The metabolic significance of the citric acid cycle in the growth of the fungus Zygorrhynchus moelleri.

Abstract

SUMMARY: Cultures of the fungus Zygorrhynchus moelleri were grown in a glucose- acetate + ammonia + salts medium in the presence of either radioactive carbon dioxide, or acetate labelled in either the methyl- or the carboxyl-positions. After growth the protein was hydrolysed and the pattern of incorporation of radiocarbon into the amino acids was determined by chromatography and radioautography; activity from carbon dioxide appeared in glutamic and aspartic acids, methionine, threonine, isoleucine, proline and arginine. With labelled acetate lysine and leucine were also radioactive. A pool of free amino acids was present in this fungus. Several amino acids were isolated from the three samples of hydrolysed protein and their specific activities were determined. Assuming that the tricarboxylic acid cycle operated in this organism, a number of predictions concerning the distribution of radiocarbon in certain amino acids which could be made were largely confirmed by the experimental findings. By partial degradation of glutamic and aspartic acids, followed by assay of the radioactivity in the products, it was possible to calculate that about 60% of the oxalacetate used for citrate synthesis was recycled C4-di- carboxylic acid; 40 % was synthesized from pyruvate and carbon dioxide. Approximately 25 % of the total respiratory carbon dioxide evolved could be accounted for in terms of decarboxylations occurring within the citric acid cycle.