Superoxide and other secondary generated reactive oxygen metabolites
are continuously generated as normal by-products of cellular metabolism.
These oxygen species are highly reactive and can attack any biochemical
components in the cells. Normal cells have a number of enzymatic endogenous
antioxidant mechanisms, for example, superoxide dismutase (SOD), catalase
and glutathions peroxidase. These enzymes quickly eliminate toxic oxygen
metabolites under normal conditions. The generation of highly toxic metabolites
of oxygen is greatly increased in pathological conditions. When the flux
of reactive oxygen metabolites exceeds the capability of the endogenous
antioxidant mechanisms, tissue injury ensures. There are three SOD isozymes,
copper- and zinc-containing SOD (Cu, Zn-SOD), manganese-containing SOD
(Mn-SOD) and extracellular-SOD (EC-SOD). EC-SOD is a secretory, tetrameric
glycoprotein with a subunit molecular mass of about 32 kDa. EC-SOD is
the major SOD isozyme in plasma, lymph and synovial fluid, whereas the
content of EC-SOD in tissues is lower than those of Cu,Zn-SOD and Mn-SOD,
which are intracellular enzymes.
A prominent feature of EC-SOD is its affinity for heparin, which differentiated
it from the other SOD isozymes. In rite vascular system, EC-SOD forms
an equilibrium between plasma phase and the endothelial cell surfaces,
but occurs primarily in the glycocalyx of cell surfaces and in the connective
tissue matrix; this form of the enzyme accounts for over 90% of the EC-SOD.
In several pathological situations the rate of superoxide formation by
activated neutrophils and xanthine oxidase-based system is increased in
the microenvironment created by the adhered neutrophil-endothelial cell
surface. The endothelial surface-associated EC-SOD would be an efficient
protector against such events in vivo.
The carboxyl-terminal position of EC-SOD, especially the cluster of
four Arg and two Lys residues at position 210-215, forms an essential
part of the heparin-binding domain. Molecular genetic studies of EC-SOD
have shown that individuals with 10-fold higher plasma EC-SOD content
have a single base substitution generating the exchange of GIy for Arg-213
(R213G). The binding of mutant EC-SOD to endothelial cells was about 50-fold
less than that of normal EC-SOD. We found that about 6% of the Japanese
population tested have the mutation. The frequency of the identified mutant
allele in hemodialysis patients was about twice of that in healthy persons.
Although the data may be insufficient, the decrease of the protective
capability on the endothelial surface by EC-SOD might accelerate the renal