The weak binding reaction between folate and human serum proteins.

New evidence is presented that folic acid in serum binds with low affinity to major serum proteins. This low affinity binding is distinct from the high affinity binding by folate binding protein (FBP), a minor protein which is known to occur in serum with great quantitative variability. These conclusions are based on results obtained by equilibrium dialysis of serum containing only negligible amounts of FBP. At 4 degrees, the equilibrium value of the ratio of bound to free folate was approximately 0.81 and remained the same even wth up to 1,000 times greater than normal folic acid concentrations; however, with higher concentrations the ratio decreased progressively. These results are predicted by the laws of mass equilibria for a binding system with low ligand concentration vis-a-vis high binder concentration and low affinity between the reactants. A rough estimate of the mean affinity constant K governing this weak folate interaction with serum proteins yielded a value of 1.12 +/- 0.13 X 10(3) M-1.