ENZYMES 



Yet it was not until 1937, some three years after Warburg had shown 

 nicotinamide to be an essential component of the coenzyme, that 

 Elvehjem and his colleagues (1) established a connection between the 

 antipellagra vitamin and nicotinamide. Elvehjem's discovery was 

 consistent with poetic justice because he had been trained both as a 

 nutritionist and as an enzyme chemist. Vitamin B2 was identified 

 in 1935 by Kuhn and Karrer with riboflavin, the prosthetic group of 

 the so-called yellow enzyme which Warburg had isolated from yeast 

 three years earlier. The time relations were reversed in the cases of 

 vitamins Bi and Eg, since the identification of the vitamins preceded 

 knowledge of their participation in enzymic reactions. The chemical 

 identification of vitamin Bi with thiamin by Williams and Cline in 

 1936 preceded by one year the demonstration by Lohmann and 

 Schuster that the prosthetic group of yeast carboxylase is a diphos- 

 phoric ester of thiamine. Peters and his group at Oxford had estab- 

 lished the role of vitamin Bj in the oxidation of pyruvic acid long before 

 the chemical nature of the vitamin was established. Vitamin Be was 

 identified with pyridoxine in 1938 by Folkers, Keresteszy et al., and 

 Kuhn et al., but it was not until 1944 that a more active form of the 

 vitamin, viz-, pyridoxal, was discovered by Snell and that the phos- 

 phoric ester of pyridoxal was shown to be the coenzyme of tyrosine 

 decarboxylase by Gunsalus. There are thus four authenticated identi- 

 fications of vitamins with prosthetic groups. In the case of vitamin 

 A, Wald has shown that it is an essential part of a photosensitive 

 pigment in the eye known as visual purple. Many will not concede 

 that visual purple has the properties of an enzyme but, whether or not 

 that point is conceded, it is at any rate admissible that vitamin A 

 fulfills the role of prosthetic group of a chromoprotein with an important 

 physiological function. 



This relation between vitamins and prosthetic groups makes 

 it easy to understand the basis for the body's continuous requirement of 

 vitamins. Since enzymes have a limited life period in consequence 

 of their destruction during activity, there is constant need for more of 

 all enzymes. The minimum amount of any of the vitamin compatible 

 with viability is therefore an approximate measure of the total amount 

 of enzymes in the body whose prosthetic groups contain that vitamin. 

 Here, again, if we knew precisely which enzymes, for example, require 

 flavin, what the normal levels of these enzymes are in different organs. 



