SUMMARY: Proteolysis of endogenous and exogenous substrates in cell-free extracts of the psychrotrophic bacterium sp. S55 has been compared. Endogenous proteins were degraded only after treatment with cyanogen bromide. The hydrolysis of exogenous proteins of high M, (i.e. casein) was optimum at alkaline pH and was stimulated by Ca, Mg, Mn and ATP. The serine protease inhibitor phenylmethylsulphonyl fluoride had no effect on ATP stimulation. Small peptides (i.e. insulin) were degraded at very high rates. This activity was optimum at slightly acidic pH and was stimulated by Ca, strongly inhibited by Mn, but not affected by ATP. Degradation of cyanogen bromide-treated cellular proteins displayed two pH optima which corresponded to the optimum pH for the degradation of insulin and casein. The characteristics of these acidic and alkaline activities were identical to those active against insulin and casein respectively. The proteases which degraded casein were much more heat resistant than those which degraded insulin.


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