Formation of an iso-1-cytochrome c-like species containing a covalently bonded heme group from the apoprotein by a yeast cell-free system in the presence of hemin.

Incubation of the 125I-labeled apoprotein, prepared from 125I-labeled iso-1-cytochrome c, with a yeast mitochondrial fraction in the presence of hemin, NADPH, and an extract of the postmitochondrial fraction at 32 +/- 1 degree C for 30 min has resulted in formation of cytochrome c-like species in yields of up to 35%. This radioactive synthesized species contains a functional group which responds to reduction with ascorbate and oxidation with K3Fe(CN)6 in that it is resistant in the reduced form and susceptible in the oxidized form to trypsin action in a manner characteristic of native cytochrome c. The functional group cannot be removed from the protein by cold HCl-acetone or 8 M urea treatment. The reduced form of the synthesized species exhibits resistance against autoxidation and the oxidized form can be reduced also by cytochrome b2. The synthesized species exhibits the same compact hydrodynamic volume of native cytochrome c. Treatment with silver sulfate followed by incubation with dithiothreitol converts the synthesized species to the original apoprotein as judged by an increase in the hydrodynamic volume. Thus, the synthesized species is indistinguishable from the original labeled iso-1-cytochrome c by these measurements; i.e. the synthesized species consists of the apoprotein to which heme is covalently attached through the thioether bond(s). The active factor of the mitochondrial fraction is heat-labile. The synthetic activity is strongly dependent on pH with a maximum approximately at pH 7.0. Hemin (or heme) appears to be required for this synthesis. The postmitochondrial fraction is inactive by itself. However, its addition markedly increases the synthetic activity. This factor is heat-stable, soluble in 80% methanol (or 75% ethanol), and insoluble in ethyl ether or ethyl acetate. Addition of NADP(H) (or NAD(H)) also increases the synthetic activity, the reduced form being more effective than the oxidized form. The postmitochondrial factor and the pyridine nucleotides appear to enhance the effect of each other. Thus, it seems that cytochrome c or a cytochrome c-like species is formed from the apoprotein and heme (or hemin) by an enzyme, cytochrome c synthetase, present in mitochondria.