The thermal denaturation (TD) of unblocked (i.e. containing 1 free SH) bovine albumin (BA) has been studied as a function of pH from pH 4 to 10 by differential scanning calorimetry. Between pH 4 and pH 8, 2 peaks are observed between 30 and 60 degrees C although at some pH's these peaks overlap. At pH 8 and above, a third peak appears between 80 and 100 degrees C; these 3 transitions may correspond to partial unfolding of each of the 3 domains of albumin. At pH 8.6 peaks occur at 45, 57, and 90 degrees C with a return to baseline between peaks 2 and 3. Reheating following heating to 72 degrees C demonstrates a lack of reversibility of transitions 1 and 2. Several blocked BA's were prepared by reacting the free SH with L-cystine (to form cys-BA), iodoacetamide, or iodoacetic acid. These blocked BA's behaved similarly when undergoing TD at pH 8.6 but very differently from unblocked BA (HS-BA). With cys-BA, peak 1 appears to be shifted upward, superimposed on peak 2, with peak 3 essentially unchanged, resulting in only 2 observable peaks at 57 and 94 degrees C. We propose that dimerization through SS bond formation between free SH's of fully or partially unfolded (but not native) monomers is responsible for the lower denaturation temperature of peak 1 (Td(1)) of the HS-BA due to the stabilization of the unfolded species through coupling between the unfolding and dimerization equilibria. Thermograms at decreasing protein concns are compatible with the model as are native and SDS (reducing and non-reducing) PAGE analyses of corresponding protein samples heated to only 72 degrees C; with decrease in concn of HS-BA, Td(1) increases and corresponding gels show a decrease in SS dimer formation whereas with cys- BA, Td(1) remains unchanged and no SS dimer forms. Urea-induced denaturation at 30 degrees C (monitored by UV difference spectroscopy) of HS-BA and cys-BA shows the same pattern as that observed with TD. HS-BA undergoes 3 discrete transitions whereas cys-BA shows only 2 (i.e. the last 2 seen with HS-BA). Incubation of HS-BA with increasing concns of urea prior to heating shows a progressive decrease in Td(1) and in peak area with the eventual disappearance of peak 1; this suggests that the most thermally labile domain is also the most susceptible to urea denaturation.
