of the outer membrane (TOM). These channels allow a controlled passage of large
molecules, including proteins, across the membrane. In contrast, the inner membrane
is a tightly sealed barrier. However, this membrane also contains proteins, such as
adenine nucleotide translocator (ANT) and the protein import translocase of the
inner membrane (TIM), which ensure that essential molecules, such as nucleotides
and proteins, can enter the mitochondrial matrix.
Normal functioning mitochondria maintain an electrochemical gradient (∆m)
across the inner membrane. The gradient is created through the efflux of H+ ions
from the matrix to the intermembrane space, resulting in a pH and voltage gradient.
The main driving force for the efflux of H+ ions is through the respiratory chain.
However, several ion channels have been identified in the inner membrane, including
the H+/K+ antiporter, the Cl-/HCO3- antiporter, the Na+/H+ exchanger and the
uncoupling proteins, which presumably also help to maintain the ion flux balance
(Brierley et al., 1994; Garlid et al., 1998). An important protein complex involved
in the regulation of the H+ ion flux is the F0F1-ATPase/H+ pump. This protein
complex normally converts ADP into ATP, and the driving force is the flux of H+ions
from the intermembrane space into the matrix. However, when the H+ gradient is
low, the protein complex can work in the reverse direction, pumping H+ ions out
from the matrix, whereby ATP is hydrolyzed into ADP.
Mitochondrial dysfunction is associated with both apoptotic and necrotic cell
death. During necrosis, mitochondrial function is compromised through the loss of
the mitochondrial membrane potential (∆m) as a result of the opening of large pores
across the inner membrane, leading to mitochondrial permeability transition. This
results in the incapacity of the organelle to synthesize ATP and provide the cells with
energy. The mitochondrial permeability transition pore (PTP) is a multiprotein
complex whose exact composition remains unclear. The complex is thought to
contain VDAC in the outer membrane, ANT in the inner membrane, the matrix
protein cyclophilin D, creatine kinase from the intermembrane space, and cytosolic
hexokinase (Beutner et al., 1998; Crompton et al., 1999). However, VDAC, ANT,
and cyclophilin D are often considered the core proteins of the pore. In addition,
members of the Bcl-2 family, Bax, Bcl-2, and Bcl-XL, have been found to co-purify
with the PTP complex (Marzo et al., 1998b; Narita et al., 1998). In apoptosis, the
mitochondrial dysfunction appears to be the result of a specific permeabilization of
the outer mitochondrial membrane to large molecules, including cytochrome c. The
loss of cytochrome c from the outer side of the inner membrane results in a
dysfunction of mitochondrial respiration, effectively blocking the electron transport
between complex III (cytochrome c reductase) and complex VI (cytochrome c
oxidase). This not only leads to a disturbance of the membrane potential across the
inner membrane but also increases the production of reactive oxygen species, resulting
in increased lipid peroxidation (Hockenbery et al., 1993; Cai and Jones, 1999).
122 GENETICS OF APOPTOSIS