SEVERAL FORMS OF CHROMAFFIN GRANULE CYTOCHROME-B-561 REVEALED BY EPR SPECTROSCOPY

Low-temperature EPR spectra of chromaffin granule membranes from bovine adrenal medulla reveal 3 different signals of the ferric cytochrome b-561. A typical g(z) signal of a low-spin cytochrome observed at g approximately 3 is comprised of a high-potential component with g(z) = 3.14 and a low-potential one with g(z) = 3.11, the low-potential signal showing significantly faster relaxation. In addition, a highly temperature-sensitive heme signal at g = 3.7 is observed which is fully retained in the preparation of granule membranes with b-561 reduced by 50% but disappears upon full reduction of the cytochrome by ascorbate. The signal is strikingly similar to that of the mitochondrial low-potential cytochrome b heme (b(L) or b-566). The presence of several forms of b-561 in chromaffin granule membranes may provide a structural basis for the transmembrane electron transfer believed to be catalyzed by this hemoprotein.