Abstract
Low-temperature EPR spectra of chromaffin granule membranes from bovine adrenal medulla reveal 3 different signals of the ferrie cytochrome b-561. A typical x signal of a low-spin cytochrome observed at g ∼ 3 is compromised of a high-potential component with g z = 3.14 and a low-potential one with g x = 3.11, the 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.
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