TERPENOIDS AND STEROIDS 169 



Biosynthesis of the carotenoids has been studied much more extensively than is the 

 case for any of the other terpenoids. Unfortunately the picture rather than being more 

 clear as a result of a great mass of data is greatly confused by many conflicting reports. 

 There is little doubt that the general pathways indicated in Figure 8-1 are followed (128). 

 However, it must be noted that Goodwin (129) found very slight incorporation of mevalonic 

 or acetic acid into the /3-carotene of maize, whereas carbon dioxide was rapidly incorpo- 

 rated. Labelled iso-pentenyl pyrophosphate was converted into carotenoids by tomato 

 fruit homoge nates 40 times faster than mevalonic acid (130). Other experiments showing 

 the incorporation of L-leucine can be accommodated into the accompanying scheme as a 

 by-pass. The greatest difficulty rests on the question as to whether the various carotenoids 

 are interconvertible or whether they arise by parallel pathways from a common (unknown) 

 C40 precursor (131). Most genetic and tracer evidence favors the idea of independent path- 

 ways for the various carotenes and xanthophylls. On the basis of genetic analysis Porter 

 and Lincoln (132) proposed a common C40 intermediate with three double bonds and diver- 

 gence after this stage resulting from successive dehydrogenations. This proposal has 

 since been modified slightly (131). Tracer experiments of Purcell et al. (133) with toma- 

 toes have pointed to an unknown precursor with four double bonds. In order of appearance, 

 phytofluene is one of the first compounds to contain label from mevalonic acid; lycopene 

 is the last; and other carotenes intermediate. It must not be inferred from this that com- 

 pounds early in this sequence are the actual precursors of compounds late in the sequence. 

 Some colorless polyenes, rather than being precursors of pigments, may be formed by 

 reduction of carotenes particularly under pathological conditions (134). Other experiments 

 (115, 116) indicate that light stimulates synthesis of carotenes from colorless polyenes; 

 and in addition, oxygen may be required. Some of these different requirements, the condi- 

 tions of the plants used, and the particular species studied may account for discrepancies 

 in the various pathways of carotenoid biosynthesis which have been proposed. Recent 

 studies with isotopically labelled oxygen have shown that hydroxyl groups of xanthophylls 

 derive their oxygen atom from the atmosphere, but in the epoxides it comes from water 

 (135). 



The introduction of nitrogen into the alkaloids of this group has not been studied. 

 In some cases, by analogy with other alkaloids, nitrogen may be supplied to the carbon 

 skeleton as ammonia from an amide. With the diterpene alkaloids the problem is compli- 

 cated by the invariable presence of an extra two-carbon residue attached to the nitrogen. 

 This three atom grouping might reasonably come intact from glycine. 



As noted above, most of the plant sterols contain an "extra" methyl or ethyl group 

 at C-24. In the ergosterol of yeast the additional methyl group is known to come from 

 formate or methionine. Nothing is known regarding the source of ethyl groups; a reason- 

 able choice might be acetate. 



GENERAL REFERENCES 



Arreguin, B., "Rubber and Latex" in Ruhland 10 223. 



de Mayo, P. , Mono - and Sesqu iterpe noids , Interscience Publishers, N. Y. , 1959. 

 de Mayo, P., The Higher Terpenoids , Interscience Publishers, N. Y. , 1959. 

 Eastman, R. H. , and NoUer, C. R. , "The Terpenes" in Organic Chemistry 4^ ed. by 



H. Oilman, John Wiley, N. Y. , 1953. 

 Fieser, L. F. and Fieser, M. Steroids , Reinhold, N. Y. , 1959. 

 Goodwin, T. W. , "Carotenoids" in Ruhland 10 186. 

 Goodwin, T. W. , The Comparative Biochemistry of the Carotenoids , Chapman and Hall, 



London, 1952. 

 Guenther, E., The Essential Oils 6 vols. , D. Van Nostrand, N. Y. , 1948-1952. 

 Haagen-Smit, A. J. , "Sesquiterpenes and Diterpenes, " Fortschr. Chem. Org. Naturstoffe 



12 1 (1955). 



