Cytochrome c release from mitochondria proceeds by a two-step process

Abstract

Cytochrome c is often released from mitochondria during the early stages of apoptosis, although the precise mechanisms regulating this event remain unclear. In this study, with isolated liver mitochondria, we demonstrate that cytochrome c release requires a two-step process. Because cytochrome c is present as loosely and tightly bound pools attached to the inner membrane by its association with cardiolipin, this interaction must first be disrupted to generate a soluble pool of this protein. Specifically, solubilization of cytochrome c involves a breaching of the electrostatic and͞or hydrophobic affiliations that this protein usually maintains with cardiolipin. Once cytochrome c is solubilized, permeabilization of the outer mitochondrial membrane by Bax is sufficient to allow the extrusion of this protein into the extramitochondrial environment. Neither disrupting the interaction of cytochrome c with cardiolipin, nor permeabilizing the outer membrane with Bax, alone, is sufficient to trigger this protein's release. This mechanism also extends to conditions of mitochondrial permeability transition insofar as cytochrome c release is significantly depressed when the electrostatic interaction between cytochrome c and cardiolipin remains intact. Our results indicate that the release of cytochrome c involves a distinct two-step process that is undermined when either step is compromised.

Cardiolipin: Setting the beat of apoptosis

“…3). Complete release of cytochrome c into the inter-membrane space requires dissociation of both the hydrophobic and the electrostatic interactions between cytochrome c and CL [69]. The final release of cytochrome c from mitochondria requires additional steps in the process, consisting of the permeabilization of the outer membrane.…”

Mitochondria as therapeutic targets for cancer chemotherapy

“…It is important to note that the composition of mitochondrial lipids changes during apoptosis, with cardiolipin oxidation as a prominent feature (Sandra et al, 2005). This might facilitate the desorption of cyt c from IM (Ott et al, 2002). Of note, it appears that a growing number of MOMP-regulatory proteins are involved in lipid metabolism.…”

Mitochondria and cancer: is there a morphological connection?

“…As mentioned earlier, rearrangements of the cristae compartments is thought to increase the susceptibility to apoptosis by enhancing the ability of mitochondria to release cytochrome c (Scorrano et al, 2002). Indeed, it seems that two pools of cytochrome c exist: a minor soluble pool, which is present in the IMS and a major pool confined in the mitochondrial cristae and which binds cardiolipin (Bernardi and Azzone, 1981;Ott et al, 2002;Scorrano et al, 2002). Mitochondrial fission is thought to be necessary for the release of the membrane-attached pool of cytochrome c.…”