in greek mythology, the Chimera was a fearsome monster with the head of a lion, the body of a goat, and the tail of a serpent, which was slain by Bellerophon. In their paper (the current article in focus, Ref. 1, see p. L917 in this issue) on the construction of a chimeric superoxide dismutase (SOD), it appears that Gao et al. may have slain the monstrous task of constructing a therapeutically beneficial SOD. Since its discovery, the family of SODs has offered the potential for an effective antioxidant therapy that would reduce undesired consequences of inflammatory diseases as well as a number of conditions associated with uncontrolled overproduction of superoxide. However, for reasons that are not entirely clear, this goal has become somewhat of a monster for researchers. By constructing a chimera of two of the isotypes of SOD, Gao et al. may have achieved the construction of a therapeutically viable form of the enzyme.
The three SOD isomers, cytosolic Cu,Zn SOD (SOD1), mitochondrial MnSOD (SOD2), and extracellular Cu,Zn SOD (SOD3), have been shown to have some therapeutic utility in protecting organ systems from oxidative stress, particularly in animal model systems of disease (5, 6). However, the success of these therapies has been limited due to a variety of reasons such as the short half-life of the protein in circulation, inability to associate with the cellular surface, and slow rates of equilibration between the vascular and interstitial spaces. Primarily due to the small molecular radius of SOD1 injected into circulation, it is rapidly (half-life of 10 min) cleared by the kidneys. Furthermore, its negative charge does not allow SOD1 to interact with cell surfaces and reduces its ability to enter the interstitium. Moreover, the therapeutic efficacy of SOD1 exhibits a bell-shaped curve after systemic administration, which, although not well understood, further limits the concentration of this protein that can be administered pharmacologically (5, 6). These limitations are partially alleviated by the use of SOD2, which is the least negatively charged SOD, and in the tetrameric form has a molecular radius of 40 Å, which retards its clearance by the kidneys (plasma half-life of 4 h). Despite its larger size, SOD2 equilibrates nearly four times faster that SOD1 within interstitial spaces (5).
SOD3 is normally tagged to the cellular surface via its hydrophilic positively charged “tail”, which gives the protein its heparin-binding ability (4, 8). Previously, it has been shown that cleavage of this tail results in the release of SOD3 from the cellular surface and that this loss may contribute to the sensitivity of the endothelium to oxidative insults. Furthermore, a major contributor of reactive intermediates near or at the endothelial plasma membrane is the NADPH oxidase. It is now recognized that a family of membrane-associated proteins (NOX) are responsible for generating superoxide and hydrogen peroxide in vascular endothelium and smooth muscle cells potentially for defense purposes and for cell signaling (9). The NOX enzymes appear to be composed of the typical low-potential membrane gp91phox flavoprotein that reduces oxygen to superoxide, as well as of cytosolic proteins, which in response to stimuli assemble into a functional oxidase (9). The generation of superoxide in the vascular compartment not only from activated inflammatory cells but also from vascular cells contributes to adverse effects of tissue injury during inflammation and other vascular disorders.
Therefore, adherence to the endothelium appears to be critical for the protective and anti-inflammatory function of SODs. The pharmacological efficacy of SOD2 may be limited by the inability of the protein to adhere to endothelial cell surface. For reasons not completely understood, SOD3 cannot be expressed and purified in large quantities, prompting investigators to utilize SOD1 and SOD2 primarily. However, on the basis of limitations of SOD1 and SOD2 discussed above, investigators have employed chemical and molecular approaches to generate SODs that combine some of the most beneficial features of SODs (2, 3). Examples of chemical modifications that extend the half-life of SOD1 and improve its pharmacological profile include coupling of polyethylene glycol, lecithin, putrescine, and sugars (for a review, see Ref. 6). Molecular approaches include the generation of chimeric proteins such as an SOD1/3 chimera protein that contains the positively charged tail of SOD3 and has been shown to be more effective in protecting tissues than SOD1 (3), although this chimera was still retained in the kidney and was not optimal for therapeutic utility.
To overcome these limitations, Gao et al. (1) have generated a new chimera utilizing the SOD3 COOH terminus linked to SOD2. This new protein combines the endothelial localization of SOD3 with the extended half-life and pharmacological profile of SOD2 to produce a novel bioactive agent against the negative effects of oxidative stress. Like the mythological Chimera, this new protein displays three different characteristics: it has the head of all SODs, namely the ability to remove superoxide; the body of a tetramer, such that its large molecular size reduces its clearance from the plasma; and the tail of positively charged residues, which allows it to be preferentially tagged to the endothelial surface. This novel chimeric protein shows considerable promise within two animal models of inflammation, IL-1-induced lung injury and carrageenan-injected paw inflammation. In the first model, administration of SOD2/3 chimera prevented the vascular leak and edema and reduced the number of infiltrating neutrophils in the lung after a IL-1 challenge. Similarly, SOD2/3 administration reduced the foot edema induced by injection of carrageenan. Thus through this novel strategy, Gao et al. (1) may have provided us with the appropriate strategy with which to utilize the power of SOD to combat oxidative stress induced by inflammatory disease. This major advancement coupled with the development of chemical and molecular approaches to target the chimeric protein to the lung (7) or other tissues could provide a powerful approach to generate a clinically therapeutic agent to fulfill the promise for SOD-based therapies.
Address for reprint requests and other correspondence: H. Ischiropoulos, Stokes Research Institute, Children's Hospital of Philadelphia, 416D Abramson Center, 34th St. and Civic Center Blvd., Philadelphia, PA 19104-4318 (E-mail:).
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