Electronically excited states during diaphorase-catalyzed benzoquinone reduction

The reduction of p-benzoquinone by the diaphorase activity of NADH:lipoamide reductase is associated with the generation of low-level chemiluminescence. The reaction proceeds by a mixed type (one- and two-electron transfer) mechanism with generation of both p-benzosemiquinone and p-benzohydroquinone. The time course of the chemiluminescence which accompanies this reaction shows two phases. Phase I develops rapidly and its termination coincides with an exhaustion of reducing equivalents (NADH). Phase II develops slowly reaching a maximum after 30 min. The association of chemiluminescence with O 2 - generation appears unlikely, whereas its association with H 2 O 2 is pointed out by the inhibitory effect of catalase. It is suggested that phase I of chemiluminescence is related to the enzymatic reduction of the quinone, whereas phase II proceeds nonenzymatically. An excited state of p-benzoquinone - probably formed in a H 2 O 2 -mediated oxidation of the quinone radical - would likely contribute to chemiluminescence.