Folic acid binding by human plasma albumin.

Binding of [3H]folate to pure human plasma albumin was studied by Sephadex G-200 gel filtration in a steady-state system. The experiments showed the presence of two binding sites per molecule of albumin and an equilibrium constant of 0.9x10(3) 1/mol. With normal human plasma, only albumin bound exogenous [3H]folate with nearly the same equilibrium constant. In human blood plasma at concentrations of less than 10(-5) mol/1, 50% of folate was free, and 50% was bound to albumin. Binding was maximal at about pH 6 and negligible above pH 8 and below pH 4.5. Neither human transferrin nor Cohn fraction II was able to bind [3H]folate.