Action spectrum and quantum yield for the photoinactivation of Mnemiospin, a bioluminescent photoprotein from the ctenophore Mnemiopsis sp

Ctenophores are bioluminescent marine invertebrates closely related to the coelenterates. The isolated bioluminescent systems of the ctenophores Mnemiopsis and Beroe and the hydrozoan jellyfish Aequorea are protein-luciferin complexes (photoproteins) which flash upon the addition of Ca2+ions. The photoprotein mnemiopsin has an oxygen-independent quantum yield for photoinactivation of bioluminescence as high as 0.5, placing it among the most light-sensitive proteins known. The action spectrum for this photoinactivation has been measured at 107 narrow (3.4 nm) wavelength bands between 230 nm and 570 nm, covering a range of four decade units in the action. The action spectrum in the visible region was identical with the absorption spectrum of native photoprotein, implicating bound luciferin. The UV action spectrum implies that absorption by aromatic amino acid residues also leads to extremely efficient photoinactivation. Although photoinactivation is a rapid first-order reaction, destruction of the luciferin is a slower, multiple-order process. Therefore, protein-bound luciferin is not the ultimate target of the photoinactivation. Absorption of light resulted in the dissociation of 'active oxygen' from the photoprotein. Therefore, the ctenophore photoprotein is a pre-charged enzyme already containing bound luciferin and oxygen. (author)