IDENTIFICATION OF HISTIDINE-RICH GLYCOPROTEIN IN HUMAN COLOSTRUM AND MILK

Histidine-rich glycoprotein (HRG) is a 74-kD glycoprotein, originally discovered in plasma, which contains an unusually large amount of histidine (13 mol%) and proline (13 mol%). The specific functions of this protein remain unclear, although it binds (reversibly) transition metal ions such as Cu(II) and Zn(II) with high capacity (10-13 equivalent) and moderate to high affinity (k(d) = 0.2-10-mu-M). Because the bioavailability of Cu(II) and Zn(II) ions in human milk is high, we have used specific antibodies from polyclonal antisera directed against purified human plasma HRG to investigate whether this or a related protein is a component of human colostrum and(or) mature milk. Fresh human colostrum (d 1-3) and milk (d 4-120) were collected in the presence and absence of multiple protease inhibitors and EDTA. Immuno "dot" blot analyses and ELISA were developed; HRG was present in both colostrum (0.13-10-mu-g/mL) and mature milk (0.1-10-mu-g/mL). Unidentified components in colostrum and milk, however, were found to depress HRG antigenicity in these assays. Western transfer and immunoblots of denatured colostrum and milk samples analyzed by SDS-PAGE revealed the presence of an immunoreactive band at 74-78 kD, with other bands at 47 and 24 kD under both reducing and nonreducing conditions; smaller immunoreactive fragments (12-14 kD) were detected in some samples. We observed at least one additional band of immunoreactivity of greater molecular mass (> 110 kD) in colostrum under nonreducing conditions; we did not observe these bands in plasma samples. Immunoaffinity and Zn(II) affinity isolation of HRG from colostrum and milk resulted in the copurification of several associated proteins. N-terminal amino acid sequence analysis of the isolated 74-kD immunoreactive protein provided further evidence for identification as intact HRG; 13 of the first 15 residues were identical. These results demonstrate the presence of HRG, or a structurally related protein, in human colostrum and milk.