BINDING OF GC GLOBULIN (VITAMIN-D-BINDING PROTEIN) TO C5A OR C5A DES ARG IS NOT NECESSARY FOR CO-CHEMOTACTIC ACTIVITY

Gc globulin (vitamin D binding protein) has been shown to augment sign ificantly the leukocyte chemotactic activity of the activated compleme nt peptides C5a and C5a des Arg, However, the mechanism of chemotaxis enhancement is not known, Natural C5-derived peptides contain a carboh ydrate side chain that comprises approximately 25% of the mass of the 11-kDa peptides, Previous studies have demonstrated that Gc globulin b inds to C5-derived peptides via sialic acid residues on this carbohydr ate side chain, The necessity of this carbohydrate side chain for chem otaxis enhancement by Gc globulin was investigated by using both natur al (glycosylated) and recombinant (carbohydrate-free) peptides, The do se-response curves of neutrophil chemotaxis to recombinant C5a or C5a des Arg plus Cc globulin were identical to those observed with the nat urally derived peptides, despite the fact that natural C5a bound to Gc globulin while the recombinant C5a failed to bind this protein, Moreo ver, neutrophils pretreated with Gc globulin then washed before additi on to the chemotaxis assay displayed significantly enhanced movement t o C5a alone, These results indicate that the binding of C5a/C5a des Ar g to Gc globulin is not necessary for their chemotactic activity to be augmented, Furthermore, these results demonstrate that the co chemota ctic activity of Gc globulin is generated on the cell surface, indepen dent of C5a binding to its receptor.