Copper-promoted overall transformation of 4-tert-butylphenol to its para-hydroxyquinonic derivative, 2-hydroxy-5-tert-butyl-1,4-benzoquinone. Biomimetic studies on the generation of topaquinone in copper amine oxidases

Cu(II)-mediated phenol oxygenation: chemical evidence implicates a unique role of the enzyme active site in promoting the chemically difficult tyrosine monooxygenation in TPQ cofactor biogenesis of copper amine oxidases

Cu(II)-mediated autoxidations of 4-tert-butylphenol under various conditions was studied, the data confirmed imidazole is the best ligand to promote phenol oxygenation. The same reaction of 2,4-di-tert-butylphenol proceeded much more quickly to lead nearly exclusively to oxidative coupling rather than oxygenation under high pressure O2. These results suggested that Cu(II)-catalyzed phenol autoxidation by activating O2 and phenol in terms of a phenoxy radical (ArO)-Cu(II)-superoxide ternary complex, whereas selectivity between oxygenation and coupling depends mainly on the electronic structure of ArO. It is appeared that CuAOs could achieve stoichiometric tyrosine monooxygenation by modulating the redox potential of Cu(II) and stabilizing the ternary complex through protein conformational adjustment.

A Theoretical Study of the Mechanism for the Biogenesis of Cofactor Topaquinone in Copper Amine Oxidases

In the present quantum chemical study, the biogenesis of the cofactor topaquinone (TPQ) has been studied using hybrid density functional theory (B3LYP). The suggested mechanism is divided into six steps and incorporates the observation of four crystallized intermediates. The experimental suggestion that the formation of the Cu(II)-peroxy species is the rate-limiting step is consistent with the results of the present study. Before the formation of the Cu(II)-peroxy species, dioxygen is suggested to first bind at the equatorial position on the copper metal center. A mechanism for the critical O-O bond cleavage is suggested, and this step is found to be driven by an unusually large exothermicity. A complex, spin-forbidden formation of H(2)O(2) with and without the involvement of the copper metal center is discussed. The results are discussed in detail, and comparisons are made to experimental findings and suggestions.

Metal catalyzed oxidation of tyrosine residues by different oxidation systems of copper/hydrogen peroxide

Metal-catalysed oxidation (MCO) reactions result in the formation of reactive oxygen species (ROS) in biological systems. These ROS cause oxidative stress that contributes to a number of pathological processes leading to a variety of diseases. Tyrosine is one residue that is very susceptible to oxidative modification and the formation of dityrosine (DT) and 3,4-dihydroxyphenylalanine (DOPA) have been widely reported in a number of diseases. However, the mechanisms of MCO of tyrosine in biological systems are poorly understood and require further investigation. In this study we investigated the mechanism of DT and DOPA formation by MCO using N-acetyl tyrosine ethyl ester as a model for tyrosine in proteins and peptides. The results showed that DT formation could be observed upon Cu2+/H2O2 oxidation at pH 7.4. Our results indicate that it is unlikely to be via Fenton chemistry since Cu+/H2O2 oxidative conditions did not lead to the formation of DT.

Biochemical and toxicological evaluation of agent-cofactor reactivity as a mechanism of action for osteolathyrism

In vitro reactivity for each of four osteolathyrogens with a model compound for the lysyl oxidase (LO) cofactor was evaluated and coupled with mixture toxicity testing to evaluate agent-cofactor reactivity as a potential mechanism of action for osteolathyrism. Reactivity of the model cofactor (mLTQ: 4-butylamino-5-methyl-o-quinone), with each of two ureides, semicarbazide (SC) and thiosemicarbazide (TSC), and each of two aminonitriles, aminoacetonitrile (AAN) and beta-aminopropionitrile (betaAPN), was assessed using UV-vis spectrophotometry; both in the absence and presence of Cu(II)-bipyridine (bipy) complex. Two sets of mixture toxicity experiments were conducted using a frog embryo assay that assessed the incidence of osteolathyrism in the notochord of tadpoles after 96-h exposure. The resulting concentration-response curves for each set were evaluated (chi(2) goodness-of-fit test) against theoretical curves for two combined effects models: dose-addition and independence, to determine the combined effect of each osteolathyrogen combination. The agents SC, TSC and AAN each showed rapid, irreversible reactivity with mLTQ, both in the absence and presence of Cu(II)-bipy complex, as indicated by bleaching of the mLTQ peak (504 nm) and formation of an adduct at 350 nm. betaAPN showed no apparent reactivity in the absence of prolonged incubation with mLTQ, whether Cu(II)-bipy complex was present or not. After prolonged incubation (24-144 h) a new peak formed at 350 nm, suggesting that betaAPN reacts weakly with the cofactor, but in a manner different from the other agents examined. The toxicity tests indicated a dose-additive combined effect for the SC:TSC, AAN:SC and AAN:SC:TSC mixtures (0.1<P<0.9; with slope and EC(50) additivity quotient values between 0.93-1.14), suggesting these agents induce osteolathyrism via a common mechanism. The betaAPN:SC and betaAPN: SC:TSC mixtures showed a greater-than dose-additive combined effect (P<0.001; slope and EC(50) additivity quotient values from 0.74 to 0.89), suggesting a different or additional mechanism of osteolathyrism for betaAPN. Taken together the results indicate that SC, TSC and AAN induce osteolathyrism primarily, if not completely, by binding to the LO cofactor, thereby inhibiting proper connective tissue fiber cross-linking. While betaAPN also has the potential to react with the cofactor, the nature of the reactivity observed suggests that betaAPN-cofactor binding is, at best, a secondary mechanism for induction of osteolathyrism in vivo.