Biochemical mechanism of keratin degradation by the actinomycete Streptomyces fradiae and the fungus Microsporum gypseum: A comparison

Two keratinolytic organisms, the procaryote Streptomyces fradiae and the fungus Microsporum gypseum, were cultured on sterile sheep's wool in a mineral solution. The loss in substrate was recorded and the degradation products in the cultivation fluid were analyzed. In M. gypseum the key reaction was the cleaving of the substrate disulfide bridges by means of sulfite excreted into the medium. Keratin denatured by „sulfitolysis’︁ was further attacked by extracellular proteases. A typical finding was the accumulation of peptides containing S‐sulfocysteine, the product of sulfitolysis of cystine. The overall excess of sulfur was removed by oxidation to sulfite and to sulfate, which was the main and final product. In S. fradiae the degradation was faster. The results did not prove that sulfite formed and the concentration of sulfate in the medium remained negligible. Neither could cysteine desulfhydration and hydrogen sulfide excretion be demonstrated. The medium was found to contain relatively high concentrations of sulfhydryl compounds, evidently cysteine‐containing peptides. Therefore, in this microorganism, keratin was most likely denatured by the direct reduction of cystine bridges. The main product of the elimination of excess sulfur was inorganic thiosulfate, which accumulated in the medium.