PIGMENTS 63 



colour faded. It is suggested that in the body the activity of tocopherol may be 

 associated with the formation of the free radical under physiological conditions. 



HERMIDINE AND ECHINOCHROME 



Cannan (1926, 1927) has successfully applied the study of electrode potentials 

 to the biological pigments — hermidine and echinochrome. Crude extracts of gr en 

 shoots of Mercurialis perennis were used in the work on hermidine. These are 

 colourless but when exposed to air become first blue and then yellow. The reactions 

 may be represented as (Haas and Hill, 1925) : — 



Hermidine ^ cyanohermidine + 2H ^ oxidant -> chrysohermidine 

 colourless blue yellow 



The first two stages are completely reversible but the last is not. The electrode 

 potential equation of the first stage was found to be, at pH 7-61, approximately 



E, = _ 0-05 + 0-03 log [ fy^^oherm idme] 



[hermidinej 



and the second, at pH 7-61, 



[oxidant] 



El, = + 0-01 + 0-03 log 



[cyanohermidine] 



It is possible that such pigments, subject to reversible oxidation-reduction 

 processes, may play an important role in cellular respiration. In the plant, hermidine 

 occurs in a condition of at least 95 per cent, reduction, i.e., the rH is 10 or less, and 

 the Eb less than — 0-09 v. at pH 7-6. 



Echinochrome, obtained from a number of echinoderms, is also subject to 

 reversible oxidation-reduction processes and the potentials observed agree with the 

 electrode equation, at pH 7-69 



Eh = — 0-263 + 0-03 log '^^^■^ 



[Red.] 



Wallenfels and Mobile (1943) consider this potential to apply to an impure 

 specimen from the shell. 



It is probable that the pigment is mainly in the oxidised condition in the cell 

 and its respiratory function is doubtful. 



PIGMENT OF CHROMODORIS ZEBRA 



The blue-purple pigment occurring in the blood and tissues of the nudibranch 

 Chromodoris zebra has been studied chemically by Crozier (1916) and potentiometri- 

 cally by Preisler (1930). It is capable of reversible oxidation-reduction, the reduced 



