Oxidation of hemoproteins by collapses the cell cytoskeleton and disrupts mitochondrial respiration leading to cytotoxicity of human lung cells.

(Spn) causes pneumonia that kills millions through acute toxicity and invasion of the lung parenchyma. During aerobic respiration, Spn releases hydrogen peroxide (Spn-H O ), as a by-product of enzymes SpxB and LctO, and causes cell death with signs of both apoptosis and pyroptosis by oxidizing unknown cell targets. Hemoproteins are molecules essential for life and prone to oxidation by H O . We recently demonstrated that during infection-mimicking conditions, Spn-H O oxidizes the hemoprotein hemoglobin (Hb), releasing toxic heme. In this study, we investigated details of the molecular mechanism(s) by which the oxidation of hemoproteins by Spn-H O causes human lung cell death. Spn strains, but not H O -deficient SpnΔ Δ strains caused time-dependent cell cytotoxicity characterized by the rearrangement of the actin, the loss of the microtubule cytoskeleton and nuclear contraction. Disruption of the cell cytoskeleton correlated with the presence of invasive pneumococci and an increase of intracellular reactive oxygen species. In cell culture, the oxidation of Hb or cytochrome c (Cyt ) caused DNA degradation and mitochondrial dysfunction from inhibition of complex I-driven respiration, which was cytotoxic to human alveolar cells. Oxidation of hemoproteins resulted in the creation of a radical, which was identified as a protein derived side chain tyrosyl radical by using electron paramagnetic resonance (EPR). Thus, we demonstrate that Spn invades lung cells, releasing H O that oxidizes hemoproteins, including Cyt , catalyzing the formation of a tyrosyl side chain radical on Hb and causing mitochondrial disruption, that ultimately leads to the collapse of the cell cytoskeleton.