, for nitrate reductase it is apparently in interaction with the oxidizing substrate that the metal is involved. In nitrogenase the role of molybdenum is still quite uncertain. 

---