Photochemical reactions of horseradish peroxidase compounds I and II at room temperature and 13 degrees K.

Some photochemical reactions of horseradish peroxidase compounds I and II (HRP-I and HRP-II, respectively) have been studied by electronic absorption spectroscopy over the temperature range 297 degrees K-10 degrees K. In glassy matrices below 80 degrees K HRP-I is rapidly converted to hrp-ii when irradiated with low power white light. The native enzyme and HRP-II are not photochemically active at these temperatures with low power irradiation. At room temperature the spontaneous decay of both HRP-I and HRP-II is catalyzed by irradiation with white light. It is shown that the photolysis is dependent upon light in the region 450-320 nm. It is concluded that the HRP-I and HRP-II conformations are closely related with only a low transition energy in the presence of electrons generated by the light. The conversion of HRP-II to HRP is accompanied by large conformational changes and so is inhibited at low temperatures.