Effects of alcohols on the autoxidation of cytochrome c-1

Microcalorimetric studies of the interactions between cytochromes c and c1 and of their interactions with phospholipids

Thermotropic properties of purified cytochrome c1 and cytochrome c have been studied by differential scanning calorimetry under various conditions. Both cytochromes exhibit a single endothermodenaturation peak in the differential scanning calorimetric thermogram. Thermodenaturation temperatures are ionic strength, pH, and redox state dependent. The ferrocytochromes are more stable toward thermodenaturation than the ferricytochromes. The enthalpy changes of thermodenaturation of ferro- and ferricytochrome c1 are markedly dependent on the ionic strength of the solution. The effect of the ionic strength of solution on the enthalpy change of thermodenaturation of cytochrome c is rather insignificant. The formation of a complex between cytochromes c and c1 at lower ionic strength causes a significant destabilization of the former and a slight stabilization of the latter. The destabilization of cytochrome c upon mixing with cytochrome c1 was also observed at high ionic strength, under which conditions no stable complex was detected by physical separation. This suggests formation of a transient complex between these two cytochromes. When cytochrome c was complexed with phospholipids, no change in the thermodenaturation temperature was observed, but a great increase in the enthalpy change of thermodenaturation resulted.

The interaction between heme and protein in cytochrome c1

The optical spectrum of reduced bovine cytochrome c1 at 77 K shows a fine splitting of the beta-band, which is indicative of the native conformation of the protein. At room temperature, this conformation is reflected in an absorbance band at 530 nm. The exposure of the heme of ferrocytochrome c1, investigated by means of solvent-perturbation spectroscopy, appears to be extremely sensitive to temperature and SH reagents bound to the oxidized protein. Addition of combinations of potential ligands to the isolated tryptic heme peptide of cytochrome c1 reveals that only a mixture of methionine and cysteine (or their equivalents) generates a beta-band at 77 K which is identical in shape to that of native cytochrome c1. In the EPR spectrum of a complex of ferrocytochrome c1 and nitric oxide at pH 10.5, no hyperfine splitting derived from a second ligated nitrogen atom could be detected. The results indicate that methionine and cysteine are the axial ligands of heme in cytochrome c1. The EPR spectrum of isolated ferricytochrome c1 is that of a low-spin heme iron compound with a gz value of 3.36 and a gy value of 2.04.