Exogenous and endogenous cytochrome c

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Abstract
The absorption spectrum of the ferricytochrome c-cyanide complex, given by Horecker and Kornberg for the visible region, has been extended to the u.-v. region and was found to be substantially different from that of ferricytochrome c itself. The complex is stable in all but very acid solns. (pH 2.5). Spectro-scopic observations showed that the cytochrome c as present in the Keilin and Hartree heart-muscle prepn. (endogenous cytochrome c) does not combine with cyanide at physiological pH. Cytochrome c, as present in the heart-muscle mince, can be transformed into the soluble exogenous form by extraction with salt solns. By removing cytochrome c from a water-washed heart-muscle mince with phosphate buffer a cytochrome c-deficient heart-muscle prepn. was obtained. Experiments with this preparation showed, in agreement with earlier results, that exogenous cytochrome c is catalytically much less active than the endogenous form. The transformation of endogenous cytochrome c into the exogenous form is reversible; when the exogenous pigment is again incorporated into the colloidal particles of the heart-muscle prepn. by suitable means, it then assumes the characteristic properties of the endogenous form. The relation of the spatial arrangement of the respiratory pigment to the activity of cytochrome c was discussed. It is suggested that endogenous cytochrome c is bound to the colloidal carrier particles through a linkage which cannot be disrupted without causing profound changes in the catalytic and other properties of endogenous cytochrome c.
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