The multifunctional oxidase activity of ceruloplasmin as revealed by anion binding studies

Chloride Control of the Mechanism of Human Serum Ceruloplasmin (Cp) Catalysis

Unraveling the mechanism of ceruloplasmin (Cp) is fundamentally important toward understanding the pathogenesis of metal-mediated diseases and metal neurotoxicity. Here we report that Cl^-, the most abundant anion in blood plasma, is a key component of Cp catalysis. Based on detailed spectroscopic analyses, Cl^- preferentially interacts with the partially reduced trinuclear Cu cluster (TNC) in Cp under physiological conditions and shifts the electron equilibrium distribution among the two redox active type 1 (T1) Cu sites and the TNC. This shift in potential enables the intramolecular electron transfer (IET) from the T1 Cu to the native intermediate (NI) and accelerates the IET from the T1 Cu to the TNC, resulting in faster turnover in Cp catalysis.

Ceruloplasmin replacement therapy ameliorates neurological symptoms in a preclinical model of aceruloplasminemia

Aceruloplasminemia is a monogenic disease caused by mutations in the ceruloplasmin gene that result in loss of protein ferroxidase activity. Ceruloplasmin plays a role in iron homeostasis, and its activity impairment leads to iron accumulation in liver, pancreas, and brain. Iron deposition promotes diabetes, retinal degeneration, and progressive neurodegeneration. Current therapies mainly based on iron chelation, partially control systemic iron deposition but are ineffective on neurodegeneration. We investigated the potential of ceruloplasmin replacement therapy in reducing the neurological pathology in the ceruloplasmin-knockout (CpKO) mouse model of aceruloplasminemia. CpKO mice were intraperitoneal administered for 2 months with human ceruloplasmin that was able to enter the brain inducing replacement of the protein levels and rescue of ferroxidase activity. Ceruloplasmin-treated mice showed amelioration of motor incoordination that was associated with diminished loss of Purkinje neurons and reduced brain iron deposition, in particular in the choroid plexus. Computational analysis showed that ceruloplasmin-treated CpKO mice share a similar pattern with wild-type animals, highlighting the efficacy of the therapy. These data suggest that enzyme replacement therapy may be a promising strategy for the treatment of aceruloplasminemia.

Assessment of oxidative stress in serum by d-ROMs test

Assessment of oxidative stress is an important but technically challenging procedure in medical and biological research. The reactive oxygen metabolites (d-ROMs) test is a simple assay marketed for analyzing the total amount of hydroperoxides in serum via the Fenton's reaction. Earlier reports have raised a suspicion that a part of the signal detected in the assay comes from sources other than metabolites generated by oxidative stress. The aim of this study was to identify which serum components interfere with the d-ROMs signal. By application of sodium azide, ethylenediaminetetraacetic acid, sodium dodecylsulphate, varying temperature, and spiking endogenous substances we demonstrate that in the case of mammalian sera the assay determines ceruloplasmin (CP) activity with potential interferences from hydroperoxides, iron level, thiols, and albumin. In sera of avian species hydroperoxides contribute more to the test outcome, but the CP part is insensitive to inhibition by azide. In conclusion, this assay has deficiencies in terms of detecting realistic concentrations of hydroperoxides, is mostly measuring CP and is also interfered with other serum components, making it very difficult to interpret in most biological systems.

Interactions of caeruloplasmin with other proteins participating in inflammation

The first detailed report of a specific interaction of CP (caeruloplasmin) with another protein described its complex with LF (lactoferrin) in 2000. Since then, several protein–protein interactions involving CP have been reported, mostly concerning iron-containing proteins. The CP–LF complex was studied thoroughly, and evidence of reciprocal effects of CP and LF was obtained. Another specific interaction investigated in detail occurs between CP and MPO (myeloperoxidase). CP–LF, CP–MPO and CP–LF–MPO complexes were found in sera of patients with inflammation. Modelling in vitro allowed understanding of which structural peculiarities of CP and partners allow the modification of their functions in a complex. The present paper reviews the latest data on complexes of CP with LF and MPO, and advances some suggestions about their role in health and disease.

Discrete Roles of Copper Ions in Chemical Unfolding of Human Ceruloplasmin

Human ceruloplasmin (CP) is a multicopper oxidase essential for normal iron homeostasis. The protein has six beta-barrel domains with one type 1 copper in each of domains 2, 4, and 6; the remaining copper ions form a catalytic trinuclear cluster, one type 2 and two type 3 coppers, at the interface between domains 1 and 6. We have characterized urea-induced unfolding of holo- and apo-forms of CP by far-UV circular dichroism, intrinsic fluorescence, 8-anilinonaphthalene-1-sulfonic acid binding, visible absorption, copper content, and oxidase activity probes (pH 7, 23 degrees C). We find that holo-CP unfolds in a complex reaction with at least one intermediate. The formation of the intermediate correlates with decreased secondary structure, exposure of aromatics, loss of two coppers, and reduced oxidase activity; this step is reversible, indicating that the trinuclear cluster remains intact. Further additions of urea trigger complete protein unfolding and loss of all coppers. Attempts to refold this species result in an inactive apoprotein with molten-globule characteristics. The apo-form of CP also unfolds in a multistep reaction, albeit the intermediate appears at a slightly lower urea concentration. Again, correct refolding is possible from the intermediate but not the unfolded state. Our study demonstrates that in vitro equilibrium unfolding of CP involves intermediates and that the copper ions are removed in stages. When the catalytic site is finally destroyed, refolding is not possible at neutral pH. This implies a mechanistic role for the trinuclear metal cluster as a nucleation point, aligning domains 1 and 6, during CP folding in vivo.

Ceruloplasmin and what it might do

The blue coloured plasma protein ceruloplasmin binds up to 95% of circulating copper, and has several possible functions. It has been proposed to function in copper transport, oxidation of organic amines, iron(II) oxidation and the regulation of cellular iron levels, and catechols, radical scavenging and other antioxidant processes. This account will consider the relative importance of these multiple functions in terms of the physiological roles of ceruloplasmin.

Ceruloplasmin revisited: structural and functional roles of various metal cation-binding sites

The three-dimensional molecular structure of human serum ceruloplasmin has been reinvestigated using X-ray synchrotron data collected at 100 K from a crystal frozen to liquid-nitrogen temperature. The resulting model, with an increase in resolution from 3.1 to 2.8 A, gives an overall improvement of the molecular structure, in particular the side chains. In addition, it enables the clear definition of previously unidentified Ca2+-binding and Na+-binding sites. The Ca2+ cation is located in domain 1 in a configuration very similar to that found in the activated bovine factor Va. The Na+ sites appear to play a structural role in providing rigidity to the three protuberances on the top surface of the molecule. These features probably help to steer substrates towards the mononuclear copper sites prior to their oxidation and to restrict the size of the approaching substrate. The trinuclear copper centre appears to differ from the room-temperature structure in that a dioxygen moiety is bound in a similar way to that found in the endospore coat protein CotA from Bacillus subtilis.

Ceruloplasmin in neurodegenerative diseases

For decades, abnormalities in ceruloplasmin (Cp) synthesis have been associated with neurodegenerative disease. From the early observation that low circulating serum ceruloplasmin levels served as a marker for Wilson's disease to the recent characterization of a neurodegenerative disorder associated with a complete lack of serum ceruloplasmin, the link between Cp and neuropathology has strengthened. The mechanisms associated with these different central nervous system abnormalities are very distinct. In Wilson's disease, a defect in the P-type ATPase results in abnormal hepatic copper accumulation that eventually leaks into the circulation and is abnormally deposited in the brain. In this case, copper deposition results in the neurodegenerative phenotype observed. Patients with autosomal recessive condition, aceruloplasminemia, lack the ferroxidase activity inherent to the multi-copper oxidase ceruloplasmin and develop abnormal iron accumulation within the central nervous system. In the following review ceruloplasmin gene expression, structure and function will be presented and the role of ceruloplasmin in iron metabolism will be discussed. The molecular events underlying the different forms of neurodegeneration observed will be presented. Understanding the role of ceruloplasmin within the central nervous system is fundamental to further our understanding of the pathology observed. Is the ferroxidase function more essential than the antioxidant role? Does Cp help maintain nitrosothiol stores or does it oxidize critical brain substrates? The answers to these questions hold the promise for the treatment of devastating neurodegenerative conditions such as Alzheimer's and Parkinson's diseases. It is essential to further elucidate the mechanism of the neuronal injury associated with these disorders.

The physiopathological significance of ceruloplasmin. A possible therapeutic approach

This article reviews and comments on the physiological roles of ceruloplasmin (Cp). We show that, in addition to its ascertained involvement in iron homeostasis, the protein, by virtue of its unique structure among multicopper oxidases, is likely involved in other processes of both an enzymatic and a nonenzymatic nature. In particular, based on the analysis of the kinetic parameters, on the one hand, and of the side-products of the oxidation, on the other, we propose that the long-recognized ability of Cp to interact with and oxidize non-iron substrates may be of physiological relevance. The striking example of 6-hydroxydopamine oxidation is presented, where we show that the catalytic action is carried out readily under physiological conditions, without release of potentially toxic oxygen intermediates.

The essential role of Glu-185 and Tyr-354 residues in the ferroxidase activity of Saccharomyces cerevisiae Fet3

The structural determinants required for ferroxidase activity by the yeast multicopper oxidase Fet3 have been partially clarified by site-directed mutagenesis based on homology modeling. Glu-185 and Tyr-354 were substituted with Ala and Phe, respectively. Fet3 E185A retained ca. 5% residual ferroxidase catalytic efficiency, and almost 40% oxidase efficiency. On the other hand, Fet3 Y354F exhibited 50% residual efficiency as a ferroxidase and more than 70% as an oxidase. These results provide new insights in the mechanism of iron binding and oxidation by Fet3, establishing the essential role of Glu-185 and Tyr-354, and allowing to dissect ferroxidase from non-iron oxidase activity.