RESPIRATORY METABOLISM 383 



portion of the cytochrome molecule through a pair of sulphur atoma. 

 Lwoff calculated that each flagellate required 520,000 molecules of pro- 

 toporhyrin in order to bring the Qoo to 55, and that each organism must 

 contain about 700,000 molecules of protoporphyrin before division 

 would take place. 



Although cytochrome C was inefi^ective alone, the action of proto- 

 porphyrin was increased when cytochrome was present (A. Lwoff, 1936) . 

 A. Lwoff (1933) found that the organisms had absorption bands at 

 555 and 530 mp, and this indicated the presence of cytochrome. There- 

 fore we may conclude that protoporphyrin is necessary for the building 

 of ( 1 ) cytochrome or ( 2 ) cytochrome oxidase. Since cytochrome C can- 

 not be substituted for protoporphyrin, it seems as if the reaction proto- 

 porphyrin -^ porphyrin C is irreversible and that protoporphyrin is neces- 

 sary for the synthesis of somethmg other than cytochrome — probably 

 cytochrome oxidase. On the assumption that all of the iron is used to 

 build the respiratory enzyme, we may calculate the rate of catalysis: one 

 gramatom of iron at 28° carries 4.83 grammolecules of O. per second. 

 For yeast, Warburg obtained a value of 100. Therefore, on the basis of 

 these assumptions, it seems as if the respiratory enzyme of yeast is 20.8 

 times as active as that of Sff/gon/onas. It has been demonstrated by M. 

 Lwoff (review, A. Lwoff, 1938) that hematin is necessary for the 

 growth of 5. 7nuscidaYum, S. cuUcidaruni var. anophelis, L. tropica, L. 

 donovani, L. agamae, L. ceramodactyli, and Schizotrypanum cniz'i, as well 

 as for the organisms discussed above. The mechanism of its action has 

 not been intensively studied, but presumably it may serve a purpose 

 similar to that it serves in 5. jasckulata. 



We may conclude from the above experiments that protoporphyrin is 

 necessary for the normal metabolism and growth of Strigomonas jascicu- 

 lata and that the organism is not capable of its synthesis. Therefore proto- 

 porphyrin may be considered a vitamin. Comparable examples are known 

 for other organisms: e.g., cholesterol for Trichomonas columhae, T. 

 foetus, and Eutrichomastix coluborum; aneurine (vitamin Bj) for 

 Glaucoma piriformis, S. oncopelti, S. fasciculata, S. culicidariim, cer- 

 tain bacteria, and fungi; pyrimidine and thiazol (parts of the aneurine 

 molecule) for Polytomella caeca and Chilomonas Paramecium; ascorbic 

 acid for Schizotrypanum cruzi; and lactoflavine, nicotinic acid, and 

 phospho-pyridine-nucleotides for various bacteria. In all cases where the 



