Endogenous reduction of intermediates

HOPhjSil O, 42 214 Horseradish peroxidase compound I formation, 43 97-98 endogenous reduction of intermediates, 43 100... 

G. Plant and Mammalian Peroxidase Superfamilies Mechanisms of Compound I Formation Endogenous Reduction of Peroxidase Intermediates Exogenous Reduction of Peroxidase Intermediates... 

The donor types D3, D4, and D6 of Keilin and Nicholls (37) all reduce compound I of Type A enzymes directly to the ferric state in a two-electron process without detectable intermediates. Each of these donors is probably also able to bind in the heme pocket of the free enzyme. Alcohols (type D3) form complexes with free ferric Type A enzymes whose apparent affinities parallel the effectiveness of the same alcohols as compound I donors (39). Formate (type D3) reacts with mammalian ferric enzyme at a rate identical to the rate with which it reduces compound I to free enz5mie (22). Its oxidation by compound I may thus share an initial step analogous to its complex formation with ferric enzyme. Formate also catalyzes the reduction of compound II to ferric enzyme by endogenous donors in the enz5mie (40, 41). Both compound I and compound II may thus share with the free enzyme the ability to ligate formate in the heme pocket. Nitrite, which is oxidized to nitrate by a two-electron reaction with compoimd I (type D4), also forms a characteristic complex with free enzyme (42). In both cases the reaction involves the donor in its protonated (HNO2) form. 

After prolonged EDTA and lysozyme treatment, Anacystis can be converted to spheroplasts (Peschek et al.,1983). We have found that the electron transport activity of these spheroplasts is dominated by a KCN and DCMU insensitive light-induced O2 uptake, which occurs via a post-photosystem I autoxidizable endogenous intermediate. This activity is reminiscent of that reported by Honeycutt, Krogmann (1970) with cell-free cyanobacteria thylakoid fragments, except that it does not require exogenous autoxidizable acceptors like methyl viologen. The spheroplasts were also found to be capable of a dark reduction of 2,6-dichlorophenol indophenol (rate, 12 pmoles/mg Chi a h), which was enhanced (2.5x) in the presence of 1 mM KCN, and was partially inhibited by 10 mM KCl. 

Despite the scarcity of direct evidence, HA is generally believed to be present as a metabolic intermediate in mammalian tissues Recent studies on the reductive detoxification of HAs both by human NADH-cytochrome i>5 reductase and by human cytochrome b5 may be considered as additional supporting evidence for the in vivo formation of HA in mammalian cells that needs to be controlled in order to avoid the toxic effects of an excess of endogenously produced HA, as well as of HA produced by detoxification of xenobiotic HA derivatives " . 

Thermoactinomyces intermedius and endogenous formate dehydrogenase. The reductive amination process was further scaled up using a preparation of the two enzymes expressed in single recombinant E. coli. The amino acid 7 can be directly protected as its boc derivative without isolation to afford intermediate 8. Yields before isolation were close to 98% with 100% EE. 

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