Migration inhibitory factor-binding sarcolectin from human placenta is indistinguishable from a subfraction of human serum albumin.

Human sarcolectin is known as growth promoter and interferon-alpha/beta antagonist. Besides N-acetylneuraminic acid-dependent cell agglutination it also binds to a macrophage migration inhibitory factor (MIF). Several types of negatively charged carbohydrates interfere with this binding, indicating importance of a negatively charged cluster. Since human serum albumin that has very similar properties in gel electrophoretic analysis can also bind to this factor with a comparatively reduced extent, sarcolectin and albumin are compared biochemically and immunologically. Their peptide maps, generated by cleavage with cyanogen bromide and N-chlorosuccinimide, reveal no differences. The N-terminal sequences are identical up to the fourteenth position that have unequivocally been determined. Reactivities to anti-human serum albumin antibody that inhibits binding of sarcolectin to MIF are similar. Fractionation of human albumin by chromatography on hydroxyapatite yields a subfraction with increased specific activity, measured by extent of inhibition of sarcolectin-MIF interaction. It exhibits the same inhibitory capacity as a similarly derived subfraction from sarcolectin. Interestingly, rabbit and pig serum albumins are as active as human albumin to inhibit binding of sarcolectin to MIF, whereas hamster, mouse, horse and bovine albumin preparations were ineffective up to 2.5 mg/ml. Thus, sarcolectin appears to be a subfraction of human serum albumin whose functionally relevant molecular peculiarities are presently unknown. Neither treatment with organic solvents nor with lipases, but exposure to trypsin, chymotrypsin and pronase can impair sarcolectin's ability to bind MIF.