Physiology 443 



which may be satisfied by cholesterol or certain other sterols. Among 66 

 different sterols tested on Trichomonas gallinae (T. cohimhae), com- 

 parable activity was shown by cholesterol, cholestanol, sitosterol, and 

 several others. Ergosterol was moderately active if not heated, whereas 

 irradiated ergosterol ("vitamin D") was inactive (47). Cholesterol also 

 seems to be required by Entamoeba histolytica (530) and is a possible 

 requirement of Trichomonas vaginalis (537). Growth of Colpidium cam- 

 pylum is slower with certain concentrations of cholesterol but reaches a 

 greater density than in the control medium (541). 



Hematin. That species of Trypanosojua and related flagellates need 

 blood in culture media was first noted many years ago. Later on Salle 

 and Schmidt (498) found that, for Leis/imania tropica, blood could be 

 replaced by hemoglobin, which they suggested as a probable growth- 

 factor. This question has been investigated extensively by M. Lwoff (349, 

 366), who has shown that certain Trypanosomidae can grow in ordinary 

 peptone media while others require supplementary blood or a more active 

 substitute, hematin (Table 8. 3). The latter are unable to synthesize 

 porphyrin groups in the production of cytochrome, cytochrome oxidase, 

 and related enzymes (347). Strigomonas fasciculata apparently can com- 

 bine iron and exogenous protoporphyrin to produce heme (341), On 

 the other hand, certain Trypanosomidae and free-living Protozoa con- 

 taining the cytochrome system apparently can synthesize porphyrins from 

 simpler materials. 



Vitamin B^n (cyano-cobalamin). It is interesting that the first evidence 

 for protozoan requirements has been obtained with a phytoflagellate. 

 Vitamin Bjo, or "cyanocobalamin" (265), tremendously stimulates growth 

 of Englena gracilis var. baciUaris in the presence of adequate thiamine 

 and is believed to be an absolute requirement (230). These findings have 

 extended earlier observations (225) that heavy growth of E. gracilis de- 

 pends upon certain factors present in crude casein. This growth-response 

 of E. gracilis has been applied to microbiological assay of cyano-cobalamin 

 (230, 579). 



Protogen. A previously undefined "Factor II," a concentrate of natural 

 origin essential for Tetrahymena pyriformis (92), has been resolved into 

 fractions IIA and IIB (540). The name, protogen, was proposed for 

 Factor IIA, which is not identical with any known vitamin or with the 

 "animal protein factor." Protogen, which may prove to be a fundamental 

 requirement of animals, is unique as the first vitamin to be discovered 

 throvigh the study of protozoan growth requirements. The search for 

 natural sources of protogen will be facilitated by the ability of T. pyri- 

 formis to digest complex foods as well as by its growth in media suitable 

 for assays. 



BiosyntJiesis of vitamins. The synthesis of vitamins by Protozoa has 

 been suggested occasionally, but specific evidence has been presented in 



