Gc globulin (vitamin D-binding protein) increases binding of low concentrations of C5a des Arg to human polymorphonuclear leukocytes: an explanation for its cochemotaxin activity.

The chemotactic activity of native human C5a des Arg is enhanced significantly by the normal serum and plasma protein Gc globulin (vitamin D-binding protein). Gc globulin attaches to sialic acid residues within the oligosaccharide chain of C5a des Arg to form a complex with potent chemotactic activity for human PMN. We investigated the mechanism whereby this phenomenon may occur and found that Gc globulin enhanced the binding of low concentrations of [125I]C5a des Arg to PMN, but had no effect on C5a-induced displacement of bound [125]C5a des Arg. Gc globulin bound to PMN, and probably acted as a C5a des Arg chaperon. Thus, it appears that Gc globulin, by complexing to C5a des Arg, increases the number of C5a des Arg molecules per unit of PMN membrane without affecting its affinity of binding. This phenomenon provides a plausible explanation for the enhancing effect of Gc globulin on the chemotactic activity of low concentrations of native human C5a des Arg.