IRON PROTOPORPHYRINS 53 



Between pH 4 and 7 the value remains constant, but there is a break at pH 6 -86 and 

 the potential falls by 0-06 v. per 1 pH unit. 



In the chapter on Polarography mention is made of the work of Carruthers on 

 the catalytic waves of cytochrome-C and other enzymes. 



PEROXIDASE AND CATALASE 



Before leaving the subject of iron-porphyrin compounds the two enzymes 

 peroxidase and catalase should be mentioned. 



Peroxidase from horse-radish has been crystallised and separated into ferri- 

 protoporphyrin (hsematin) and protein. In the presence of hydrogen peroxide 

 peroxidase effects oxidation by the following mechanism : — 



Peroxidase + HgOg ^ Peroxidase HgOg 

 Peroxidase HgOa + A ^ Peroxidase + AO + HgO 



In these reactions the iron in the peroxidase remains in the ferric state but 

 dihydroxymaleic acid may be oxidised by peroxidase functioning as an aerobic 

 peroxidase ; in this case carbon monoxide inhibits the reaction. 



The characteristic oxidation-reduction potential of the reactants seems not to 

 be the crucial factor since in the case of hydroquinone and pyrogallol, despite a 

 similar E^ value, the reaction rate differs tenfold. By special methods of observation 

 of rapid colour changes, three colours of peroxidase-HgOa compounds, green, pale red 

 and deep red, are described by Chance (1949) and Roughton. 



Catalase from liver, erythrocytes and bacteria has been crystallised ; it is a 

 protohaematin-protein compound. It remains a puzzle why organisms such as 

 Micrococcus lysodeikticus should produce such large amounts of catalase, far in excess 

 of the need to break down peroxide (Herbert). The mechanism by which catalase 

 decomposes hydrogen peroxide to oxygen and water is still disputed, the reactions 

 suggested by Keilin and Hartree (1938) being : 



4 Fe®®© + 2H2O2 ^ 4 Fe®® + 4H® -f 20^ 

 4 Fe®© -f O2 -f 4H® ^ 4 Fe®©© -f 2H2O 



It is suggested that catalase can function as a secondary oxidation catalyst 

 (Theorell, 1947). 



A catalytic peroxidation which proceeds in vitro but has not been detected in vivo 

 proceeds as follows (Albert and Falk, 1949). Cytochrome-C and methaemoglobin 

 catalyse the oxidation by hydrogen peroxide of amino- and hydroxy-acridines and 

 quinolines. Quinonimines and quinones are formed and these act as hydrogen carriers 

 which oxidise ascorbic acid and cysteine autocatalytically. 



J Dehydro-ascorblc Acid 



'^'^^^-^ Ascorbic Acid 



I 



OH \^0 



NH2 >^ >^ .NH 



yy- 



