CARBON SOURCES 



143 



this substance. AVe have noted in this laboratory that pyruvic acid 

 ordinarily disappears from culture medium as the time of incubation is 

 increased. The disappearance of the pyruvic acid in the culture medium 

 is usually correlated with a rise in pH. Some typical reactions of pyruvic 

 acid are shown in scheme VII. For a review of pyruvate metabolism see 

 Stotz (1945). 



Scheme VIIo Some Typical Transformations of Pyruvic Acid 



CH3-CO— COOH 



H ^^ Pl7ri11M^ rtmA ^\ NH3+H 



CH3-CHOH-COOH 



Lactic acid 







Pyruvic acid 



-CO2 



CH3— CHO 

 Acetaldehyde 



CH3-CH(NH2)-COOH 



ot - Alanine 



H 



CH3-COOH 

 Acetic acid 



CH3-CH2OH 



Ethyl alcohol 



It is probable that most intermediates used in the synthesis of proto- 

 plasm are synthesized from low-molecular-weight compounds. Acetate 

 is used by yeasts for the synthesis of fats and other cellular constituents. 

 Weinhouse and Millington (1947) studied the metabolism of isotopic 

 acetate by yeast depleted of endogenous nutrients. Acetate was rapidly 

 utilized. The distribution of the carbon from the isotopic acetate was 



Scheme VIII. Oxidation of Acetate by Yeast by Means of the Krebs Citric 



Acid Cycle* 



COOH 



Hooc— CH2— c— CHj— coon 



CH3— COOH -I HOOC— CHr— CO— COOH - 



T 



2CH3— COOH 



HOOC— CH2— CHO H— COOH 



T 



HOOC— CH=CH— COOH 



T 



HOOC— CH2—C Ho— COOH <— 



HOOC— CH: 



-CO2 



—CH— CHO H— COOH 



COOH 

 CO2 



HOOC— CH2—CH2— CO— COOH 



* Original scheme modified according to Weinhouse. Courtesy of Weinhouse and Millington, Jour. 

 Am. Chem. Soc. 69: 3093, 1947. Published by permission of the American Chemical Society. 



determined by analysis. A portion of the acetate was oxidized; another 

 portion was found in the lipide fraction and cell residue; some was con- 

 verted to citric acid. It was calculated that from one-fourth to one-third 

 of the lipides found in the yeast cells at the end of the experiment (a 7-hr. 

 period) were newly synthesized from acetate. The cell residue (after 

 extraction of the fats) contained only a little isotopic carbon. This is not 

 surprising, since nitrogen was not furnished during these experiments. 



