monoxide (CO) is produced in mammalian cells during heme metabolism and serves as an important signaling messenger. Here we report the bioactive properties of selective CO scavengers, hemoCDl and its derivative R8-hemoCD1, which have the ability to detect and remove endogenous CO in cells. HemoCDl is a supramolecular hemoprotein-model complex composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(II) and a per-O-methylated beta-cyclodextrin dimer having an pyridine linker. We demonstrate that hemoCDl can be used effectively to quantify endogenous CO in cell lysates by a simple spectrophotometric method. The hemoCDl assay detected ca. 260 pmol of CO in 10(6) hepatocytes, which was well-correlated with the amount of intracellular bilirubin, the final breakdown product of heme metabolism. We then covalently attached an octaarginine peptide to a maleimide-appended hemoCDl to synthesize R8-hemoCDl, a cell-permeable CO scavenger. Indeed, R8-hemoCDl was taken up by intact cells and captured intracellular CO with high efficiency. Moreover, we revealed that removal of endogenous CO by R8-hemoCDl in cultured macrophages led to a significant increase (ca. 2.5-fold) in reactive oxygen species production and exacerbation of inflammation after challenge with lipopolysaccharide. Thus, R8-hemoCDl represents a powerful expedient for exploring specific and still unidentified biological functions of CO in cells.