SUMMARY: Suspensions of various mesophilic strains of show absorption bands attributable to a cytochrome and a green protein; there are small differences in the position of absorption maxima depending on the strain and culture medium. Both pigments have been extracted, together with flavins rich in flavinadenine dinucleotide; an electrophoretically and chromatographically pure preparation of the cytochrome has been obtained and is designated . The green protein has been termed ‘desulphoviridin’. Cytochrome is a soluble autoxidizible thermostable haemoprotein (reduced bands at 553, 525 and 419 mμ.) of low redox potential (- 205 mV.), high iso-electric point (pH>10) and containing 0·9 % Fe. Degradation studies indicate that it is a bifunctional haemato-haematin with the thio-ether haem-apoprotein links also found in cytochromes and its is approx. 13,000 (S = 14493 × 10). Spectroscopic data for various derivatives including haemin and a porphyrin derivative are recorded. Material purified to at least 94 % by cellulose and ion-exchange chromato-graphy acts as carrier in the reduction in hydrogen of sulphite, thiosulphate, tetrathionate or dithionite by detergent-treated bacterial preparations; a similar role has been demonstrated with cell-free systems which reduce sulphite, thiosulphate and tetrathionate. Benzylviologen would replace cytochrome . No preparation has been obtained showing -linked sulphate reduction; the evidence for this depends on difference spectra and competition by known sulphate antagonists. Oxidation of H or organic compounds with O has been demonstrated with these bacteria; the H/O reaction takes place fastest in an atmosphere containing 4 % O, when oxygen is frequently reduced faster than sulphate. The reaction requires the mediation of cytochrome and is probably a consequence of the autoxidizibility of . Desulphoviridin is a thermolabile, soluble, acidic porphyroprotein absorbing at 630, 585 and 411 mμ.; no metabolic function has been detected. It is stable over a limited pH range and decomposes readily, yielding a chromophoric group which fluoresces red in ultraviolet light, absorbs at 595 mμ. in neutral and alkaline solution (solution red) and at 612 mμ. in acid solution (solution blue-green). This material can be purified by chromatography on ‘Florisil’ or paper. It is very photo-sensitive and water-soluble. Its character is obscure; it may be a highly carboxylated chlorin. Spectroscopic data are recorded.


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