148 CARBON METABOLISM III 



Virtually all modern theories of acid biosynthesis assume a pre- 

 liminary conversion of carbohydrate to pyruvic acid, except, of course, 

 in the biosynthesis of the sugar acids. Whether this assumption repre- 

 sents the facts or merely the limitations of current biochemical knowl- 

 edge is uncertain. The phosphogluconate pathway of glucose metab- 

 olism (Chapter 7) provides to the cell a group of compounds, includ- 

 ing the 4- and 7-carbon sugar phosphates, previously unknown and 

 quite possibly of direct importance in acid synthesis. 



Table 1 summarizes the reported formation of acids by fungi and 

 actinomycetes acting in culture on carbohydrates. So far as possible, 

 duplicating reports have been omitted, and some of those included are 

 based on questionable or at least inadequate evidence. Satisfactory 

 proof for the identity of an acid should involve more than color tests 

 or the chromatographic constant in a single solvent; on the other hand, 

 the results of these presumptive tests are often correct and reports 

 based on them cannot, therefore, be dismissed out of hand. Ideally, 

 an acid or other metabolic product should be isolated as a crystalline 

 derivative; if this is not possible, several independent criteria should 

 be utilized. Enzymatic identification promises to be the most sensitive 

 and accurate for metabolites which are found only in micro amounts. 



2. THE METABOLISM OF LIPIDS 



The lipids may be defined as those substances which are soluble in 

 fat solvents; a few, like the lecithins, are somewhat water soluble, but 

 most are insoluble in water. Lipids which are of importance to the 

 physiology of the fungi include the true fats, the fatty acids, the 

 compound lipids (phospholipids), the sterols, the carotenoids and 

 triterpenes, and a few hydrocarbons. The general chemistry of the 

 lipids is reviewed by Deuel (164). Those lipids which occur as constit- 

 uents of the cell are considered in Chapter 2; here we are primarily 

 concerned with synthesis and breakdown, although for convenience the 

 occurrence of some lipids is included. 



The Synthesis of Fats, Fatty Acids, and Phospholipids. The neutral 

 fats are triglycerides of the higher fatty acids. These are synthesized 

 by many fungi in amounts up to 50 per cent of the mycelial dry 

 weight. One rather striking difference between the lipids of fungi and 

 those of animals is the high content of free fatty acids in most fungal 

 lipids (Chapter 2). This means that most experiments on the factors 

 affecting gross lipid content deal indiscriminately with both neutral 

 fats and fatty acids. 



