Summary: Catalase and peroxidase were markedly decreased in quantity in molybdenum-deficient felts of Neurospora crassa. Addition of molybdenum aseptic- ally and in vivo to mats deficient in molybdenum restored the activities of the two enzymes and the yield of mycelium to the amounts in the control treatments. Infiltration of hydrogen peroxide to growing felts, deficient in molybdenum, after 3 days of growth, resulted in a partial reconstitution of catalase to 18 % and peroxidase to 50 % of the control levels after a further 2 days growth but the weight of mycelium remained unchanged. There was no positive correlation between the activity of either enzyme at various stages of purification and its molybdenum content as determined by a radioassay method. The purified enzymes were not activated by molybdenum. The effect of molybdenum deficiency on the two iron- containing enzymes is probably indirect, resulting from a decrease in the activity of molybdenum-dependent flavoproteins which produce hydrogen peroxide, the common substrate for catalase and peroxidase.
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