Thermal stability of human albumin measured by differential scanning calorimetry. II. Effects of isomers of N-acetyltryptophanate and tryptophanate, pH, reheating, and dimerization

Abstract

Thermal stability of undefatted human albumin preparations at 5% concentration and neutral pH in 145 mM Na+ was investigated by differential scanning calorimetry. At 30 and 4 mM N-acetyltryptophanate, the thermogram for previously unheated albumin monomer is independent of the stereochemistry of this ligand; thus, the affinity of the protein must be the same for the L- and D-isomers in the temperature range of thermal denaturation (62-86 degrees C). 26 mM L-tryptophanate bestows a slight increase in stability on previously unheated monomer, whereas 27 mM D-tryptophanate has no effect, a result consistent with the reported weaker binding of the D-isomer. Previously unheated albumin monomer shows slightly greater thermal stability at pH 6.4 than at pH 7.4. The tracing of differential heat capacity versus temperature (thermogram) for monomer from once heated albumin, i.e. once processed and once heated normal serum albumin (NSA), is almost identical to that for previously unheated monomer. Monomer prepared from outdated, multiply reprocessed, multiply reheated NSA (old monomer) had the same corresponding denaturation temperatures as previously unheated monomer; the corresponding dimer (old dimer) is slightly less stable. Thermograms for old monomer and old dimer, like those for previously unheated monomer, comprise two denaturation peaks (endotherms). Their endotherms, however, are very broad, reflecting the great heterogeneity of the old proteins. The thermogram for old monomer remains broad even in the presence of N-acetyltryptophanate and/or caprylate. Old dimer does not dissociate when undergoing thermal denaturation.