The effect of osmotic 'shock' on the swelling pattern and respiratory control of rat-liver mitochondria

Localization of myoglobin in mitochondria: implication in regulation of mitochondrial respiration in rat skeletal muscle

Mitochondria play a principal role in metabolism, and mitochondrial respiration is an important process for producing adenosine triphosphate. Recently, we showed the possibility that the muscle-specific protein myoglobin (Mb) interacts with mitochondrial complex IV to augment the respiration capacity in skeletal muscles. However, the precise mechanism for the Mb-mediated upregulation remains under debate. The aim of this study was to ascertain whether Mb is truly integrated into the mitochondria of skeletal muscle and to investigate the submitochondrial localization. Isolated mitochondria from rat gastrocnemius muscle were subjected to different proteinase K (PK) concentrations to digest proteins interacting with the outer membrane. Western blotting analysis revealed that the PK digested translocase of outer mitochondrial membrane 20 (Tom20), and the immunoreactivity of Tom20 decreased with the amount of PK used. However, the immunoreactivity of Mb with PK treatment was better preserved, indicating that Mb is integrated into the mitochondria of skeletal muscle. The mitochondrial protease protection assay experiments suggested that Mb localizes within the mitochondria in the inner membrane from the intermembrane space side. These results strongly suggest that Mb inside muscle mitochondria could be implicated in the regulation of mitochondrial respiration via complex IV.

Lowered respiratory activity in hypo-osmotically shocked mitochondria without loss of cytochrome c

Brief exposure of rat liver mitochondria to hypo-osmotic sucrose media caused a decline in the rates of succinate and ascorbate-NNN'N'-tetramethyl-p-phenylenediamine oxidation without loss of cytochrome c when assayed in iso-osmotic media. Lowered respiration rates in mitochondria after brief exposure to hypo-osmotic media may reflect a modification of cytochrome c binding.

The effect of cytochrome c and its 'dimer' on electron transfer and energy transformation

A preparation that contained cytochrome c, mainly in the form of its ;dimer', was studied and compared with native cytochrome c with respect to its ability to support electron transfer and energy transformation in cytochrome c-depleted rat liver mitochondria. When the depleted mitochondria were titrated with either cytochrome c or the ;dimer', the extent of coupling between respiration and phosphorylation was enhanced, as manifested by an increase in the P/O ratio. The ;dimer' was relatively ineffective as an electron carrier in the respiratory system, but it was as effective as cytochrome c in reconstitution of oxidative phosphorylation in depleted mitochondria. Addition of ;dimer' to the depleted mitochondria, in the presence of a low, non-saturating concentration of cytochrome c, increased the P/O ratio without concomitant stimulation of respiration. Both cytochrome c and the ;dimer' stimulated spontaneous swelling and electron transport-driven proton translocation in depleted mitochondria. The pattern of action of cytochrome c and its ;dimer' is in accord with the assumption that they affect an early step in energy conservation.