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High and Low Energy States of Cytochromes

I. IN MITOCHONDRIA

From the ¹ From the Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19104

Methods for the observation of reversed electron transfer reactions by a quick freezing, trapped steady state method are described. Adenosine triphosphate-activated cytochrome oxidation in the trapped steady state is comparable to that observed at room temperature.

Cytochrome oxidation by ATP activation of reversed electron transfer in terminally inhibited mitochondria containing endogenous substrate shows difference spectra of recognized components of the respiratory chain; no unusual compounds are observed.

Supplements of oxalacetate or malonate to terminally inhibited mitochondria show on ATP addition not only oxidation of cytochromes in reversed electron transfer, but the formation of an absorption band between those of cytochromes b and c, here interpreted as the formation of the compound b₅₅₅.

Addition of uncoupling agents to terminally inhibited mitochondria containing endogenous substrate causes the decomposition of a new compound which has been tentatively designated as cytochrome b₅₅₅, on the basis of (a) the positions of the and bands at 77° K, 555 and 425 mµ, respectively; (b) the high ratio of the intensities of the to the bands characteristic of cytochromes of type b; (c) the positions of the and bands (560 and 428 mµ, respectively) at room temperature; and (d) the shift of the band to a wave length shorter by 5 mµ on cooling from room temperature to a low temperature.

With the assumption that the molecular extinction coefficient and low temperature enhancement factor of cytochrome c are applicable to cytochrome b₅₅₅, the amount of this b₅₅₅ compound present in pigeon heart mitochondria can be calculated and approaches 50% of that of cytochrome c.

The b₅₅₅ compound is relatively stable, reaching a maximum concentration less than 30 sec after sulfide is added and being detectable for approximately 15 min thereafter. The compound appears at a very low concentration in the presence of highly active reductants such as succinate.

The concentration of Dicumarol required for half-maximal decomposition of the compound is 5 µm at room temperature. Other uncoupling agents are effective, for example, 2,4-dinitrophenol and m-chlorodicyanocarbonylphenylhydrazone.

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