In this laboratory we discovered the UV photoactivation of Urocanase in Pseudomonas putida. We demonstrated the dark reaction: sulfite addition to the bound nicotinamide adenine dinucleotide. This established a new type of photobiochemical reaction that may relate to photoreception. Our goal is to elucidate the photochemistry and protein chemistry. Associated with this is another aim, to study the structure and mechanism of this unique enzyme. In progress is a study of the role of -SH groups. When p-chloromercuribenzoate was added to the enzyme, two sulfhdryl groups reacted at once and the enzyme was inactivated. However, if NAD ion (10 microns) was added with the thiol, 60-90 percent of the initial activity was restored. Restoration of activity by NAD ion was concentration dependent and specific. This is the first demonstration for any urocanase of dependence on NAD ion and confirms the coenzyme role. It is suggested that this enzyme contains two reactive -SH groups and that an essential -SH group contributes to NAD ion binding. Another goal is to demonstrate the nature of the dark reversion in vivo. Although sulfite stimulates the dark reversion in cells, it is possible that another nucleophile participates in the reversion. Studies on this are in progress.