SUMMARY: A chemically defined culture medium was used to study the effect of Parified lactoperoxidase and thiocyanate on the growth of several cultures of and While not inhibited by either component alone, growth was completely inhibited when both components were present in the medium. The growth inhibition was annulled completely by glutathione, thioglycollic (mercaptoacetic) acid of catalase. glyceraldehyde phosphate dehydrogenase was inhibited by lactoperoxidase when hydrogen peroxide was present. The inhibition was annulled with cysteine and glutathione which suggested this dehydrogenase to be a possible site of inhibition. The inhibition was Postulated to be a peroxidatic conversion of essential enzymic sulphydryl groups to inactive disulphide groups, thus interfering with the energy metabolism up of With cultures a delay in growth inhibition up to 6 hr resulted instead of complete growth inhibition. Catalase neutralized this effect. The extent of growth inhibition was greatest in those strains which were unable to adapt to an oxidative pathway for their energy supply. In becoming independent of the fermentative pathway, the cultures were no longer as sensitive to peroxidase, thioeysnate and hydrogen peroxide. The necessity for thiocyanate in the is inhibitory system is not yet clear. Thiouracil and thiourea were ineffective replacements for thiocyanate.


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