Allosteric properties of the oxyphenbutazone--human serum albumin complex

The conformational change of albumin which occurs around physiological pH, the so-called N-B transition, has been studied by measuring the induced circular dichroic signal of the oxyphenbutazone-albumin complex. This N-B transition has been characterized by a set of parameters according to the two-state model of Monod, Wyman and Changeux. The influence of calcium ions on the N-B transition has been interpreted in terms of a change in some of the parameters describing the two-state model, viz. a decrease of the apparent pK value of the histidines and of the apparent allosteric constant of the oxyphenbutazone-albumin complex. This apparent pK change increases with increasing Ca2+ concentration, whereas the apparent allosteric constant approaches a final value at 5 mM Ca2+. From acid-base titration curves of albumin in the presence and in the absence of Ca2+ it could be concluded that in the presence of Ca2+ less histidines are titratable than in the absence of Ca2+. Assuming that these histidines are not involved in the N-B transition it follows that at least four to five histidines are involved in the N-B transition.