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The first experiments in which protohaem was removed from haemoglobin and the native globin used to prepare haemoglobins containing protohaem, mesohaem and deuterohaem were reported by Hill & Holden (1926). The only reports of work on the oxygen equilibria of reconstituted protohaemoglobin which we have been able to trace appear to be those of Hill, quoted by Lemberg & Legg (1949), and of Davies, quoted by Wyman (1948). Hill found that the oxygen-dissociation curve was hyperbolic, a finding confirmned by Davies, who also found that the oxygen pressure for half-saturation was decreased to about onequarter of that required for natural haemoglobin, and that the Bohr effect was absent. More recently Rossi-Fanelli, Antonini & Caputo (1958) have prepared globin from human haemoglobin which could be coupled with protohaematin to give a product having the same absorption spectrum, sedimentation constant and electrophoretic behaviour as natural human methaemoglobin. After reduction by means of an enzyme system the oxygen-dissociation curve was the same as that for natural haemoglobin; the oxygen concentration for half-saturation, the degree of inflexion of the curve and the Bohr effect all agreed closely (Rossi-Fanelli & Antonini, 1959a). Although the evidence presented by RossiFanelli & Antonini (1959a) suggests strongly that haem-haem interaction had survived unchanged in the reconstituted protein, the form of the dissociation curve over the middle range of saturations is not, by itself, fully conclusive. Thus Roughton (1949) has pointed out that on the Adair (1925) hypothesis many sets of equilibrium constants describing the consecutive reactions of oxygen molecules with reduced haemoglobin can be found which yield dissociation curves scarcely differing from each other or from a given model over the range 10-90% saturation. Values of the individual equilibrium constants can, so far, only be obtained by exacting gasometric procedures which require prolonged training and experience of the operator (see, for example, Roughton, Otis & Lyster, 1955). Procedures have, however, been devised which will allow the determination of several kinetic constants corresponding to individual partial reactions of haemoglobin with gases. Such determinations offer not only information about haem-haem interaction similar to that embodied in the values of the individual equilibrium constants, but in addition give a further insight into the mechanism of the reactions. In this paper it is shown that reconstituted human protohaemoglobin prepared by the methods of Rossi-Fanelli et al. (1958) gives the same values for the individual kinetic constants as the natural protein in every case which we have been able to examine. Comparative measurements have also been made on haemoglobins in whichmesohaem and deuterohaem have replaced protohaem, thus extending the work already initiated by RossiFanelli & Antonini (1959b), and Rossi-Fanelli, Antonini & Caputo (1959) in the kinetic field.

Some observations on the kinetics of the reactions with gases of natural and reconstituted haemoglobins