Bicarbonate, incorporation into orotic acid

Oxipurinol leads to an increase of bicarbonate- C incorporation due to an inhibition of the second part of pyrimidine biosynthesis as shown above. This is in keeping with the orotic aciduria after administration of oxipurinol observed in vivo. It is also in agreement with our results obtained in cultured human lymphoblasts (Banholzer et al. 1980, Banholzer et al. 1981). Purines did not reduce bicarbonate incorporation into orotic acid as they did in human cultured lymphoblasts. Conversely they increased... 

Measurements of the incorporation of labelled bicarbonate into orotic acid established the occurrence of the complete pathway of de novo... 

By measuring bicarbonate- C incorporatic into orotic aci as well as the incorporation of orotic acid- -C or uridine- C into nucleic acids it was possible to study the first and second part of pyrimidine biosynthesis and the salvage pathway separately. 

Although both orotic acid and uracil are utilized by Ehrlich ascites tumour cells in mice [52], the presence of orotic acid completely inhibits the incorporation of bicarbonate into C2 of the acid-soluble nucleotides in the same tumour system [181]. 

The small-molecule precursors of orotic acid were identified by Reichard and Lagerkvist [72]. During in vitro incubation, liver slices incorporated labelled ammonia, bicarbonate and aspartate into pyrimidine components of RNA. When the slices were incubated with the labelled precursors in the presence of extracellular orotate, isotopes were found in this extracellular fraction. In this way, orotic acid was recognized as an intermediate in pyrimidine synthesis de novo. The labelled orotate was chemically degraded and small molecules identified as the precursors of particular atoms of the orotate ring, as shown in the following scheme ... 

F8 Fedorov, N. A. and Matveenko, V. N. Levels of free pyrimidines and purines in rat liver and incorporation of carbon-14-labeled sodium bicarbonate into these compounds under the action of a single intraperitoneal injection of orotic acid. Vopr. Med. Khim., 17, 260-263 (1971) (Russ.)... 

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