The fluorescence decay of human serum albumin and its subfractions.

Human serum albumin does not decay monoexponentially although it contains a single tryptophan residue per molecule. The molecular population is thus heterogeneous with respect to the tryptophan emission. The separated monomeric and dimeric molecules of this protein, as well as various fractions isolated by the procedures of Foster and his coworkers, exhibit deviations from monoexponential decay which are comparable to those of the unfractionated protein; thus, the heterogeneity in molecular population of human serum albumin persists in the various fractions. By comparing the fluorescence decay data of this protein in the presence of thyroxine with the corresponding quenching data it was found that the fluorescence of the protein does not respond uniformly to the binding for all protein molecules. Qualitatively similar behavior was found for bovine serum albumin. In view of the above, binding studies followed by fluorescence should be viewed as averages over a heterogeneous population of the molecules of the serum albumin.