Cardiolipin modulates allosterically the nitrite reductase activity of horse heart cytochrome c

Upon cardiolipin (CL) liposomes binding, horse heart cytochrome c (cytc) changes its tertiary structure disrupting the heme-Fe-Met80 distal bond, reduces drastically the midpoint potential, binds CO and NO with high affinity, displays peroxidase activity, and facilitates peroxynitrite isomerization. Here, the effect of CL liposomes on the nitrite reductase activity of ferrous cytc (cytc-Fe(II)) is reported. In the absence of CL liposomes, hexa-coordinated cytc-Fe(II) displays a very low value of the apparent second-order rate constant for the NO₂ ^--mediated conversion of cytc-Fe(II) to cytc-Fe(II)-NO (k _on = (7.3 ± 0.7) × 10^-2 M^-1 s^-1; at pH 7.4 and 20.0 °C). However, CL liposomes facilitate the NO₂ ^--mediated nitrosylation of cytc-Fe(II) in a dose-dependent manner inducing the penta-coordination of the heme-Fe(II) atom. The value of k _on for the NO₂ ^--mediated conversion of CL-cytc-Fe(II) to CL-cytc-Fe(II)-NO is 2.6 ± 0.3 M^-1 s^-1 (at pH 7.4 and 20.0°C). Values of the apparent dissociation equilibrium constant for CL liposomes binding to cytc-Fe(II) are (2.2 ± 0.2) × 10^-6 M, (1.8 ± 0.2) × 10^-6 M, and (1.4 ± 0.2) × 10^-6 M at pH 6.5, 7.4, and 8.1, respectively, and 20.0°C. These results suggest that the NO₂ ^--mediated conversion of CL-cytc-Fe(II) to CL-cytc-Fe(II)-NO could play anti-apoptotic effects impairing lipid peroxidation and therefore the initiation of the cell death program by the release of pro-apoptotic factors (including cytc) in the cytoplasm.