Unexpected difference between human serum albumin and human serum toward l-tryptophan binding

The binding of l-tryptophan to human serum albumin (HSA) Fr. V in Krebs-Ringer phosphate buffer and to HSA in serum was studied by equilibrium dialysis. At constant albumin concentration (in the in vivo range) and various l-tryptophan concentrations, binding was greater in serum, with an apparent association constant (k) equal to 4.88 × 10⁴ I/mole and the number of binding sites (n) equal to 0.62. The corresponding kinetic parameters obtained at constant HSA Fr. V concentration and various l-tryptophan concentrations in Krebs-Ringer phosphate bufferwere 7.61 × 10³ I/mole and 0.28, respectively. The Scatchard plot obtained from experiments with various albumin concentrations and a constant l-tryptophan concentration had a negative slope in serum and a positive slope in Krebs-Ringer phosphate buffer.