The generation of an organic free radical in substrate‐reduced pig kidney diamine oxidase‐cyanide

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Abstract
When the cyanide complex of the copper protein, pig kidney diamine oxidase, is reduced anaerobically by cadaverine (1,5-diaminopentane), the broad, 480 nm absorption band characteristic of the resting enzyme is bleached and a new absorption spectrum with features at 457, 429, 403 (shoulder), 360 (shoulder) and 332 nm appears. Concomitantly, the EPR spectrum of the enzyme Cu(II)-CN complex decreases in intensity and a new signal is observed that is attributable to an organic free radical. The g values and hyperfine splittings are similar to those previously assigned to a free radical observed when the cyanide complex of lentil seedling diamine oxidase is reacted with the substrate p-dimethylaminomethylbenzylamine [(1984) FEBS Lett. 176, 378–380]. The optical absorption and EPR spectra of the organic radical observed in both proteins are consistent with the same semiquinone-type structure, as expected if pyrroloquinolinequinone (PQQ) is the bound cofactor found in both enzymes.