Plasma protein binding of riboflavin and riboflavin‐5′‐phosphate in man

Ultrafiltration studies with aqueous solutions of riboflavin (FR) or riboflavin‐5′‐phosphate (FMN) and each of the major human plasma protein fractions (in concentrations normally found in plasma) show that these flavins interact mainly with albumin. Equilibrium dialysis and ultrafiltration experiments with human plasma containing added FR or FMN, or with serum obtained after parenteral administration of FMN, show that the extent of binding of these flavins is essentially the same as in aqueous solutions containing albumin in a concentration equal to that in the plasma or serum. Both FR and FMN appear to be bound on only a single site on the albumin molecule. The FMN‐albumin association constant (3.2 × 10⁴ l./mole at 30°) is considerably larger than the FR‐albumin association constant (1.3 × 10³ l./mole at 30°). There is indication that the interaction between FR and albumin is nonionic, but that electrostatic forces contribute appreciably to the binding of FMN to albumin.