Abstract
Escherichia coli B was shown to contain a pigment with a single symmetrical absorption band at 503 nm, which cannot be attributed to a cytochrome. The absorption of tetrahydroporphyrin in vitro closely resembled that of P-503 in intact cells. The compounds which rendered P-503 colorless, such as cyanide, azide, hydrazine, thiocyanate, hydroxylamine, dithionite, sulfite, and methylcyanide, also rendered tetrahydroporphyrin colorless. The pigment was present when the cells were grown aerobically or anaerobically in glucose minimal medium, or aerobically in either lactate or succinate minimal medium, but the pigment was not found in cells grown in complex media or in minimal media supplemented with methionine. A model is presented to suggest the involvement of methionine in the conversion of coproporphyrinogen to protoporphyrin. A variety of evidence suggesting that the 503-nm chromophore is in kinetic equilibrium with flavoprotein is discussed. However, it is not a participant in main line respiration, as its rate of reduction upon exhaustion of oxygen was too slow, and the rate of respiration in resting-cell suspensions was independent of P-503 concentration.