] (39 CARBON M I T ABOLISM III 



supporting the second hypothesis (361). It must be concluded that 

 the question is still open. 



It is usually assumed that the oxidized carotenoids, neutral and 

 acidic xanthophylls, arise by oxidation of the hydrocarbon polyenes 

 (264). 



Although /?-ionone increases carotenogenesis in Pliycomyces blakes- 

 leeanus, isotope evidence indicates that the molecule is not, as one 

 might expect, incorporated as such into the /^-carotene structure (182). 



Two functions have been suggested for the carotenoids: a direct 

 role as receptor for the light stimulus in phototropism, and an un- 

 defined role in sexual differentiation. 



Phototropism is discussed later (Chapter 11), and it is concluded that 

 there is no satisfactory proof of the direct involvement of carotenoids. 



Although the carotenoid pigments and the colorless polyenes of 

 Allomyces javanicus are restricted to the male gametangia (178, 529), 

 an essential role in the sexual process is rendered less probable by the 

 finding (527) that diphenylamine inhibits carotenogenesis in A. 

 javanicus without appreciably affecting the sexual reaction. It seems 

 likely that the carotenes are side products of a metabolism which is 

 in some respects at least distinctive to the male gametangium. The 

 (+) and (— ) strains of some of the Mucorales show quantitative 

 differences in carotenoid content; studies, as reviewed by Goodwin 

 (232), have been confined to the fact itself, and it would appear that 

 in Pliycomyces blakesleeanus the difference between (+) and (— ) 

 strains is not consistent (219). Zygospore formation or merely pairing 

 of compatible strains in some of the Mucorales is accompanied by in- 

 creased formation of carotene (29, 270a). 



The most serious deficiency in the study of fungal carotenoids is 

 the failure to determine whether or not carotenes occur in association 

 with proteins or lipoproteins. Such an association is theoretically 

 necessary if they are to function in metabolism, e.g., in the reception 

 of light stimuli. Carotenoproteins are well known in animals (234), 

 and the now classical studies on rhodopsin (147, 537) provide a model 

 for the investigation of other pigments. 



The Metabolism of Hydrocarbons and Hydrocarbon Derivatives. 



It has been mentioned (Chapter 3) that hydrocarbons support growth 

 of some actinomycetes and fungi. The oxidation of n-dodecane and 

 other hydrocarbons by Proactinomyces opacus can be demonstrated 

 manometrically (Figure 6); the organism also oxidizes long-chain 

 saturated fatty acids, which may be intermediates in hydrocarbon 

 oxidation. Details of the breakdown of hydrocarbons are not known; 



