Affinity and Rate Constants for Interactions of Bovine Folate-Binding Protein and Folate Derivatives Determined by Optical Biosensor Technology. Effect of Stereoselectivity

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Abstract
The interactions between bovine folate-binding protein (FBP) and different folate derivatives in pure diastereoisomeric forms were studied at pH 7.4 by a surface plasmon resonance technology (Biacore). The results show that folic acid had the most rapid association rate (ka = 1.0 × 106 M-1 s-1), whereas (6S)-5-HCO-5,6,7,8-tetrahydrofolic acid had the most rapid dissociation rate (kd = 3.2 × l0-3 s-1). The equilibrium dissociation constant (KD), calculated from the quotient of kd/ka, showed that the two forms of folates not occurring in nature, that is, folic acid and (6R)-5-CH3-5,6,7,8-tetrahydrofolic acid, had the highest affinities for FBP, 20 and 160 pmol/L, respectively. The results thus show that there were great differences in the interactions between folate-binding protein and the major forms of folate derivatives. The nutritional implications of these differences are discussed. Keywords: Folate-binding protein; optical biosensor; rate constant; equilibrium constant; folic acid; 5-methyltetrahydrofolate diastereomers