The preparation and some properties of mammalian cytochrome c modified with 2-hydroxy-5-nitrobenzyl bromide.

2-Hydroxy-5-nitrobenzyl bromide reacts with horse heart cytochrome c at acid pH to yield a chemically modified protein. Chromatography of the protein on CM-cellulose allows separation of a single chemically modified species. This species is shown by gel chromatography to be monomeric, and isoelectric focusing shows the pI to be lowered from 10.5 to 9.8 on introduction of the reagent molecule. The changes observed in the u.v. region of the spectrum are consistent with the introduction of a single residue of the reagent, and the normal fluorescence of tryptophan is lost. The chemically modified protein exhibits marked changes in its functional properties as compared with native cytochrome c. Unlike the native monomer, the modified cytochrome c has a pH-dependent spectrum which is typical of a high-spin species in the alpha/beta region at low pH, changing to a low-spin species with an apparent pK of 7.5. The modified protein is autoxidizable and the ferrous form binds CO at neutral pH with an affinity constant of 2.6 X 10(5)M-1. The ferrous form of the modified cytochrome c binds CN- at pH 10.0 with an affinity constant of 3.5 X 10(2)M-1. The modified cytochrome c was incapable of restoring the electron-transfer activity to mitochondria depleted of cytochrome c.