Abstract
1. One molecule of HCN combines with the haematin iron in one molecule of ferri-eytochrome a. From spectrophotometric ana-lyses the dissociation constant of the complex was 9.2×10-5M at pH 7.4 and 20°C.
2. Under strictly anaerobic conditions, one molecule of ferrocytochrome a combines with at least two molecules of HCN, one combining through the haem iron and the other through a formyl group. The latter combination was stable in acidic media but not in alkaline media and the combination was reversed on addition of aldehydes. Cyanhydrin formation was confirmed with porphyrin a and effects of aldehyde and al-kali were also observed.
3. The absorption spectrum of ferrocyto-chrome a was markedly altered by aldehyde reagents such as hydroxylamine, hydrazine and nitromethane. The alteration was suggest-ed to be due to formation of an addition compound through a formyl group, which was reactive only in ferrocytochrome a.
4. From these findings, the nature of the interaction between cytochrome a and cyanide was schematically illustrated and ar-guments brought forward against the inter-pretation of the phenomena on the basis of the cytochrome a3 hypothesis. The possibili-ty that the reactivity of the formyl group is dependent on the redox state of haem iron in cytochrome a was also discussed.
