j 52 CARBON METABOLISM III 



of Neurospora crassa (336), and direct analyses of the mycelium of 

 Aspergillus nidulans (475)— indicate that the routes of formation of 

 the two types are independent; evidence to the contrary (146) is less 

 convincing. The only fatty acid dehydrogenase known in the fungi 

 acts only on the short-chain acids (387). In animal tissues it seems 

 likely that the saturated and the unsaturated fatty acids arise in- 

 dependently (216). 



Phospholipid synthesis has been studied by Raveux (442), who found 

 the phospholipid content of Aspergillus niger to be independent of 

 the fat content and to parallel the available nitrogen of the medium. 

 The time course of phospholipid formation has been determined in 

 A. niger. lipoidal phosphorus of the mycelium rises to a peak and 

 then declines during the later — probably autolytic— phases of culture 

 (222). 



The Breakdown of Fats and Phospholipids. Although the neutral 

 fats are relatively poor sources of carbon, they are utilized by many 

 fungi (Chapter 3). The first step in such utilization is the hydrolysis 

 of the fat to glycerol and fatty acids (288); it is assumed that endoge- 

 nous fats are split in the same way before utilization. The hydrolysis 

 is catalyzed by lipase or lipases. 



It is common practice to assay for lipase activity by measuring the 

 hydrolysis of simple esters, e.g., ethyl acetate or p-nitrophenyl acetate. 

 It is doubtful, however, whether the simple esterases so determined 

 are active in hydrolysis of the true fats (7). Simple esterases occur in 

 most fungi investigated; the esterase of Lenzites sepiaria fails to 

 hydrolyze fats (574). Lipase proper, as distinct from esterases, can 

 probably best be studied with water-soluble synthetic substrates (60, 

 168). 



Virtually all fungi which have been studied for lipase production 

 appear to form the enzyme (187, 317). Lipase is primarily intra- 

 cellular, but both it and esterase do appear in the medium late in the 

 culture cycle (153, 488, 570). It has been suggested (198) that Aspergil- 

 lus niger and Penicillium roqueforti form two lipases, one in the 

 mycelium and the other released into the medium. 



Data on the relation of pH to activity and on the stability of fungal 

 lipases vary widely from one organism to another (195, 198, 317, 440, 

 471, 512), so much so that the formation of different enzymes seems 

 likely. The amount of extracellular lipase formed by Mucor mucedo 

 is influenced by the temperature of cultivation and is increased by 

 inclusion of a lipid in the growth medium (488). Like lipase from 

 other sources, fungal lipase under appropriate conditions catalyzes the 



