Acyl-CoA-induced generation of reactive oxygen species in mitochondrial preparations is due to the presence of peroxisomes.

Preparations of rat liver mitochondria, but not of brain and heart mitochondria, produce large quantities of reactive oxygen species (ROS) in the presence of palmitoyl-CoA and other long-chain acyl-CoAs. Palmitoyl-CoA inhibited respiration of rat liver mitochondria with glutamate plus malate or with succinate as substrate. However, ROS production induced by acyl-CoA was independent of respiration inhibition, as it was also observed in antimycin A- and rotenone-inhibited mitochondria and in submitochondrial particles in the absence of respiratory substrates (other than acyl-CoA). Increased ROS production by acyl-CoA in rat liver mitochondrial preparations was observed when measured in the external medium using Amplex red as a probe, but not inside mitochondria using the internal fluorescent probe MitoSOX or aconitase activity as the "intrinsic" indicator of ROS generation in the matrix compartment. Stimulation by acyl-CoA of ROS generation was higher in "light" mitochondrial preparations that were enriched in peroxisomes, as assayed by urate oxidase. It is concluded that stimulation of ROS production in preparations of rat liver mitochondria could be ascribed to contaminating peroxisomes. Preparations of rat brain and heart mitochondria were not or were much less contaminated with peroxisomes, as indicated by low urate oxidase activity.