FUNCTION OF CAROTENOIDS IN PHANEROGAMS 



is 7-8 per cent, of the total pigment present, whilst enzymic destruction 

 amounts to 27-28 per cent. With stems different values were obtained, 

 sunlight accounting for about the same loss as with leaves whilst 

 the enzymic destruction increased to as much as 70 per cent. 



LIPOXIDASE 



The enzyme responsible for carotenoid oxidation in plants is pro- 

 bably lipoxidase,^'^^ which has been the subject of a number of recent 

 investigations. 



In 1939 Sumner and Dounce^^a demonstrated that a carotene solu- 

 tion in oil is rapidly bleached (oxidised) by an enzyme present in soya 

 beans and other legumes. Later Sumner and Sumner ^ ^ * and Sumner 

 and Smith 2 26 noted that xanthophylls and bixin were similarly 

 oxidized, that a necessary adjuvant was an unsaturated fat, which was 

 simultaneously converted into a peroxide, and that the optimum pH 

 and temperature were 6-5 and 40-45° respectively. Reinvestigating 

 the problem, Tauber ^ 2 e considered the enzyme to act directly on the 

 unsaturated fatty acid and the carotene to be indirectly oxidized by the 

 unstable peroxides so formed. He thus renamed the enzyme " un- 

 saturated fat oxidase'' Siillmann^s? ^nd Mikhlin and Pshennova^a* 

 reached conclusions similar to those of Tauber but pointed out that 

 the enzyme was identical with lipoxidase first described in 1932 by 

 Andre and Hou. ^29 Siillmann also found that the enzyme had no 

 prosthetic group and that, when a neutral fat was the substrate, lecithin 

 and a-tocopherol acted as inhibitors ; on the other hand lecithin was 

 without effect when a free fatty acid was the substrate. 



Strain 2 3 0, 2 3 1 considered that only fats containing a — HC=CH. 

 (CHg)?. C=0 group with a cw-configuration were substrates for lipoxi- 

 dase. Recently Holman and his colleagues, 2 3 2, 2 3 3 whilst confirming 

 the necessity for the presence of a m-configuration in the substrate, 

 found that the position of the ethylenic double bond is not critical for 

 arachidonic acid [CH 3(CH 2)4CH - CHCH ^CH = CHCH ^CH - CH 

 CH2CH=CH(CH2)3, COOH] is oxidized at the same rate as are 

 linoleic and linolenic acids [CH3.(CH2)4 . CH=CH . CHg . CH= 

 CH . (CH2),. COOH and CH3 . CH^ . CH-CH . CH2 . CH = 

 CH . CH2 . CH=CH . (CH2)7COOH, respectively.] They consider that 

 the structural necessity for lipoxidase activity is the methylene inter- 

 rupted doubly unsaturated system, — CH = CH . CHj . CH = CH — , 

 for conjugated unsaturated systems are not attacked. 2 3 3, 2 3 4 Pigments 

 other than carotenoids {e.g., chlorophylls a and b and haem) can also 

 be inductively oxidized by lipoxidase. 2 3 1, 2 3 6 



93 



