Reports from this laboratory have shown that urocanase of Pseudomonas putida is photoactivated by UV light. In the present study, an action spectrum for photoactivation of purified inactive enzyme was determined. This spectrum shows a major peak at 275 nm and a secondary peak at 320 nm. Reciprocity of time and dose rate was obeyed within the limits tested. The absorption spectrum of α-ketobutyrate, the coenzyme of urocanase, has some resemblance to the action spectrum. Therefore, it is proposed tentatively that the coenzyme is the chromophore for photoactivation. When active urocanase was irradiated with light in the range from 240 to 310 nm, the enzyme was inactivated, the action spectrum showing a peak at 275-280 nm. Inactivation was not found above 320 nm. This spectrum corresponded closely with the absorption spectrum of a simple protein, suggesting that tyrosine or tryptophan may be the chromophore for inactivation. Urocanase has characteristics which could be relevant to enzymatic photoregulation by sunlight.

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