COMPETITION OF PYRIDINE AND CYANIDE TO HEME AND HEMATIN

抄録

The competitive binding of pyridine and CN^- to heme and hematin respectively was quantitatively studied through the absorption change in the visible as well as in the Soret region of the spectrum by means of electric spectrophotometer. The following results were obtained.
1. Both in heme and hematin the substitution reaction of pyridine to CN^- proceeded in two steps of typically consecutive reaction:_??_This is suggesting that the bonding of pyridine and CN^- to Fe seems to be of the same nature. Their binding to heme and hematin has been well assumed to be coordinative and the ionic bonding of CN^- seems to be unprobable though in monocyanoferri-protoporphyrin an ionic bonding of CN^- may also be suggested to be possible. But if the bonding ofCN^- in_??_is ionic, the compound should have two negative charges.
The equilibrium constants in every reaction steps were determined as follows:
_??_
The results are indicating that pyridine combines more easily to heme-Fe^II than to hematin-Fe^III, while CN^- more easily to hematin-Fei11 than to heme-Fe^II. The difference of the pK' values of the first and the second step in both of the reaction of heme and hematin were determined as 2.142 and 2.152 respectively. This means that the second step beginns after that the formation of_??_reached up to 50 per cent in the first step reaction. The dissociation curves of the first and the second step make a continuous curve without any flexion, indicating that the substitution occurs simply following the mass law of the reaction.
2. The ideal absorption figures of the intermediary compounds in their pure state, _??_could be determined based on the experimental data obtained. Their individual figures are not the mathematical means of the absorptions of both of the limiting figures;_??_they are indicated by more specific characterisation of the absorption.
A similar series of experiment was reported by Horiguchi (6) in our laboratory dealt with the competitive binding of alkali denatured globin and CN^- to heme-Fe. It was noted in his experiment also that the absorption figure of_??_was not indicated by the mathematical mean of the absorptions of_??_and that of_??_, but it was more specific.
3. Both in heme and hematin, the affinity of pyridine to Fe seems to be somewhat strengthened in the intermediary compound_??_than in_??_. As already mentioned, the difference of the pK values in the first and second steps were found to be 2.142 and 2.152. The difference of the pK values, however, are not exactly 2.0, but they were obtained somewhat in higher value. This is indicating that the affinity of pyridine to Fe is somewhat intensified in the intermediary compound of_??_.
This work was aided partly by a Grant from the Scientific Research Fund of the Ministry of Education for which we wish to thank.