Characterization of a folate receptor in parotid gland and a folate binding protein in saliva from humans. Epitope relatedness to human milk folate binding protein.

The present study was performed to establish the antigenic identity and origin of the folate binding protein in human saliva. We identified a folate receptor in human parotid and submandibular gland which immunoreacted with antibodies against human milk folate binding protein, as evidenced by ELISA and immunostaining of ductal epithelium and secretory glandular material. The receptor concentration was 0.4-1.4 nmol 3H-folate bound/g protein. Ligand binding was of a high-affinity (K=10(10) M(-1)) type, exhibited positive cooperativity, a slow radioligand dissociation at pH 7.4, and inhibition by folate analogues. The concentration of immunoreactive folate binding protein in saliva as determined by ELISA with antibodies against human milk folate binding protein was several fold higher than that determined by radioligand binding (nil - 1 nM). This indicates that a major fraction of the immunoreactive material does not bind 3H-folate, and could represent a precursor form of the protein. In conclusion, the folate binding protein in human saliva seems to be a secretory product of the salivary glands. The protein is also epitope-related to folate binding proteins in other human mucosal secretions.