Carbon monoxide (CO) is produced during the heme catabolism by heme oxygenase. In brain or blood vessels, CO functions as a neurotransmitter or an endothelial-derived relaxing factor. To verify whether crystallographically proposed CO-trapping sites of rat and cyanobacterial heme oxygenase-1 really work, heme catabolism by heme oxygenase-1 from rat and cyanobacterial Synechocystis sp. PCC 6803 has been scrutinized in the presence of 2-propanol. If 2-propanol occupies the trapping sites, formation of CO-bound verdoheme should be enhanced. Although effects of 2-propanol on the rat heme oxygenase-1 reaction were obscure, the reaction of cyanobacterial enzyme in the presence of NADPH/ferredoxin reductase/ferredoxin was apparently affected. Relative amount of CO-verdoheme versus CO-free verdoheme detected by optical absorption spectra increased as the equivalent of 2-propanol increased, thereby supporting indirectly that the hydrophobic cavity in cyanobacterial enzyme traps CO to reduce CO inhibition of verdoheme degradation. (c) 2005 Elsevier Inc. All rights reserved.