Functional characterization of mitochondiral respiratory complex I by delayed luminescence spectroscopy

Delayed luminescence (DL) spectroscopy proved successful in several clinical applications, but its cellular origin is still unclear. Our results obtained with human leukemia Jurkat T cells challenged with Complex I targeting agents like rotenone, menadione and quercetin support the notion that DL is produced primarily within the mitochondrial electron transfer system at the level of Complex I. The data provide novel insights into the structural and functional organization of respiratory Complex I, which appears to function predominantly as a dimer and less frequently as a tetramer in this cell type. States emitting blue or green/yellow light are most likely excited states of Complex I-associated NADH or FMN, respectively, which are produced after a series of forward and backward electron transfer steps involving inner Fe/S clusters of Complex I. Furthermore, the timescales of various electron transfer steps involving the formation of flavin and ubisemiquinone radicals with subsequent production of superoxide can be estimated from delayed red-light emission.