Small heparin fragments regulate the amplification pathway of complement

Abstract Heparin is a highly sulfated, polydisperse and heterogenepus glycosaminoglycan which has been well characterized for its ability to regulate multiple sites in the complement cascade. Although previous studies demonstrated the relationship between degree of sulfation, particularly O-sulfation, and complement capacity, they left unclear the relationship between the size of the heparin molecule and its ability to inhibit complement. Therefore, although the structure-activity relationship for heparin is well understood for anticoagulant activity, it is ill defined for the complement system. The present studies were designed to examine depolymerized heparin to determine which fragments were capable of inhibiting amplification pathway activation. We found that as the size of the molecule increases the ability to regulate complement increases; below 1000 Da the fragments were essentially inactive and above 3500 Da they had the same activity as does commercial heparin. Furthermore, we examined the five major tetrasaccharides of heparin and found that the degree of sulfation did correlate with the ability to inhibit complement. These studies have for the first time begun to examine the minimal structural requirements for heparin to regulate complement.