We propose two different pathways for the production
We propose two different pathways for the production of this species: either that described by Dooley , where the phenol ring is first activated by binding of its oxygen to copper, or one where a copper-peroxo species would directly react with an active site tyrosine residue (Scheme 1B). This second pathway would exhibit differences between inactivation by ascorbate and inactivation by H2O2. Generation of a copper-peroxo species by ascorbate would start with the reduction of Cu(II) to Cu(I) by ascorbate, this species then reacting with oxygen. On the other hand, production of such an adduct caused by H2O2 would imply direct reaction between peroxide and the active site copper. We believe that the less intense formazan stains obtained with H2O2 could either prove that DPQ formation occurs through various pathways, or that H2O2 is the cause for further degradations of the quinone, for example to muconic Dihydroeponemycin .
DbH activity was followed by oxymetry, and we showed that tyramine was responsible for a partial protection from H2O2-mediated inactivation. Since a peroxo-copper species could form, samples of DbH incubated with H2O2 and tyramine were probed for activity by HPLC, but octopamine could only be detected in very small amounts under such conditions, and its formation could not be correlated to H2O2 and tyramine concentrations (data not shown). The substrate role in the protection of the enzyme can be explained in various ways: first, it could be thought as a purely physical role. Indeed, if H2O2 and substrate bind the enzyme at a same site, assumed as being CuB, they could compete for this site. Introduction of tyramine to the inactivation incubate should thus cause hydrogen peroxide to be partly excluded from this site, and therefore decrease its inactivating capacity. Second, recognition of substrate tyramine by the enzyme could induce structural modifications in its tertiary structure that would prevent any attack by hydrogen peroxide. Protection of catalytic activity by substrate tyramine, at about a 50% rate, gives us some hints about the way H2O2 induces inactivation of the protein. Indeed, substrate tyramine is only recognised by CuB, and H2O2 is not specific regarding copper. Therefore we believe that introduction of tyramine in the incubation buffer prior to activity measurement protects CuB from an attack by hydrogen peroxide, while CuA or any residue in its environment still is available for oxidation, thus producing an enzyme that exhibits partial activity. In the presence of hydrogen peroxide only, some residues in the CuB environment are also likely to be modified under the action of H2O2. For instance, it was observed using model copper complexes that a copper-bound thioether group could be oxidised into a sulfoxide after introduction of H2O2. The methionine residue located near CuB could undergo such a modification in our inactivation experiments. Thus, inactivation by H2O2 could occur through various mechanisms, while inactivation by ascorbate would only lead to DPQ formation.
We showed that substrate tyramine is able to protect DbH from H2O2-mediated inactivation. DbH inactivation by either H2O2 or ascorbate/dioxygen produced a protein-bound quinone derivative, which formation was also evidenced on ascorbate-inactivated DbH by UV–visible spectroscopy. This derivative is proposed to be DPQ. Its formation suggests the involvement of a tyrosine residue during catalysis.
Introduction Egr1 (Zif268, NGFI-A, TIS8, or Krox24) is a transcription factor with three zinc fingers of the Cys2His2 class (reviewed by (O\'Donovan et al., 1999)). Egr1 binds to a GC-rich motif (5′-GCG(T/G)GGGCG-3′) through its three zinc finger DNA-binding domains (Christy and Nathans, 1989), and modulates transcription of a number of genes that participate in various cellular functions (reviewed by (Silverman and Collins, 1999, Thiel and Cibelli, 2002)). As an immediate early gene, many stimuli such as growth factors, hormones, neurotransmitters, stress, and cytokines induce Egr1 expression (Cohen, 1996, Dieckgraefe and Weems, 1999, Kaufmann et al., 2001, Liu et al., 2005, Park and Koh, 1999). This response is presumed to have a key role in orchestrating a second wave of gene expression that underlies long-term effects of these factors on cell growth and differentiation.