Co(II) is not oxidized during turnover in the copper amine oxidase from Hansenula polymorpha.

Co(II) substitution into the copper amine oxidases (CAOs) has been an effective tool for evaluating the mechanism of oxygen reduction in these enzymes. However, formation of hydrogen peroxide during turnover raises questions about the relevant oxidation state of the cobalt in these enzymes and, therefore, the interpretation of the activity of the metal-substituted enzyme with respect to its mechanism of action. In this study, Co(II) was incorporated into the CAO from Hansenula polymorpha (HPAO). The effect of hydrogen peroxide on the catalytic activity of cobalt-substituted HPAO was evaluated. Hydrogen peroxide, either generated during turnover or added exogenously, caused a decrease in the activity of the enzyme but did not oxidize Co(II) to Co(III). These results are in strong contrast with results from the CAO from Arthrobacter globiformis (AGAO), where hydrogen peroxide causes an increase in the activity of the enzyme as the Co(II) is oxidized to Co(III). The results of this study with HPAO support previous reports that have shown that this enzyme acts by transferring an electron directly from the reduced TPQ cofactor to dioxygen rather than passing the electron through the bound metal ion. Furthermore, these results provide additional evidence to support the idea that different CAOs use different mechanisms for catalysis.

Cobalt substitution supports an inner-sphere electron transfer mechanism for oxygen reduction in pea seedling amine oxidase.

Copper amine oxidases (CAOs) are a large family of proteins that use molecular oxygen to oxidize amines to aldehydes with the concomitant production of hydrogen peroxide and ammonia. CAOs utilize two cofactors for this reaction: topaquinone (TPQ) and a Cu(II) ion. Two mechanisms for oxygen reduction have been proposed for these enzymes. In one mechanism (involving inner-sphere electron transfer to O(2)), Cu(II) is reduced by TPQ, forming Cu(I), to which O(2) binds, forming a copper-superoxide complex. In an alternative mechanism (involving outer-sphere electron transfer to O(2)), O(2) is directly reduced by TPQ, without reduction of Cu(II). Substitution of Cu(II) with Co(II) has been used to distinguish between the two mechanisms in several CAOs. Because it is unlikely that Co(II) could be reduced to Co(I) in this environment, an inner-sphere mechanism, as described above, is prevented. We adapted metal replacement methods used for other CAOs to the amine oxidase from pea seedlings (PSAO). Cobalt-substituted PSAO (CoPSAO) displayed nominal catalytic activity: k(cat) is 4.7% of the native k(cat), and K(M) (O(2)) for CoPSAO is substantially (22-fold) higher. The greatly reduced turnover number for CoPSAO suggests that PSAO uses the inner-sphere mechanism, as has been predicted from (18)O isotope effect studies (Mukherjee et al. in J Am Chem Soc 130:9459-9473, 2008), and is catalytically compromised when constrained to operate via outer-sphere electron transfer to O(2). This study, together with previous work, provides strong evidence that CAOs use both proposed mechanisms, but each homolog may prefer one mechanism over the other.

Atypia predicting prognosis for intracranial extraventricular neurocytomas.

OBJECT: The literature, at present, provides limited information about extraventricular neurocytomas (EVNs) and is almost exclusively composed of case reports or small case series. Treatment for EVNs has largely been guided by results from central neurocytoma outcome studies. The authors present an analysis of all reported intracranial EVN cases to establish if tumor histopathological features can substratify EVN into groups with differing prognosis and help guide treatment decisions. METHODS: The authors identified studies reporting histology, treatment modality, and outcomes for patients with intracranial EVN. The rates of recurrence and survival for patients were compared using Kaplan-Meier analysis. Atypical tumors, defined by MIB-1 labeling index exceeding 3% or atypical histological features, were compared with typical tumors, and patients 50 years of age or older were compared with those younger than 50 years of age. RESULTS: Eighty-five patients met the inclusion criteria, and 27% of them had an atypical histology. Typical EVNs had a better prognosis than atypical EVNs after primary treatment, with a 5-year recurrence rate of 36% compared with 68% (p < 0.001), and a 5-year mortality rate of 4% compared with 44%, respectively (p < 0.001). Age younger 50 years was associated with a better prognosis than age equal to or greater than 50 years, with a 5-year recurrence rate of 33% and 74%, respectively (p < 0.001), and a 5-year mortality rate of 4% and 52%, respectively (p < 0.001). Multivariate analysis demonstrated that atypical EVNs carried significantly increased risk for recurrence (hazard ratio [HR] 4.91, p < 0.001) and death (HR 22.91, p < 0.01). Gross-total resection was superior to subtotal resection (STR) alone in tumor control rates for typical EVNs (95% and 68%, p < 0.05), and there was a trend for adjuvant external-beam radiotherapy to benefit STR. There was suggestion of similar trends in patients with atypical EVNs. CONCLUSIONS: There are at least 2 distinct histological subtypes of EVN, with different prognostic significances. Atypia or MIB-1 labeling index greater than 3% is a significant predictor of poor prognosis for EVNs. Complete resection or more aggressive attempts at providing adjuvant therapy following STR appear to improve the prognosis for patients with EVNs. Although the authors' results are informative, there are limitations to their analysis. Given the relatively modest total number of cases reported, as well as the nature of the disaggregated analysis, the authors were not able to use formal meta-analytical methods to limit the impact of between center heterogeneity. Additionally, they were not able to control for individual differences in data analysis and presentation across the different studies included in their analysis.

Structure and activity of CPNGRC: a modified CD13/APN peptidic homing motif.

Asn-Gly-Arg peptides have been designed as vehicles for the delivery of chemotherapeutics, magnetic resonance imaging contrast agents, and fluorescence labels to tumor cells, and cardiac angiogenic tissue. Specificity is derived via an interaction with aminopeptidase N, also known as CD13, a cell surface receptor that is highly expressed in angiogenic tissue. Peptides containing the CNGRC homing sequence tethered to a pro-apoptotic peptide sequence have the ability to specifically induce apoptosis in tumor cells. We have now identified a modification to the Asn-Gly-Arg homing sequence motif that improves overall binding affinity to aminopeptidase N. Through the addition of a proline residue, the new peptide with sequence, CPNGRC, inhibits aminopeptidase N proteolytic activity with an IC(50) of 10 microM, a value that is 30-fold lower than that for CNGRC. Both peptides are cyclized via a disulfide bridge between cysteines. Steady-state kinetic experiments suggest that efficient aminopeptidase N inhibition is achieved through the highly cooperative binding of two molecules of CPNGRC. We have used NMR-derived structural constraints for the elucidation of the solution structures CNGRC and CPNGRC. Resulting structures of CNGRC and CPNGRC have significant differences in the backbone torsion angles, which may contribute to the enhanced binding affinity and demonstrated enzyme inhibition by CPNGRC.

Metal binding characteristics and role of iron oxidation in the ferric uptake regulator from Escherichia coli.

The ferric uptake regulator is a metal-dependent transcription repressor that is activated by divalent transition metal cations. Fe(II) is believed to be the primary functional metal in vivo; however, the ability of other divalent cations to activate Fur brings into question the true physiological metal. Furthermore, the role of different oxidation states of iron in activating Fur has not been determined. Comparison of the affinity of different metals with intracellular metal concentrations would suggest which metals activate Fur in vivo; however, no accurate determinations of the affinity of Fur for metals have been reported. In this study, methods for reconstituting Fur with Fe(II), Fe(III), Co(II), and Zn(II) are described. Reconstituted protein was assayed for DNA affinity by gel shift assays. Fur is activated for DNA binding when reconstituted with Fe(III), as well as Fe(II), Zn(II), Co(II), and Mn(II), with little difference in DNA affinity for the different metallo forms of Fur. The affinity of Fur for the different metals was determined and ranges over several orders of magnitude in the following order: Zn(II) >> Co(II) > Fe(II) > Mn(II). Only Fe(II) binds with sufficient affinity to activate Fur significantly at physiological metal concentrations, when compared to previously determined total metal concentrations in Escherichia coli.

Mechanistic comparison of the cobalt-substituted and wild-type copper amine oxidase from Hansenula polymorpha.

A recent report by Mills and Klinman [Mills, S. A., and Klinman, J. P. (2000) J. Am. Chem. Soc. 122, 9897-9904] described the preparation and initial characterization of a cobalt-substituted form of the copper amine oxidase from Hansenula polymorpha (HPAO). This enzyme was found to be fully catalytically active at saturating substrate concentrations, but with a K(m) for O(2) approximately 70-fold higher than that of the copper-containing, wild-type enzyme. Herein, we report a detailed analysis of the mechanism of catalysis for the wild-type and the cobalt-substituted forms of HPAO. Both forms of enzyme are concluded to utilize the same mechanism for oxygen reduction, involving initial, rate-limiting electron transfer from the reduced cofactor of the enzyme to prebound dioxygen. Superoxide formed in this manner is stabilized by the active site metal, facilitating the transfer of a second electron and two protons to form the product hydrogen peroxide. The elevated K(m) for O(2) at the dioxygen binding site in Co-substituted HPAO, relative to that of wild-type HPAO, is proposed to be due to a change in the net charge at the adjacent metal site from +1 (cupric hydroxide) in wild-type enzyme to +2 (cobaltous H(2)O) in cobalt-substituted HPAO.