ALIPHATIC ORGANIC ACIDS 



143 



berg-Wieland reaction. This pathway has been proposed in fungi on 

 the basis of both chemical and isotope data (30, 49, 102, 202, 203, 450). 

 As noted elsewhere (Chapter 7), the evidence is not conclusive, al- 

 though there is of course some mechanism for the conversion of acetate 

 and other 2-carbon compounds to larger molecules. 



4. The cleavage of isocitric acid directly to succinic and glyoxylic 

 acids (405). This system is probably identical with one known in bac- 

 teria (477); its functions during glucose metabolism and its quantita- 

 tive importance in fungi are not yet known. 



Formic and Acetic Acids. These two volatile acids are more char- 

 acteristically bacterial than fungal metabolites, and under normal con- 

 ditions of culture neither accumulates in such large amounts as do the 

 non-volatile acids so far considered. 



Formic acid has not been reported to accumulate during carbohy- 

 drate utilization, but several fungi form it during the metabolism of 

 lactic and pyruvic acids (133, 454). These experiments suggest the 

 well-known transacetylase reaction: 



Pyruvic acid + coenzyme A — » acetyl-coenzyme A + formic acid (1) 



The entrance of formate, or a derivative, into metabolism is demon- 

 strated by isotope results (302) and by direct utilization of formate 

 (137, 262). It has been suggested that formate may enter through the 

 sequence of reactions: 



Glycine + formate -» serine -> pyruvate (2) 



Until active cell-free preparations are studied these reactions of 

 formate must remain hypothetical in the fungi. 



Acetic acid is found in traces in the medium of many fungi and 

 actinomycetes; the list in Table 1 omits non-specific determinations 

 of "volatile acids" (54, 73). Larger amounts of acetate are formed by 

 Streptomyces fradiae (421) and by Fusarium spp. (351). As also shown 

 in Table 1, two fungi form ethyl acetate from carbohydrate. 



In view of the central role of acetate and acetyl-coenzyme A in 

 metabolism we may assume their presence even if isolable amounts are 

 not formed; this generalization is borne out by isotope dilution studies 

 on Penicillium chrysogenum (368) and Streptomyces coelicolor (144). 

 In particular organisms acetate may be formed from pentose (225), 

 from ethanol by oxidation (398, 415), or, possibly, from glycine by 

 deamination (562). Direct studies have shown that exogenous acetate 

 is oxidized by Merulius niveus (30), Penicillium chrysogenum (278), 

 and Zygorhynchus moelleri (382). 



