52 PHYSIOLOGY OF THE FUNGI 



drogenase) ; oxidases (cytochrome oxidase, tyrosinase, polyphenol oxi- 

 dase). Succinic acid dehydrogenase oxidizes succinic acid to fumaric 

 acid by the removal of two hydrogens ; but this reaction takes place only 

 in the presence of another system (cytochromes) which "carries" the 

 hydrogen to an oxidizing enzyme, which converts the hydrogen to water 

 and regenerates the cytochrome system so that it can transport more 

 hydrogen. In the cell, succinic acid dehydrogenase is said to be cyto- 

 chrome-linked. In the laboratory, hydrogen carriers other than cyto- 

 chrome may be used. Various other dehydrogenases are linked to the 

 cytochrome system. 



Another oxidase, tyrosinase, is found in many fungi. It is well estab- 

 lished that copper is an essential constituent of this enzyme system 

 (Kubowitz, 1937) and may be removed by dialyzing the enzyme against 

 cyanide solutions. The activity which is lost by this treatment is restored 

 by cupric ion, Cu++, but other divalent metals do not replace copper. 

 Various reagents which react with copper, such as cyanide, diethyl 

 dithiocarbamate, salicylaldoxine, and carbon monoxide, inhibit the action 

 of tyrosinase. Among the fungi which produce tyrosinase are the follow- 

 ing species (Nelson and Dawson, 1944) : Boletus luridis, Russula foetens, 

 R. niger, Lactarius piperatus, and PsalUota campestris. It is probable that 

 the darkening and coloration of the fruit bodies of these fungi depend upon 

 the activity of tyrosinase. 



Pyruvic acid, CHs — CO — COOH, is a key compound in carbohydrate 

 utilization, and perhaps in other metabolic processes as well. The 

 enzyme, carboxylase, catalyzes the decomposition of pyruvic acid in 

 the following way: 



carboxylase 



CHs— CO— COOH > CO. + CHs— CHO 



Pyruvic acid Carbon dioxide Acetaldehyde 



The carbon dioxide formed escapes, while the acetaldehyde formed may 

 be either oxidized to acetic acid or reduced to ethyl alcohol. The enzyme 

 which catalyzes the decarboxylation of pyruvic acid to carbon dioxide and 

 acetaldehyde is abundant in yeast and other fungi. This enzyme con- 

 sists of three moieties, a specific protein, a magnesium ion, and thiamine 

 pyrophosphate. 



CHEMICAL NATURE OF ENZYMES 



In the past there has been a great deal of controversy over the chemical 

 nature of enzymes. Sumner (1926) was the first to isolate an enzyme 

 (urease) in pure crystalline condition. Since then a dozen or more 

 enzymes have been prepared in pure crystalline form. All the enzymes 

 which have been isolated in pure crystalline condition have proved to be 

 proteins. 



