Binding of macrophage colony-stimulating factor to serum proteins

Although three molecular forms of macrophage colony-stimulating factor (M-CSF) have been reported, Western blot analysis of immunoaffinity-purified M-CSF from human blood has shown that the major species of M-CSF in the serum has a molecular weight (MW) of 85 kD. Superose-12 gel filtration chromatography of immunoaffinity-purified serum M-CSF showed the presence of M-CSF-positive fraction in a higher MW area compared with the elution profile of recombinant human (rh) 85-kD M-CSF. Western blot analysis of the higher MW fraction showed that the M-CSF was the same as rh 85-kD M-CSF (not proteoglycan form of M-CSF), indicating that, in the serum, M-CSF exists bound to some serum proteins. To detect the serum proteins, we performed M-CSF-bound column chromatography. The eluate contained at least three serum proteins including albumin and IgG. This result was supported by chromatography using biotinylated M-CSF and avidin-agarose. The binding between rhM-CSF and albumin or IgG was also demonstrated by high-performance liquid chromatography (HPLC) fractionation of the mixture and enzyme-linked immunosorbent assay (ELISA) of the fractions. In the serum, a fraction of M-CSF seems to be complexed with serum proteins such as albumin and IgG.