QUENCHING OF TRYPTOPHAN PHOSPHORESCENCE IN ALCOHOL-DEHYDROGENASE FROM HORSE LIVER AND ITS TEMPERATURE-DEPENDENCE

The phosphorescence of alcohol dehydrogenase from horse liver (LADH) can be observed at room temperature. The quenching of this long-lived light emission, which comes from a tryptophan residue well buried within the interior of the enzyme structure, was measured. The rate constants for the quenching by the small O2 molecule and by the I- ion were 1.4 .times. 108 M-1 s-1 and 108 M-1 s-1, respectively, at room temperature. The temperature dependence of the quenching yields an activation energy of about 14 kcal/mol. This activation energy and the meaning of the accompanying large pre-exponential factor in the Arrhenius equation, A = 1018 M-1 s-1, are discussed in terms of a model in which the quencher threads its way through the protein network.