Peroxidized coelenterazine, the active group in the photoprotein aequorin.

The photoprotein aequorin emits light by an intramolecular reaction when Ca2+ is added under either aerobic or anaerobic conditions. Previously reported evidence has indicated two possibilities: (i) the functional group of aequorin is coelenterazine itself, a compond that plays key roles in the bioluminescence of various other types of organisms, or (ii) it is the enolized form of this compound. Present data rule out both of these possibilities, through elucidation of the structure of the yellow compound that is split off aequorin by treatment with NaHSO3. The yellow compound is now shown to be a tertiary alcohol of coelenterazine on the basis of chemical reactions, mass spectral data, and relationships to known derivatives of coelenterazine. From this structure and the method of forming the yellow compound from aequorin, aequorin evidently contains a peroxide of coelenterazine as the active group. The presence of such a peroxide is consistent with the fact that aequorin yields free coelenterazine upon treatment with Na2S2O4. Although there is no applicable technique at present to determine with assurance the specific state of the peroxide in the protein, a study with 18O tracer indicates that a linear peroxide structure is more likely than the alternative possibility of a dioxetane structure.