Identification of a serum glycoprotein that increased in OK-432-treated mice as hemopexin

LB, the antitumor protein whose level is increased in the serum of OK-432-treated mice, was identified as hemopexin, which is a serum beta-glycoprotein having a heme-binding capability. When incubated with heme, LB showed several absorption maxima in the ultraviolet and visible regions, and the addition of dithionite increased the extinction of the Soret band, which was shifted to a longer wavelength. The ultraviolet circular dichroism spectrum of LB exhibits a positive maximum at 292 nm and a shoulder near 280 nm. The far ultraviolet CD spectrum of LB has a prominent positive maximum at 231 nm. These spectral characteristics are identical with those of rabbit, rat or human hemopexin. In addition, the relative mobility on polyacrylamide gradient gel electrophoresis and the results of amino acid analysis ae in very good agreement with results obtained previously for rabbit and human hemopexins. Since LB has enhancing effects on the tumor cell killing and binding by OK-432-elicited macrophages, these results strongly imply that the increase of hemopexin in serum plays an important role in the host-mediated antitumor activity of certain antitumor drugs.