292 THE BIOCHEMISTRY OF B VITAMINS 



readily diffuse out of the intact cell. In this manner a mechanical barrier 

 materially influences the availability of the thiamine. Boiling, or other 

 measures which fracture the yeast cells effectively, destroy the anti- 

 thiamine effect. In general, the other B vitamins of live yeast appear to 

 be similarly unavailable for nutritional purposes by virtue of the same 

 effect. Effects such as these are by no means limited to yeast, and empha- 

 size the desirability of obtaining information regarding the availability 

 of the B vitamins in any particular food source for any species dependent 

 upon that source. 155 * 



Natural Inhibitors and Inactivators 



It has been discovered in recent years that there exists in nature a 

 group of substances which are able to exercise a pronounced effect upon 

 the nutritional value of B vitamins, either by destroying them, by irre- 

 versibly binding them, or by competing with them for some enzyme 

 system involved in the function of the vitamin. Typical of these effects 

 are the thiaminases, which enzymatically destroy thiamine; avidin, which 

 tightly binds biotin; and a possible pellagragenic factor in corn, which 

 competes with nicotinic acid. In addition to these rather clear-cut cases, 

 a large number of less thoroughly studied examples are known where the 

 nature of the effect is not as yet clear. The consideration of these various 

 effects is here undertaken in the order mentioned, to emphasize the type 

 of mechanism involved, rather than the details of any specific case. 



The effects of a large group of synthetic inhibitors are considered in a 

 later section; the application of the use of inhibitors and inactivators to 

 the assessment of B vitamin requirements has already received passing 

 mention (p. 260). 



Enzymatic Inactivation of B Vitamins. It was in 1932 on the farm 

 of J. S. Chastek that a fatal disease of domestic foxes was first observed, 

 although it was not until some years later, when the affliction had reached 

 major economic proportions, that it was realized that the fatal symptoms 

 followed closely upon the incorporation of raw fish into the animal stock 

 diet. The careful studies of Green et al. 156 indicated clearly that the afflic- 

 tion, which generally terminated fatally in from two to three months, 

 was probably due to a thiamine-splitting enzyme in the raw fish. This 

 was soon shown to be so, 15T and the thiaminase was found in most un- 

 cooked fresh water fish and molluscs, and in a few salt-water forms. It 

 was also found that the enzyme produced similar effects in the diet of 

 cats 158 and could potentially act in this manner in man. It was also active 

 in suppressing the growth-promoting effects of thiamine on yeast (En- 

 domyces vernalis) . 159 The enzyme consists of a heat-labile, nondialyzable 



