CAROTENOIDS 



carotenoid metabolism when he found that the photochemical destruc- 

 tion of carotene solutions in vitro occurs in the presence of chlorophyll 

 which is directly involved and does not act merely as a catalyst ; 

 sodium cyanide partly inhibits this reaction. It is interesting that about 

 the same time it was noticed that carotene inhibits the photodestruction 

 of chlorophyll. ^ i » In vivo, however, chlorophyll does not apparently 

 take part in the photochemical destruction of carotenoids, for Bernstein 

 and Thompson - ^ ^ showed that photodestruction is as great in etiolated 

 leaves as in green leaves. Photodestruction occurs equally well in all 

 regions of the spectrum and is influenced by atmospheric oxygen. 

 Between oxygen concentrations of 0-5 and 20 per cent, destruction is 

 proportional to the oxygen concentration ; between 0-5 % and 0-02 % 

 no change in destruction rate is noted, probably owing to the replace- 

 ment of atmospheric oxygen by some cellular constituent. Below 0-02 % 

 destruction ceases. 



(B) Enzymic Destruction. Enzymic degradation of carotenoids 

 occurs in plant tissue when the pigments and/or enzymes are liberated 

 by destroying the cells by maceration. ^^^ Strain ^^ found that, apart 

 from blanching, this oxidation could be inhibited by small amounts of 

 zinc dust, magnesium oxide, sodium hydroxide, ammonium hydrox- 

 ide, lead acetate and mercuric chloride ; cyanide is also an inhibitor. ^ ^ ^ 

 Mitchell and Hauge ^ 2 consider that the enzyme responsible is lipoxi- 

 dase {vide infra) and that cell permeability controls oxidation in the 

 intact plant, for, as just stated, oxidation is very rapid when the cells 

 are ruptured by freezing or when the plant wilts. The occurrence of 

 natural protective substances in the intact cell should not be overlooked, 

 for Weier22i found that if blanched carrots were leached with cold 

 water prior to storage the stability of the carotenoids was considerably 

 reduced, indicating the presence of a protective substance soluble in 

 cold water after liberation from the cells by blanching ; this has 

 recently been confirmed, ^ 2 ia 



Mitchell and Hauge 22 2 found that the increased oxidative action 

 in sunlight is not completely explained by the possible catalytic effect 

 of chlorophyll, but may be due to two other factors, {a) increased trans- 

 piration due to the opening of stomata in light, which would produce 

 more rapid wilting, and (b) production of leaf temperatures above that 

 of the surrounding air, which would result in both accelerated wilting 

 and increased enzymic activity. The effect of temperature on carotene 

 destruction has been more recently examined by Bernstein and Thomp- 

 son, 2 1 8 who showed that between 4° and 25° the temperature coefficient 

 (Q 10) for the enzymic destruction is 1-6-1 -7. Griffith and Thompson22iA 

 have shown that in lucerne leaves, the sunlight-sensitized destruction 



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