Charge heterogeneity of macrophage migration inhibitory factor (MIF) in human liver and breast tissue

Macrophage migration inhibitory factor (MIF) is an ubiquitous protein playing various immunological and hormonal roles. Theoretical electrophoretic coordinates calculated from protein sequence in the SWISS‐PROT database (AC P14174) are 12 kDa and p I 8.24. Using two‐dimensional (2‐D) immunoblotting, we have detected isoelectric forms at ca. 11.9 kDa, with p I values of 7.8 and 6.98 in human liver tissue, breast tissue and a cell line and in preparations of human MIF expressed in E. coli. This evidence suggests that MIF charge heterogeneity originates from a post‐translational modification not requiring euka‐ryote‐specific enzymes. We have also detected in human liver a minor immunoreactive spot at p I 6.23, which coincides with the MIF spot in the liver map in SWISS‐2DPAGE. The p I 6.23 isoform also conceivably derives from post‐translational modification, as MIF is known to be encoded in the human genome by a single copy gene.