328 Mineral Nutrition of Plants 



dence of such validity that it would have established the essential status 

 of iron as a nutrient, even if results of growth experiments were not 

 available. Two other enzymes found in plants, peroxidase and catalase, 

 have as their prosthetic groups iron-porphyrin compounds. Another 

 enzyme widely distributed in plants and capable of participating in the 

 respiratory process is polyphenoloxidase. This enzyme depends for its 

 activity on the reversible oxidation and reduction of copper. Copper is 

 the prosthetic group of the enzyme and cannot be replaced by any other 

 metal (•?■?, jo). Other copper enzymes in plants are laccase, noted 

 previously, and ascorbic acid oxidase. In recent years the enzyme car- 

 bonic anhydrase was isolated from red blood corpuscles and was shown 

 by Keilin and Mann (^2) to have zinc as a prosthetic group. Present 

 indications are that carbonic anhydrase also occurs in higher plants 

 (/6). 



In the examples just cited the metal micronutrient is the prosthetic 

 group of the enzyme. Its place cannot be taken by any other element. 

 In this it fully meets the test of specificity which has been previously 

 designated as a criterion of essentiality in growth experiments. It was 

 already implied, however, that the identification of an element with a 

 specific function in no way excludes other roles which the element may 

 perform. An excellent illustration of this principle is the well-estab- 

 lished property of many divalent ions to serve as activators of enzyme 

 systems. The first stage in the metabolic transformation of hexose is 

 the transfer to it of a phosphate ester group from adenosinetriphosphate. 

 This is mediated by the enzyme hexokinase, which has been isolated 

 from yeast. The enzyme is inactive in the absence of magnesium, and 

 a relatively high concentration of this ion is required for full activity, 

 (14). Arginase from both plant and animal tissues is activated by ad- 

 dition of Mn++, Co++, or Fe++; of these M11++ is the most effective 

 (18). The carboxylase of Proteus vulgaris which catalyzes the oxidative 

 decarboxylation of pyruvic acid to acetic acid and carbon dioxide is 

 activated by the addition of Mn++, Mg++, Fe++, Co++, Ni++, and 

 Zn++ (50). Of special interest is the manganese activation of P-car- 

 boxylases since these enzymes catalyze reactions causing assimilation 

 of carbon dioxide and leading to the formation of di- and tricarboxylic 

 acids of importance in intermediary metabolism (46). 



