Binding of methylfolate and pteroylglutamic acid by the specific serum folate binder.

The binding sites for methylfolate of the FABP in serum are fully saturated in the physiological state. When the FABP of serum from pregnant subjects is stripped of its endogenous folate, it is capable of binding an average of 549 +/- 12 pg/ml (range 464 to 723) of 14C-methylfolate, compared with 736 +/- 57 pg/ml (range 491 to 1610) of 3H-PGA. When 3H-PGA is added to untreated serum, an average of 258 +/- 6 pg/ml (range 218 to 325) is bound by FABP; this binding represents sites available to PGA but not to methylfolate, as well as sites from which methylfolate is displaced by 3H-PGA. This displacement averages 71 +/- 10 pg/ml (range 9 to 142), the amount which the standard technique for measurement of the UFBC for 3H-PGA overestimates the true UFBC for PGA.