FACTORS INFLUENCING B VITAMIN REQUIREMENTS 295 



of plant materials have anti-thiamine activity, and it was at first thought 

 that a similar enzymatic principle was involved in these plants. Later, 

 however, it was found that these antithiamine effects were due to ther- 

 mostable principles, so that it now appears, but not unequivocally, that 

 an antimetabolite principle is involved. 



Bhagvat and Devi m found that chloroform-water extracts of rice 

 polishings, ragi {Eleasine coracana) , green grain {Phaseolus radiatus) , 

 mustard seed {Brassica juncea) , yellow cotton seed (Gossipum sp.), and 

 linseed (Linum usitatissi?num) have an inactivating effect on both free 

 and combined thiamine. The active factor contains a dialyzable and a 

 nondialyzable portion. Incubation for short periods of the crude material 

 or organic extract with thiamine or thiamine-containing products indi- 

 cated a loss of thiamine activity, as shown by thiochrome tests and rat 

 and pigeon assays. The product was, however, active for mosquito larvae, 

 suggesting the possibility of a chemical cleavage analogous to the bisulfite 

 cleavage, in which the fission products have activity for some species. 

 This would not involve inhibitory effects in the sense that we are con- 

 sidering them, and chemical nonenzymatic inactivation would be more 

 analogous to enzymatic destruction if this mode of action is proved to 

 exist. Unfortunately, further information upon this point is lacking. "Fern 

 poisoning," which afflicts horses and cattle that consume considerable 

 amounts of the fern Pteris aquilina, has also been shown to exert its toxic 

 effect by producing a thiamine deficiency, 174 and has indeed been proposed 

 as another way of bringing about such deficiencies experimentally. In 

 this case the highly thermostable material seems to differ from the active 

 substance in Bhagvat and Devi's work in some characteristics, and 

 although the mode of action is unknown, an antimetabolite effect seems 

 unlikely. 



A naturally occurring vitamin B 6 inhibitor has been shown to exist in 

 linseed oil meal, 174a and vitamin B 6 deficiency is rapidly induced in 

 chicks when their diet contains 30 per cent of this meal. Usual dietary 

 levels of vitamin B 6 do not affect the inhibitor, but synthetic pyridoxine 

 added in higher levels completely prevents the anti-pyridoxine activity. 

 Water pretreatment of the meal also abolishes the activity, but it does 

 not change the vitamin B (i content. 



Rat growth on a low casein diet is inhibited by threonine, and this 

 inhibition is prevented by either tryptophan or niacin. 174b Both threonine 

 and phenylalanine are said to intensify the symptoms of niacin-trypto- 

 phan deficiency in the rat. 174c 



The possible presence in synthetic vitamin preparations of impurities 

 which have inhibitory effects should be given some consideration. A 

 recent example of this was the report that folic acid inhibited the activity 



