SUMMARY: All strains of and tested, were found to contain a blue protein; this has been named ‘Azurin’. It has been extracted from the cells of all three species and it has also been isolated from culture supernatant fluids of . The azurin was purified by chromatography on ion-exchange cellulose derivatives and obtained in physically pure form; the materials from each species were identical. In one strain of azurin comprised 0·1% of the bacterial dry weight. Azurin is a water-soluble autoxidizable protein of high oxidation-reduction potential (+395 mV.). It has an absorption maximum at 625 mμ in the oxidized form, with a millimolar extinction coefficient of 3·5 × 10. It contains 0·45% copper which can be completely removed by dialysis against cyanide. The molecular weight is 14,600 ( = 1·58 × 10, = 10·6 × 10). Azurin can be reduced by cysteine, glutathione and other reducing agents; the blue colour disappears on reduction. Azurin undergoes reduction in the presence of concentrated cell-free extracts of and succinate, and is reoxidized on aeration.


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