Redogenic donors

The stoichiometric reduction of iron can be explained only by assuming that in the presence of peroxidase H2O2 produces two free radicals which are very reactive and effectively reduce the added electron acceptor. There are two types of free radicals which appear in the peroxidase reaction. We have classified them into redogenic and oxidogenic ones (26). Redogenic donors can give rise to two-equivalent oxidized forms, but oxidogenic donors cannot. 

The two adjectives derive from Yamazaki s theory of peroxidase activity, that the oxidogenic donors generate radicals with oxidizing ability and the redogenic donors radicals with reducing power, after their reaction with peroxidase peroxide compounds. This extension of George s idea (Section II.A above) will be examined together with some other theories in Section II.E below. 

The studies of Kenten and Mann (1952, 1953) served to emphasize the complexity of some of the processes promoted by peroxidase. Although the three substances in group (a) of the redogenic donors (Table IV) remain the only ones undergoing oxidation in the presence of peroxidase alone, other reactions are possible in the presence of manganese. As explained earlier, the oxidation of manganous ions to higher oxidation states is promoted by peroxidase. Under these conditions not... 

A similar system to that for dicarboxylic acids has been described by Akazawa and Conn (1958) for pyridine nucleotides. Chance (1951) had shown that DPNH was a hydrogen donor for the peroxidatic activity of the enzyme, albeit not a very effective one. Akazawa and Conn found that an oxidatic reaction was also catalyzed by peroxidase if manganous ions and a monohydric phenol were present. In its sensitivity to catalase, copper salts, and cyanide, this system closely resembles the classical peroxidase-catalyzed oxidations. Although these investigators did not analyze their system in the terms put forward by Yamazaki, they found that oxidogenic donors were stimulatory and redogenic donors inhibitory in this reaction. 

In 1953, Kenten and Mann, during their survey of various donors active in the oxidatic reaction (see Section III), showed that p-cresol promotes the oxida tion of dihydroxyfumarate. Kenten later (1955) demonstrated that in the case of indoleacetic acid, some phenolic compounds (monophenols, resorcinol, etc.) are stimulatory while others are inhibitory (most polyphenols) despite the fact that all were substrates for peroxidase. Similar results have been obtained for pyridine nucleotides (Akazawa and Conn, 1958). Yamazaki analyzed the behavior of phenols and other hydrogen donors effective in the peroxidatic activity of the enzyme, and has shown that they fall into two classes, which he terms redogenic and oxidogenic. Table IV gives a list of the donors... 

Redogenic hydrogen donors Oxidogenic hydrogen donors... 

These free radicals were all obtained from the hydrogen donors in the redogenic class (Table IV). This contains those donors that undergo oxidatic reactions (group a) as well as others which promote certain reductions other than that of oxygen all except />-phenylene-diamine and catechol will reduce cytochrome c in presence of peroxidase and peroxide. Yamazaki suggests reactions of the type shown (Eqs. 16 and 17) to account for this behavior. 

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