442 Physiology 



reduced the density of populations (283), evidence that this vitamin is 

 essential for Tetrahymena pyriformis is still lacking (290). However, it 

 seems to be important in the metabolism of malarial parasites (535, 556). 



Pteroylglutamic acid and p-aminobenzoic acid. In the first report on 

 Protozoa, Kidder (275) found this vitamin essential to growth of T. 

 pyriformis. Calculated on the basis of free pteroylglutamic acid, the 

 vitamin has about the same activity as it conjugates, pteroylglutamylglu- 

 tamic acid and pteroylhexaglutamic acid (289). Apparently, p-amino- 

 benzoic acid cannot be substituted for folic acid. This ability to use 

 conjugates and the holozoic nature of T. pyriformis suggest the probable 

 value of this ciliate in assays of natural products. 



The action of sulfadiazine against P. gaUinaceum in chickens is re- 

 versed by pteroylglutamic acid (166); p-aminobenzoic acid has the same 

 effect on sulfonamides used against P. lopliurae (520) and P. gaUinaceum 

 (376). Some of these sulfonamides, such as sulfanilamide (68, 562) and 

 sulfathiazole (562), inhibit oxygen consumption of malarial parasites. In 

 addition to the evidence obtained with analogues, growth of P. knozolesi 

 is stimidated in vitro by p-aminobenzoic acid (8, 11). For the phytoflagel- 

 lates, a reversal of sulfanilamide action by p-aminobenzoic acid has been 

 reported in Polytojuella caeca (359). 



Nucleic acid derivatives. Ribonucleic acid contains certain purines 

 (adenine, guanine), pyrimidines (cytosine, uracil) and D-ribose; in de- 

 soxyribonucleic acid, uracil is replaced by thymine and D-ribose by d-2- 

 desoxyribose. Several nucleic acid derivatives have been tested on Tetra- 

 hymena pyrijorynis (284). Together with folic acid, the purines (guanine 

 apparently being essential) form the active components of 'Tactor I," an 

 undefined concentrate previously found essential to growth of T. pyri- 

 formis (92). Although adenine and hypoxan thine show a guanine-sparing 

 action (286), neither can replace guanine. However, the inhibitory action 

 of an adenine analogue (adenazolo) on growth of T. pyriformis is re- 

 versed specifically by adenine (293). Among various substituted purines, 

 1-methyl-guanine is about 75 per cent as active as guanine, several are 

 inert, and others are inhibitory (291). 



'Tactor III," another concentrate which appeared necessary to growth 

 of T. pyriformis, has been resolved into the pyrimidine derivatives, uracil 

 and cytosine, or their ribosides or ribonucleotides (281, 291, 293). T. 

 pyriforjnis is believed to synthesize thymine from non-pyrimidine precur- 

 sors in reactions involving pteroylglutamic acid (293), as reported previ- 

 ously for bacteria (539). 



Ascorbic acid. Although a need for this vitamin has been attributed to 

 Haematococcus pluvialis and several parasitic flagellates (Table 8. 3), 

 there is at present no conclusive evidence that ascorbic acid is essential 

 to growth of Protozoa. 



Sterols. Several flagellates (Table 8. 3) require sterols, a requirement 



