[Carbon monoxide--a toxic gas and ...a signal molecule with therapeutic potential].

The toxic gas, carbon monoxide (CO), is produced endogenously during the catabolism of heme, resulting in the co-production of biliverdin, and iron (Fe2+) in equimolar amounts. This process is catalysed by heme oxygenase (HO). HO exists in three different isoforms. Inducible HO-1 may be induced by a wide variety of stimuli. HO-2 and HO-3 are constitutively expressed. The toxic effects of CO are well known. At low concentrations, CO appears to be cytoprotective. CO shares some chemical and biological properties with nitric oxide (NO), which together with some experimental findings has led to the suggestion that CO also possesses physiological functions; a role as signalling molecule has been proposed. Subsequently, it has been demonstrated that CO is a mediator in the autonomic nervous system and also in the immune system. NO and CO production appears to be reciprocally modulated by the respective synthesizing enzyme, which indicates a complicated interrelationship between the two mediators.