Reductive nitrosylation of the cardiolipin-ferric cytochrome c complex.

Native horse heart cytochrome c (cytc) displays a very low reactivity toward ligands and does not exhibit catalytic properties. However, upon bovine cardiolipin (CL) binding, cytc achieves myoglobin-like properties. Here, NO binding to CL-cytc(III) between pH 7.2 and 9.5, at 20 °C, is reported. At pH 7.2, CL-cytc(III) undergoes reversible nitrosylation, whereas between pH 7.9 and 9.5 CL-cytc(III) undergoes irreversible reductive nitrosylation leading to the formation of CL-cytc(II)-NO. Over the whole pH range explored, first-order kinetics of NO binding to CL-cytc(III) (k = 9.3 s(-1) ) indicates that ligand binding is limited by the cleavage of the weak heme-Fe distal bond. Between pH 7.9 and 9.5, nitrosylated CL-cytc(III) converts to the ligand-free ferrous derivative (CL-cytc(II)), this process being pH-dependent (hOH-  = 3.0 × 10(3) M(-1) s(-1) ). Then, CL-cytc(II) converts to nitrosylated CL-cytc(II), in the presence of NO excess. The value of the second-order rate constant for CL-cytc(II) nitrosylation is essentially pH-independent, the average value of lon being 1.4 × 10(7) M(-1) s(-1) . These results agree with the view that CL-cytc nitrosylation may play a role in apoptosis regulation.