TURNOVER OF EXTRACELLULAR-SUPEROXIDE DISMUTASE IN TISSUES

BACKGROUND: The secretory glycoprotein, extracellular-superoxide dismu tase (EC-SOD) is in the body, primarily located to the tissue intersti tial space, and in tissue is almost completely composed of homotetrame ric high-heparin-affinity C-type. The aim of the present study was to determine the turnover rate of EC-SOD C in tissue and the importance o f the heparin-affinity for the retention. EXPERIMENTAL DESIGN: EC-SOD C and two EC-SOD carboxyterminal truncation variants with reduced and absent heparin-affinities, respectively, were labeled with I-125 and t hen subcutaneously and intramuscularly injected into rats. The retenti ons were followed with repeated determinations with a gamma camera. RE SULTS: EC-SOD C displayed a tissue half-life of about 85 hours, wherea s the EC-SOD variants with reduced and absent heparin-affinities displ ayed half-lives of about 20 and 7 hours, respectively. The half-lives were remarkably similar in the intramuscular and subcutaneous injectio n sites, suggesting rather small overall differences between tissues i n EC-SOD C retention. CONCLUSIONS: The findings established that EC-SO D C in the tissue interstitium exists almost completely anchored to he paran sulfate proteoglycan via the carboxyterminal heparin-binding dom ains, and that this binding is the determinant of the long tissue rete ntion of the enzyme. The findings further suggest that reductions in h eparin-affinity, e.g., by proteolytic truncation of the highly suscept ible heparin-binding domain, may be an important mechanism of eliminat ion of EC-SOD from tissues, both physiologically and as enhanced under pathologic conditions.