CAROTEKOIDS, LIPIDES, PROTEINS 475 



This capacity of carotenoids to serve as reversible oxygen acceptors has caused U. 

 and H. von Euler and Hellstrora (1928), von Euler and Ahlstrom (1932) and Joyet- 

 Lavergne (1935) to discuss the possibility that carotene (or the related vitamin A) may 

 have a catalytic function in the binding of oxygen in respiration. Equally feasible is 

 their participation in the reverse process— the hberation of oxygen in photosynthesis 

 (cf. Chapter 11, page 292). 



It has recently been proved (cf. Vol. II, Chapter 30) that light ab- 

 sorbed by the carotenoids can be fully or partially utilized for photo- 

 synthesis — probably by a primary transfer of excitation energy to chloro- 

 phyll. However, this subsidiary function of the carotenoids is unlikely 

 to provide an adequate explanation of their ubiquitous occurrence in all 

 photosynthesizing cells, since in many of them — particularly the green 

 cells of the higher plants — the contribution of the carotenoids to the total 

 light absorption is almost negligible. 



3. Carotenoids, Lipides, and Proteins 



The affinity of carotenoids for lipides, which has lead to their desig- 

 nation as "lipochromes," should cause them to associate with lipides in 

 the chloroplasts. If it is true that the lipides are concentrated in the 

 grana (arguments in favor of this assumption have been mentioned in 

 chapter 14), the carotenoids also must be accumulated in the grana. 



The association of carotenoids with lipides is indicated, e. g., by 

 the protective action of lecithin on carotene colloids (Karrer and Strauss 

 1938). Zechmeister (1934) suggested that this association may be 

 caused by esterification of the polyene alcohol by the fatty acid. He 

 even spoke of the possible existence in leaves of a carotenoid-chlorophyll 

 ester, with chlorophyll as the acid component, and carotenol replacing 

 phytol as the esterifying alcohol. However, carotene cannot form esters, 

 yet associates itself with lipides; and luteol, although able to esterify, 

 must be present in leaves in the free state, since it can be extracted 

 without saponification. It thus appears that the carotenoid-lipide 

 binding in lipochromes is of a less "chemical" character than the alcohol- 

 acid link in an ester; it is probably due to "van der Waals' attraction" 

 between the long carbon chains in the carotenoids and in the lipides. 



The carotenoids are also capable of forming complexes with proteins. 

 Such complexes are known in nature (the brown pigment of lobsters is 

 an example), and can be obtained artificially, by coprecipitating carote- 

 noids with proteins from colloidal solutions. The pigment cannot be 

 extracted from these precipitates by means of the usual organic solvents. 

 In contrast to carotene-lecithin complexes, carotene-protein complexes 

 are not protected from oxidation; nor does this association appreciably 

 shift the position of the absorption bands of carotene (in contrast to the 

 behavior of chlorophyll in artificial chlorophyll-protein complexes; cf. 

 page 388). 



