A Spectroscopic Study on the Intermediates of Heme Degradation by Heme Oxygenase

Summary

Heme is degraded by heme oxygenase to biliverdin with the aid of NADPH-cytochrome P-450 reductase serving as an electron donor. The process involves three monooxygenase reactions and produces one molecule of carbon monoxide. Several intermediates such as oxygenated heme and verdoheme bound to heme oxygenase are spectrally distinguishable during the degradation of heme to biliverdin. We investigated the kinetics of heme degradation by stopped-flow spectrophotometry; the conversion of heme into its oxygenated form was four times as fast as the formation of verdoheme, and its cleavage to biliverdin was three times slower than the preceding step. These results indicate that the opening of the porphyrin ring should be considered the rate-determining step in the heme degradation reaction. We also measured the redox potential of the heme-heme oxygenase complex. The potential proved to be −76mV at pH 7.0, which was far lower than those of ascorbate and cytochrome b ₅. This is consistent with the adoption of NADPH-cytochrome P-450 reductase as the physiological reducing system.