10-FOLD INCREASE IN HUMAN PLASMA EXTRACELLULAR-SUPEROXIDE DISMUTASE CONTENT CAUSED BY A MUTATION IN HEPARIN-BINDING DOMAIN

Extracellular superoxide dismutase (EC-SOD) is a secretory SOD isoenzy me. 99% of EC-SOD is anchored to heparan sulfate proteoglycans in the tissue interstitium, and 1% is located in the vasculature in equilibri um between the plasma and the endothelium. Analysis of EC-SOD in plasm a samples from 504 random blood donors revealed a common (2.2%) phenot ypic variant displaying 10-fold increased plasma EC-SOD content. The E C-SOD in the plasma of these individuals, collected both before and af ter intravenous injection of heparin, displayed a reduced heparin affi nity when compared with samples from normal individuals. The specific enzymatic activity was the same as that of normal enzyme. Nucleotide s equence analyses of two of the affected subjects revealed a nucleotide exchange resulting in a substitution of Arg-213 by Gly. The substitut ion is located in the center of the carboxyl-terminal cluster of posit ively charged amino acid residues, which defines the heparin-binding d omain. Polymerase chain reaction-single-strand conformational polymorp hism and allele-specific polymerase chain reaction showed that all 11 affected individuals are heterozygous, carrying the same single-base m utation. Recombinant EC-SOD containing this mutation had a reduced hep arin affinity similar to that of EC-SOD C from variant persons. The hi gh plasma activity can be explained by an accelerated release from the tissue interstitium heparan sulfate to the vasculature and should thu s be accompanied by significantly reduced tissue EC-SOD activities.