Radicals from enzymes

Rather scanty evidence exists for the participation of free radicals in Alzheimer s disease and Down s syndrome. However, more recendy, reports have appeared that suggest possible free-radical involvement in the pathogenesis of these two conditions. Zemlan et al. (1989) repotted that the activity of the free-radical scavenging enzyme, SOD, was significantly increased in fibroblast cell lines derived from familial Alzheimer s and Down s patients. They hypothesized that the elevation in SOD activity observed in the Alzheimer patients supports the theory that paired helical filaments are formed by free-radical hydroxylation of proline residues. They further su ested that SOD levels might also be increased in the brains of Alzheimer s and Down s patients, and that the increase in SOD may reflect an enhanced generation of free radicals. 

If XO is an undoubted historical pioneer among free radical-producing enzymes, whose capacity to catalyze one-electron transfer reactions opened a new era in biological free radical studies, NADPH oxidase is undoubtedly the most important superoxide producer. This enzyme possesses numerous functions from the initiation of phagocytosis to cell signaling, and it is not surprising that its properties have been considered in many reviews during last 20 years [56-58]. 

The MTHFRs of Arabidopsis and maize have recently been cloned by genomics-based approaches, based on homology with the enzymes from other organisms.17 Like mammalian MTHFRs, the plant enzymes were found to be homodimers of two-domain subunits that are homologous to the mammalian enzymes throughout both domains. However, when the recombinant plant proteins were expressed in yeast, they were found to differ radically from the mammalian MTHFRs in both their pyridine nucleotide preference and their regulatory properties plant enzymes prefer NADH to NADPH, and they are insensitive to AdoMet.17... 

The aforementioned behavior of P450 as a one-electron oxidant needs to be explained. Since P450 is overall a two-electron oxidant, it is a priori unbelievable that this enzyme would release a highly reactive cation-radical from its active site. The authors cited in the following text considered different aspects of such unexpected resistivity of cation-radicals to further oxidation. 

SCHEME 13. Redox generation of an aminoxyi radical from the hydroxylamine by an enzyme... 

Peroxidases are haem proteins that are activated from the ferric state to one-electron oxidants by H202. They play a significant role in the generation of radicals from xenobiotics. The compound I state contains one oxidising equivalent as an oxoferryl-haem entity and the second as a porphyrin -radical cation. Upon the oxidation of a substrate the porphyrin radical is repaired, giving the compound II. Reduction of the oxoferryl haem back to the ferric state by a second substrate molecule completes the enzyme cycle. In addition to the classical peroxidases, several other haem proteins display pseudo-peroxidase activity. The plant enzyme horseradish peroxidase (HRP) is often employed in model systems. 

The activating enzyme will also generate radicals from short peptides such as Arg-Val-Ser-Gly-Tyr-Ala-Val, which corresponds to residues 731-737 of the pyruvate formate-lyase active site. If Gly 734 is replaced by L-alanine, no radical is formed, but radical is formed if D-alanine is in this position. This suggests that the pro-S proton of Gly 734 is removed by the activating... 

This compound undergoes a two-step ATP-dependent cyclization352-355 to form dethiobiotin. The final step, insertion of sulfur into dethiobiotin, is catalyzed by biotin synthase, a free-radical-dependent enzyme related to pyruvate formate lyase (Fig. 15-16). It transfers the sulfur from cysteine via an Fe-S cluster.3553 Biosynthesis of lipoic acid involves a similar insertion of two sulfur atoms into octanoic acid.356 See also p. 1410. 

For GAOX, structural analysis is particularly complicated, because of the existence of multiple states of the enzyme differing essentially only in the number of electrons, i.e., the oxidation state of the metalloprotein complex. Three distinct oxidation states can be prepared, each with properties and reactivities dramatically different from the others, as indicated in Fig. 10 (Whittaker and Whittaker, 1988). When isolated from culture medium, GAOX is a mixture of two of these states a blue, one-electron reduced, catalytically inactive form (lAGO) that contains a Cu(II) ion and no radical and a green form that is catalytically active (AGO) and contains both Cu(II) and a free radical. The enzyme may be converted to... 

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