Several cations have been shown to affect protein and porphyrin formation by the G 12/6 strain of . Cobaltous ion (Co), at concentrations only slightly inhibiting growth, caused a marked inhibition of toxin formation. This effect was not produced by vitamin B or annulled by increases of cysteine or histidine concentration. Only a small proportion of added Co, at concentrations above 0·2 μg./ml., was found in washed organisms, and the effect of Co was more marked than that of Fe when based on the weight of metal in the organisms. No appreciable proportion of the Co existed as a porphyrin complex. The growth inhibition caused by 5 μg. Co/ml. was annulled by 0·6 μg. Fe/ml. Co caused no increase in the cytochrome content of the organism, nor did it affect the iron concentration required for maximum toxin formation. The combined toxin-inhibitory effect of Fe Co was considerably less than the sum of their independent effects. The bearing of these findings on the thesis that diphtheria toxin is the apoenzyme of cytochrome is discussed.


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