Haptoglobin binds haemoglobin so firmly that there is practically no dissociation. It would be expected that the heat of the reaction would be relatively large. The development of the microcalorimeter by Benzinger offered the opportunity to measure the heat of reaction. The experiments were carried out in the Beckman 190B Microcalorimeter in two ways: (1) a constant amount of haptoglobin (Kabi; 65mg.) with different amounts of haemoglobin, and (2) a constant amount of haemoglobin (32·5mg.) with different amounts of haptoglobin. The proteins, each in 5ml. of 0·15m-phosphate buffer, pH7·4, were placed in equal-volume calorimeter cells. The heat produced/mg. of haemoglobin was calculated from the slope of the curve for a constant amount of haptoglobin and from the maximum heat for a constant amount of haemoglobin. This heat is about 70kcal./mole at 37°. ΔH varies with temperature, being −70·2 at 37°, −29·7 at 20° and 7·2 at 4°. From the amount of haptoglobin required to attain maximum heat with 32·5mg. of haemoglobin and the amount of haemoglobin required to attain maximum heat with 65mg. of haptoglobin, it appears that at excess of haptoglobin there is competition between the reactions of 2moles of haptoglobin with 1mole of haemoglobin (or 2 αβ-chains) and 1mole of haptoglobin with 1mole of haemoglobin.

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