466 M. Wheldale. 



The general distribution of peroxydases is favourable to the view 

 that ox3'dases are universall}' formed; it is the oxygenase element 

 which is sensitive to inhibitors and the oxydase tests are solely 

 indicative of the activity of this component of the enzyme. Some 

 authors, notably Gonn ermann^) and Bertel-) consider the 

 darkening of extracted plant juices to be due to the presence of 

 homogentisic acid, one of the oxidation products of tyrosinase on 

 tyrosin. Solutions of this acid rapidly darken when exposed to air. 

 Gon nermann's results are based chiefly upon experiments with 

 Beet-root sap. Schulze^) on the other hand, was unable to detect 

 homogentisic acid in the Beet- root and only found small quantities 

 of tyrosin. 'I'yrosinase is undoubtedly present in potato tubers, beet- 

 root and fructifications of many Fungi and is probably the cause of 

 the darkening of these bodies; at the same time, other oxydases, in 

 addition, must be present, since the extracts, like those from all other 

 plants which turn brown on autolysis, give the blueing with guaiacura 

 which tyrosinase itself is unable to produce. 



P a 1 1 a d i n 's Respiration-Pigments. 



The conception of an oxidation mechanism consisting of an 

 aromatic chromogen in conjunction with an oxydase forms in outline 

 the basis of Palladin's respiration theory.*) I am not in a 

 position to criticise the validity of this theory solely as an exposition 

 of the mechanics of respiration. It must be admitted, however, that 

 in bringing forward his h3'pothesis, Pa 11a din has done much to 

 emphasize the connection existing between oxidation processes and 

 pigment formation. According to this author, aromatic chromogens, 

 frequently in combination with sugars in the form of glucosides, are 

 ver}^ widely distributed in plants. His evidence is largely based 

 upon the results of autolj'sis and of the action of artificially prepared 

 Peroxydase from the Horse-radish root upon plant juices. He found 

 that when a little peroxydase solution, together witli hydrogen per- 

 oxide, was added to the extracted sap (in which all ferments had 



1) Pfläg. Arch.. Bd. LXXXII p. 289 (l&OO). Über die Honiogentisinsäure. Ber. 

 d. d. bot. Gesell, Bd. XXI. 1903. 



2) Über Tyrosinabbau in Keimpflanzen. Ber. d. d. bot. Gesell., Bd. XX. 1902. 

 ^) Über die bei Luftzutritt eintretende Dunkelfärbung- des Eübeusaftes. Hoppe- 



Seyler's Zeitschr. f. phys. Chemie, Bd. 50. 1906—07. 



*j Über das Wesen der Pflanzeuatinuug-. Biochem. Zeitschr. 1909 and literature 

 quoted therein. 



