Carbon monoxide (CO) is a cytoprotective endogenous gas that is ubiquitously produced by the stress response enzyme heme-oxygenase. Being a gas, CO rapidly diffuses through tissues and binds to hemoglobin (Hb) increasing carboxyhemoglobin (COHb) levels. COHb can be formed in the erythrocytes or in plasma from cell-free Hb. Herein, it is discussed whether endogenous COHb is or is not an innocuous and inevitable metabolic waste product and it is hypothesized that COHb has a biological role. In the present review literature data is presented to support this hypothesis based on two main premises: (i) there is no direct correlation between COHb levels and CO toxicity; and (ii) COHb seems to have a direct cytoprotective and antioxidant role in erythrocytes and in hemorrhagic models in vivo. Moreover, CO is also antioxidant by generating COHb, which protects against the pro-oxidant damaging effects of cell free Hb. Up to now COHb has been considered as a sink for both exogenous and endogenous CO generated during CO intoxication or heme metabolism, respectively. Hallmarking COHb as an important molecule with a biological (and eventually beneficial) role is a turning point on CO Biology research, namely in CO intoxication and CO cytoprotection.
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