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1. Iron protoporphyrin IX was required for the growth of H. influenzae. It could be replaced by protoporphyrin IX. When grown on protoporphyrin evidence was obtained for the presence of Fe porphyrin in the organism. It was concluded that the organism could insert iron into the protoporphyrin ring. 2. In the smooth strains, other porphyrins containing no iron such as deutero-, hemato-, meso-, and coproporphyrins could not replace protoporphyrin for growth. Since protoporphyrin has two vinyl groups which other porphyrins lack, it was concluded that the two vinyl groups were essential for growth. 3. When porphyrins lacking vinyl groups were converted chemically into iron porphyrins and then supplied to the organisms it was found that these iron porphyrins supported growth. It was concluded that the "smooth" organisms were able to insert iron only into the porphyrin containing the vinyl groups; i.e., protoporphyrin. One function of the vinyl groups then was to permit iron to be inserted biologically into the porphyrin ring. 4. An anomalous behavior in the rough Turner strain was observed and discussed. This organism was able to insert iron into mesoporphyrin at low concentrations but was inhibited by this compound at higher concentrations. In all other reactions with the porphyrins this rough strain behaved in the same was as did the smooth strains. 5. All strains which were grown on iron porphyrins lacking vinyl groups could not reduce nitrate to nitrite. When grown on protoporphyrin or Fe protoporphyrin reduction of nitrate occurred. It was concluded that the nitrate-reducing mechanism required the presence of the vinyl groups either for its formation or function. 6. The porphyrins lacking iron and lacking vinyl groups inhibited the growth of H. influenzae on Fe protoporphyrin. The inhibition between a porphyrin and Fe protoporphyrin was a competitive one. It was suggested that the porphyrin inhibited the growth-promoting properties of Fe protoporphyrin by attaching on to a particular apoprotein, thus preventing the formation of a heme catalyst. Likewise, competition between two growth-promoting Fe porphyrins for apoenzymes could be shown to occur. 7. Protoporphyrin and Fe protoporphyrin supported growth. When their propionic acid side chains were esterified they no longer supported growth. It was suggested that the esterified carboxyl groups could not attach to the specific apoproteins to form the heme enzymes and so could not act to support growth. For the same reason the inhibitory action of porphyrins lacking vinyl groups could be prevented by esterifying their propionic acid groups.

THE PORPHYRIN REQUIREMENTS OF HAEMOPHILUS INFLUENZAE AND SOME FUNCTIONS OF THE VINYL AND PROPIONIC ACID SIDE CHAINS OF HEME