Human blood contains circulating cell-free mitochondria, but are they really functional?

Dache et al. (2020, FASEB J. 15, e2002338–15) recently reported the presence of respiratory-competent cell-free mitochondria in human blood (up to 3.7 x 106 per mL of blood), providing exciting perspectives on the potential role of these extra-cellular mitochondria. While their evidence for the presence of cell-free mitochondria in human blood is compelling, their conclusion that these cell-free mitochondria are respiratory-competent or functional has to be re-evaluated. To this end, we evaluated the functionality of cell-free mitochondria in human blood using high-resolution respirometry and mitochondria extracted from platelets of the same blood samples as positive controls. While cell-free mitochondria were present in human plasma (i.e. significant complex IV activity), there was no evidence suggesting that their mitochondrial electron transport system (ETS) was functional (i.e. respiration rate not significantly different from 0; no significant responses to ADP, uncoupler or mitochondrial inhibitors oligomycin and antimycin A). Yet, in vitro complex IV activity was detectable and even slightly higher than levels found in mitochondria extracted from platelets, suggesting that cell-free mitochondria in human blood only retain a non-functional part of the electron transport system. Despite being unlikely to be fully functional in the narrow-sense (i.e. capable of oxidative phosphorylation), circulating cell-free mitochondria may have significant physiological roles that remain to be elucidated.