VITAMINS 257 



pteroylglutamic acid). According to this view antivitamins are com- 

 petitive enzyme inhibitors (see p. 272 for the analogous case of malonate 

 versus succinate). 



Substances are also known which act against other types of essential 

 metabolites. For example, methionine sulfoximine, a substance produced 

 in fiour by a formerly used bleacliing agent (nitrogen trichloride) has been 



0- NH2 



CH3— S-CHjCHj— C-COOH 



I I 



HN- H 



L-Methionine sulfoximine 



found to cause "running fits" in dogs by acting as an antagonist of the 

 essential amino acid methionine. In general, such materials are called 

 antimetabolites. Many additional examples of antimetabolites are listed 

 by Woolley. 



The concept of competition for enzyme surfaces offers a reasonable 

 explanation for the action of antivitamins and other antimetabolites and 

 furthermore may well serve as a guiding principle in the search for new 

 drugs to combat disease. In theory, it should be possible selectively to 

 poison any unwanted organism with a drug patterned after the chemical 

 structure of some metabolite essential for that organism. Injury to the 

 host would be avoided if the metabolite were peculiar to the parasite 

 only. 



REVIEW QUESTIONS ON VITAMINS 



1. What are vitamins? Name those about the existence of which there is no 

 controversy. 



2. Discuss for each of the commonly accepted vitamins: (1) occurrence; (2) 

 symptoms caused by lack of the vitamin. 



3. Which vitamins have been obtained in ciystalline form? Give briefly the 

 chemical nature of each. Which have been synthesized in the laboratory? 



4. Discuss the anti-infective properties of vitamin A. 



5. Account for differences in need for vitamin D supplements in northern and 

 southern regions. Why is sunlight transmitted through an ordinary window pane 

 ineffective in preventing rickets? 



6. In addition to man, which animals suffer from scur^jy? How is the nonsus- 

 ceptibility of other animals explained? 



7. Discuss incidence of the various deficiency diseases in the United States. 



8. Which vitamin is formed from a plant pigment? Which from sterols? Which 

 one is particularly susceptible to oxidation? 



9. Explain the following terms: (1) "Viosterol," (2) ascorbic acid, (3) pro-vitamin 

 A, (4) riboflavin, (5) calciferol, (6) nicotinic acid, (7) pantothenic acid, (8) folacin. 



10. What effect is produced by ingestion of massive doses of vitamin D? 



11. What is the nature of the tissue changes responsible for noticeable respiratory 

 trouble in A-deficient animals? 



12. List the factors that influence the vitamin D requirement of au aaimal. 



