Mn2+-Activated Luminescence of the Photoprotein Obelin

The light emission of obelin may be initiated by Mn2+ under alkaline conditions. The luminescence takes place in a pH range from 7 to 12 with a sharp optimum at 11.75. The first-order rate constant for Mn2+-activated luminescence decay is more than 9 s−1, while that for Ca2+-activated luminescence decay is only 6.9 s−1. The Mn2+ concentration-effect curve for obelin determined with simple dilutions of manganese salt is a sigmoid curve, The slope of the curve is moderately dependent on the pH and was not more than 1 within the pH range tested. The maximal light emission, which is initiated by 3.6 × 10−5 M Mn2+ at pH 11.75 was about 10% of the maximal Ca2+-activated luminescence. Mg2+ ions inhibit the Mn2+-activated luminescence of obelin. The addition of OH. and O−2 scavengers did not influence the Mn2+-activated luminescence, but when singlet oxygen quenchers were added, the Mn2+-dependent light emission was inhibited. This suggests that the 1O2 might be formed and itself be responsible for chromophore oxidation attended with light emission. NEM and Na2S2O4 inhibit the Mn2+-initiated light emission of obelin completely, showing that endogenous hydroperoxide and SH-group(s) of the photoprotein are essential for both Ca2+-activated and Mn2+-activated light emission of obelin.