Photo-Reversal by Monochromatic Light of the Carbon Monoxide-Inhibited Heme Degradation Catalyzed by the Reconstituted Heme Oxygenase System

Abstract

Photo-reversal of the carbon monoxide inhibition of the heme oxygenase reaction by monochromatic light was investigated. Heme degradation in either the microsomal or the reconstituted heme oxygenase system was inhibited by CO. In both systems the extents of CO inhibition were dependent on the CO/O₂ ratio and were nearly equal at a given CO/0₂ ratio. In the reconstituted heme oxygenase reaction using a highly purified heme oxygenase preparation the relationship between the intensity of light and the degree of reversal of the CO inhibition of heme degradation as expressed in terms of ΔK/Kd was not linear, but the tentatively obtained photochemical action spectrum exhibited the peaks of reversal at about 420, 540, 570, and 640 nm and suggested the occurrence of at least two steps of CO inhibition in the overall sequence of heme degradation. One could be ascribed to protoheme and the other was supposed to be the 688 nm compound which is an intermediate locating between hydroxyheme and the biliverdin-iron complex in the sequence of heme degradation.