ANALOGS OF THIAMINE 529 



tually be utilized in thiamine synthesis. Supporting the importance of a 

 pyrithiaminase in resistance is the observation by Woolley (1944 a) that a 

 pyrithiamine-resistant strain of Endomyces vernalis, obtained by subcul- 

 turing in increasing concentrations of the analog and capable of withstand- 

 ing 25 times the concentration initially depressing growth 50%, contains 

 such an enzyme. Indeed, pyrithiamine is capable of stimulating growth in 

 the absence of thiamine since the pyrimidine portion (which is all that is 

 required by this organism) is provided by the splitting reaction. However, 

 destruction of pyrithiamine is not the only factor involved, since enough 

 of the analog remains unsplit to inhibit completely the parent strain. This 

 enzyme may be functional in the pathway biosynthesizing thiamine, which 

 would be the reason for the correlation with thiamine requirements. It might 

 also be well to consider another possible mechanism of inhibition, a block 

 of the transport of thiamine into the cells; only those organisms requiring 

 intact thiamine would be susceptible. A strain of S. aureus adapted to py- 

 rithiamine exhibits a variety of changes: the pigment color changes from 

 orange to lemon yellow, glucose utilization is severely depressed, and ace- 

 tate utilization is increased (Das and Chatterjee, 1962). A partial blocking 

 of the pentose-P pathway was also observed. These results indicate the 

 complex alterations occurring during the development of resistance. 



Some of the effects on microorganisms will be briefly summarized, since 

 most of this work has no direct bearing on the mechanism of inhibition. 

 In most cases the growth depression by the analogs is counteracted by thia- 

 mine, as in the inhibition of Neurospora crassa by oxythiamine (Sankar, 

 1958), and, at least in some cases, the inhibition is formally cimpetit"'e 

 with thiamine (Quesnel, 1956). Growth depression can depend on various 

 factors. For example, Phycomyces blakesleeanus becomes more resistant to 

 pyrithiamine with culture age, the concentration required for 50% inhibi- 

 tion being 8 times greater at 13 days than at 4 days (Fluri, 1959). Is this 

 due to an alteration of thiamine metabolism with age, or to different me- 

 tabolic requirements for thiamine ? No change in sensitivity to oxythiamine 

 with age was noted. Furthermore, oxythiamine seemed to induce a thiamine 

 deficiency, determined by changes in carbohydrate content, whereas pyri- 

 thiamine did not. Euglena gracilis occurs in a normal green form and a 

 white form (chlorophyll-deficient from streptomycin treatment): The white 

 form is about 5 times more sensitive to pyrithiamine than is the green form 

 (Schopfer and Keller, 1951). Thiamine analogs have been considered as 

 possibly useful in certain infections. The growth of Microsporum audouini 

 is very strongly inhibited by 0.0012 mM pyrithiamine and the use of the 

 analog in tinea capitis was suggested (Ulrich and Fitzpatrick, 1951). The 

 infection of wheat with leaf rust (Puccinia) might be controlled with oxy- 

 thiamine inasmuch as this substances exerts a selective action on the fungus 

 when isolated infected leaves are tested (Samborski and Forsyth, 1960). 



