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New evidence for a binding principle specific for folates as a normal constituent of human serum.

The binding of folate by human serum was investigated by incubating serum with [3H]pteroylglutamic acid followed by charcoal separation of free and bound fractions. Sera from all of 99 normal volunteers and 9 16 patients exhibited some degree of folate binding, ranging from less than 0.1 to greater than 8 ng/ml. This binding phenomenon was found, by competitive inhibition, to be specific for folate derivatives when we used pooled serum as well as 10 individual sera representative of the entire range of folate binding as encountered in the total of 1015 sera. Studies of the association and dissociation reactions of folate and serum indicated that the binding reaction is of the reversible type. Hence, a mass-action equilibrium must also exist in vivo between bound and free folate fractions. The binding principle appears to be protein in nature. The implications of the interference of this binder in competitive binding assays of serum folates are discussed.