Fatty acids are metabolised by mitochondria in a series of cyclic enzymatic reactions known as mitochondrial fatty acid  beta-oxidation (FAO). Deficiencies in FAO can result in metabolic disorders characterized by liver dysfunction, cardiomyopathy and rhabdomyolysis. Interestingly, secondary defects in mitochondrial oxidative phosphorylation (OXPHOS) have been observed in some FAO patients, however it is not understood why this occurs. Recently, physical interactions between FAO and OXPHOS protein complexes have been described. These interactions may be important for both FAO and OXPHOS complex stability and activity, however the mechanisms involved are unknown.
We are characterizing native FAO and OXPHOS complexes in mitochondria from patients with FAO deficiencies to better understand how these complexes interact. In fibroblast mitochondria from a patient with medium chain acyl-CoA dehydrogenase (MCAD) deficiency, steady-state levels of the MCAD protein were reduced to only 3%. Using blue native (BN)-PAGE and Western blotting, we also indentified reduced steady-state levels of the OXPHOS complexes I (52% of control) and IV (48%) in MCAD patient mitochondria. This suggests that MCAD plays a role in stabilizing OXPHOS complexes I and IV, possibly via an interaction with the OXPHOS supercomplex as previously described¹.
We are now performing in vitro mitochondrial import assays, coupled with Blue Native (BN)-PAGE analysis, to examine whether loss of MCAD can disrupt the assembly of OXPHOS complex I structural subunits into the mature complex. These experiments will help us to define the native FAO-OXPHOS protein complex interactions that are crucial for the activity of each pathway and also how disruption of these interactions contributes to disease pathogenesis.