1 36 CARBON METABOLISM 111 



ance of pcntonic acids in cultures of Fusarium lini (265) has not been 

 confirmed and the method of identification used leaves something to 

 be desired. 



Lactic Acid. The formation of lactic acid in fungi is restricted 

 almost entirely to phycomycetes (Table 1), and all fungi which con- 

 vert a major fraction of the carbohydrate to lactate are in this group. 

 The early history of the problem and the cultural conditions favoring 

 lactic acid accumulation are reviewed by Foster (200), Lockwood et al. 

 (353), and Ward et al. (541, 542). 



The optical isomer formed by fungi is usually L-lactic (dextro- 

 rotatory) (536, 541), although formation of the d- and DL-forms has 

 been reported (453, 457, 501). 



As discussed in Chapter 7, the anaerobic formation of lactate by 

 Rhizopus oryzae conforms to the postulate that glucose is broken down 

 by the Embden-Meyerhof sequence of reactions to yield lactate, 

 ethanol, and carbon dioxide in equimolar amounts; the mechanism by 

 which the equimolar ratio is maintained is not yet known. More 

 lactate is formed aerobically than anaerobically, and the biosynthesis 

 of this "extra" lactate is not understood. If it is assumed that all of 

 the glucose used is converted to pyruvate, the aerobically produced 

 lactate must arise from it or from ethanol, perhaps with 4-carbon 

 acids as intermediates (109). 



Some carbon dioxide enters into the lactate molecule (226). Lactic 

 acid production in high yields is obtained from cultures which are 

 aerated either by shaking (54) or by the rotary drum method (542). 

 It should be noted that Sakaguchi and co-workers (460) could not 

 confirm the stimulation of lactate formation by aerobiosis. 



Lactate-forming organisms other than Rhizopus spp., that is, the 

 lower phycomycetes and Streptomyces, do not, so far as is known, form 

 "extra" lactate aerobically. Again in contrast to RJiizopus, some 

 lower phycomycetes anaerobically convert 85 to 100 per cent of the 

 glucose consumed to lactic acid (106, 231, 292). Evidently the bio- 

 chemical basis here differs in important respects from that of Rhizopus; 

 the difference recalls that between the homofermentative and the 

 heterofermentative lactic acid bacteria. 



Lactic acid is not an end product of aerobic metabolism but is con- 

 sumed after exhaustion of the principal carbon source (106, 171). An 

 aerobic lactic dehydrogenase is known in Penicillium chrysogenum 

 (111), but the enzymatic basis of lactate utilization in those fungi 

 which accumulate the acid is not known. The oxidation of lactic 

 acid by resting cells of P. chrysogenum is suppressed by glucose (278). 



