Characterization of a folate receptor in parotid gland and 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 ori gin of the folate binding protein in human saliva. We identified a folate r eceptor in human parotid and submandibular gland which immunoreacted with a ntibodies against human milk folate binding protein, as evidenced by ELISA and immunostaining of ductal epithelium and secretory glandular material. T he receptor concentration was 0.4-1.4 nmol H-3-folate bound/g protein. Liga nd 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 b y folate analogues. The concentration of immunoreactive folate binding prot ein in saliva as determined by ELISA with antibodies against human milk fol ate binding protein was several fold higher than that determined by radioli gand binding (nil - 1 nM). This indicates that a major fraction of the immu noreactive material does not bind H-3-folate, and could represent a precurs or 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 s ecretions.