Harnstoff-Metabolismus bei Basidiomyceten

Abstract

Summary In fruit-bodies of the commercial mushroom, Agaricus bisporus L ange , uracil is metabolized both in anabolic and catabolic reactions. The main radioactive degradation product in uracil-2- 14 C feedings is urea. The relationship between pyrimidine breakdown and urea biosynthesis might be an indirect one. Uracil is obviously reductively degraded, resulting carbon dioxide is introduced into urea formation by way of the Krebs-Henseleit ornithine cycle. There is no evidence concerning any mode of oxidative pyrimidine catabolism. - Urea biosynthesis from uracil-2- 14 C in Agaricus caps is inhibited by pretreatment with DL- s -methylaspartate, hadacidine, DL- δ -Nacetylornithine, DL-N-allylasparate, or KF. In experimentes using NaH 14 CO 3 and L-citrullineureido- 14 C, radioactive citrulline accumulates after treatment with s-methylaspartate, suggesting inhibition of argininosuccinate synthesis. N-Allylaspartate and potassium fluoride may influence carbamylphosphate synthesis. KF quantitatively inhibits nucleotide synthesis from uracil. 14 CO 2 formed by reductive degradation of uracil-2- 14 C is only partly released. Most of the evolved carbon dioxide is captured and used in urea synthesis. 5-Fluorouracil-2- 14 C is incorporated into urea, too. Pretreatment of Agaricus caps with nonlabelled fluorouracil results in decreased incorporation of radioactivity from uracil-2- 14 C into urea, suggesting some mode of „substrate competition“ at the active site of dihydrouracil dehydrogenase. 5-Bromouracil and 5-nitrouracil may act in a similar manner. Thymine-2- 14 C feedings do not result in urea labelling, supposing a permeability barrier. 2-Thiouracil-2- 14 C is metabolized but does not result in urea labelling. 6-Azathymine is an effective inhibitor of urea biosynthesis from uracil-2- 14 C possibly by affecting dihydrouracil dehydrogenase. A crude enzyme was prepared from acetone dried fruit-bodies of Agaricus by phosphate buffer extraction possessing dihydrouracil dehydrogenase aktivity as revealed by optical methods. Incubation of the preparation with uracil-2- 14 C results in carbon-dioxide- 14 C evaluation. Part of the evolved 14 CO 2 is fixed in urea synthesis. Uracil is not used in anabolic reactions. The enzyme preparation must contain the complete enzyme systems of both the reductive pyrimidine catabolic path and of urea biosynthesis by way of the ornithine cycle reactions. - In fruit-bodies of Agaricus bisporus L ange , pyrimidines are presumably formed according to the wellknown orotic acid scheme of pyrimidine biosynthesis. Preliminary experiments show that carbamylaspartateureido- 14 C is introduced into orotic acid and not identified nucleotides. Orotic acid-6- 14 C and orotic acid-2- 14 C are converted into nucleotides and free uracil. In the presence of 6-azauracil known as inhibitor of orotidylic acid decarboxylase orotate accumulates. There is no evidence that the observed urea formation from carbamylaspartate or orotate is a result of reversibility of orotic acid synthesis in fruit-bodies of Agaricus bisporus L ange . Orotate-2- 14 C is more effective in urea labelling than orotic acid-6- 14 C that might be used in urea formation via s -alanine. - In fruitbodies of Lycoperdon perlatum P ers . ( Lycoperdaceae ) orotic acid-6- 14 C gives rise to labelled s -alanine, suggesting the operation of the usual reductive pyrimidine catabolism. s -Alanine is not formed by aspartic acid decarboxylation. - In fruit-bodies of Panus tigrinus (Fr.) S ing . ( Tricholomataceae ) supplied pyrimidines are metabolized in a similar manner as in sporophores of Agaricus and Lycoperdon . Urea formation from pyrimidines, however, is not detectable, for an active urease is present.