Studies on the electron transfer system. XLV. Some effects of antimycin on cytochrome b.

It has been known for some time that antimycin A modifies the enzymic properties of cytochrome b. Chance (1) reported that cytochrome b was reduced at a much faster rate in heart muscle preparation blocked by antimycin A than in a preparation blocked by cyanide; moreover, cytochrome b was reduced to a greater extent in the presence than in the absence of antimycin A. From a study of difference spectra it appeared that: (1) succinate reduced a form of cytochrome b with an or-band at 562 rnp; (2) more b-like material with an a-band at 566 rnp was reduced in the presence of antimycin A; (3) still more b-like material, with a rather diffuse band at 560 mp, could be reduced by dithionite. It was suggested that antimycin A was in some way able to stimulate the reduction of cytochrome b by flavoprotein, and that the increased rate of reduction enabled a relatively inactive form of the cytochrome having an or-band at 566 rnp to become reduced. Although cytochrome 6 is reduced fairly rapidly in some mitochondrial and submitochondrial preparations (2, 3)) its reduction in others appears to be much slower than the reduction of flavoproteins and cytochromes c and cl and a (1, 4). On this basis, it has been inferred that cytochrome b is not an essential component of the main oxidative pathway and hence is not itself the antimycin-sensitive factor. Nevertheless, the disparity between the behavior of cytochrome b in the presence, and in the absence, of antimycin was too great to be easily explained as a direct consequence of the antimycin block, i.e. as a result of inhibition of the reoxidation of cytochrome b by the next member of the chain. It looked as if antimycin induced a change in the properties of cytochrome b above and beyond its action on the antimycin-sensitive factor. In the present paper an attempt is made to clarify this problem. The effects of antimycin A on some properties of cytochrome b are described, and possible relationships between b and the antimycin-sensitive factor are discussed.