Orotic acid biosynthesis

In this review several aspects relating to the biochemical and physiological effects of orotic acid wiU be discussed. Since there are many studies devoted to orotic acid biosynthesis, incorporation, metabolic transformation, physiological and therapeutic roles, only about 500 references, which are believed to cover the main findings, will be presented. Data on the transformation and biological activity of orotic acid (summarized here) can be found elsewhere [20,21,64-69]. 

F14 Fitsner, A. B. and Mardashev, S. R. Possible mechanism of the inhibitory effect of barbiturate on orotic acid biosynthesis in microscopic sections of rat liver. Vopr. Med. Khim.,... 

T15 Tremblay, G. C., Crandall, D. E., Knott, Ch. E. and Alfant, M. Orotic acid biosynthesis in rat liver studies on the source of carbamoylphosphate. Arch. Biochem. Bio-phys., 178, 264-277 (1977)... 

Fig. 44 Pathways for uridylate biosynthesis. Mutants lacking enzymes PRTase or ODCase can complete a route to UMP provided by an antibody orotate decarboxylase in conjunction with the naturally occurring uracil PRTase. Decarboxylation of orotic acid [135] is thought to proceed through the transition state [136], for which the hapten [137] was developed (Smiley and Benkovic, 1994).

Apparently the acceleration of de novo purine biosynthesis by orotic acid results from a release of feedback inhibition imposed by hepatic purine nucleotides. In a related study, it was found that orotic acid feeding can prevent hyperlipaemia, which normally follows the administration of Triton WR-1339, a surface active agent [152]. The influence of orotic acid on lipid metabolism can be readily shown by the fact that depression of serum lipoproteins and milk production were observed in lactating goats when an aqueous suspension of orotic acid was administered orally [164]. 

Orotic acid added to rat diet also provokes an. excessive biosynthesis of porphyrins in liver, erythrocytes and bone marrow. Administration of adenine monophosphate (AMP) counteracted this effect of orotic acid intoxication [165]. Haemorrhagic renal necrosis in rats, caused by choline deficiency, can be relieved by orotic acid [166], Simultaneous supplementation of the diet with adenine does not influence the protective effect of orotic acid. It has been suggested that orotic acid may lower the body requirement for choline through a metabolic interaction—orotic acid may stimulate the cytidine phosphate choline pathway of lecthin biosynthesis [166]. 

These three compounds exert many similar effects in nucleotide metabolism of chicks and rats [167]. They cause an increase of the liver RNA content and of the nucleotide content of the acid-soluble fraction in chicks [168], as well as an increase in rate of turnover of these polynucleotide structures [169,170]. Further experiments in chicks indicate that orotic acid, vitamin B12 and methionine exert a certain action on the activity of liver deoxyribonuclease, but have no effect on ribonuclease. Their effect is believed to be on the biosynthetic process rather than on catabolism [171]. Both orotic acid and vitamin Bu increase the levels of dihydrofolate reductase (EC 1.5.1.4), formyltetrahydrofolate synthetase and serine hydroxymethyl transferase in the chicken liver when added in diet. It is believed that orotic acid may act directly on the enzymes involved in the synthesis and interconversion of one-carbon folic acid derivatives [172]. The protein incorporation of serine, but not of leucine or methionine, is increased in the presence of either orotic acid or vitamin B12 [173]. In addition, these two compounds also exert a similar effect on the increased formate incorporation into the RNA of liver cell fractions in chicks [174—176]. It is therefore postulated that there may be a common role of orotic acid and vitamin Bj2 at the level of the transcription process in m-RNA biosynthesis [174—176]. 

A common intermediate for all the nucleotides is 5-phosphoribosyl-l-diphosphate (PRPP), produced by successive ATP-dependent phosphorylations of ribose. This has an a-diphosphate leaving group that can be displaced in Sn2 reactions. Similar Sn2 reactions have been seen in glycoside synthesis (see Section 12.4) and biosynthesis (see Box 12.4), and for the synthesis of aminosugars (see Section 12.9). For pyrimidine nucleotide biosynthesis, the nucleophile is the 1-nitrogen of uracil-6-carboxylic acid, usually called orotic acid. The product is the nucleotide orotidylic acid, which is subsequently decarboxylated to the now recognizable uridylic acid (UMP). 

Orotic acid (uraciI-6-carboxylic acid), an intermediate in the biosynthesis of uracil, also reacts smoothly with CF3SCI in pyridine to give the 5-substituted compound. The pyridinium salt initially formed can be easily cleaved with dilute hydrochloric acid ... 

The second step in pyrimidine synthesis is the formation of car-bamoylaspartate, catalyzed by aspartate transcarbamoylase. The pyrimidine ring is then closed hydrolytically by dihydroorotase. Thi resulting dihydroorotate is oxidized to produce orotic acid (onotate, Figure 22.21). The enzyme that produces orotate, dihydroorotate dehydrogenase, is located inside the mitochondria. All other reactions in pyrimidine biosynthesis are cytosolic. [Note The first three enzymes in this pathway (CPS II, aspartate transcarbamoylase, and dihydroorotase) are all domains of the same polypeptide chain. (See k p. 19 for a discussion of domains.) This is an example of a multifunctional or multicatalytic polypeptide that facilitates the ordered synthesis of an important compound.]... 

The stimulatory effect of l-(chloromethyl)silatrane on the DNA synthesis in the cells of the regenerating liver is shown in Table 10 and Fig. 3. In a similar way (according to incorporation of 3H-thymidine, 14C-orotic acid and 3H-leucine) it has been found that l-(chloromethyl)silatrane intensifies the DNA, RNA and protein biosynthesis in other developing cells (by 20—60%). This has shown the importance of further investigation of this preparation as a stimulator of cell division and biosynthesis of nucleic acids and proteins. 

Treatment of HCN-polymer with 6.0NHC1 afforded adenine 1, AICN, 3,4-dihydroxypyrimidine 9 and 5-hydroxyuracil 10, while 1, AICN and orotic acid 11 were recovered after reaction with sodium hydroxide (Scheme 5). A reaction mechanism involving the formation of different aminopyridines as intermediates and reduction steps was proposed to explain the distribution of the obtained products. In accordance with the chemomimetic concept, orotic acid is a key intermediate in the current biosynthesis of pyrimidine nucleotides [62],... 

Orotic acid (2,6-dioxo-l,2,3,6-tetrahydropyrimidine-4-carboxylic acid) was first isolated in 1905 from the whey of cows milk. It is the key substance in the biosynthesis of probably all naturally occurring pyrimidines starting with L-aspartic acid <57MI 602-01 >. 

C] aspartie and orotic acids for the biosynthesis of pyrimidine nucleotides. 

Pyrimidines play a central role in cellular regulation and metabolism. They are substrates for DNA and RNA biosynthesis, regulators of biosynthesis of some amino acids, and cofactors in the biosynthesis of phospholipids, glycolipids, sugars, and polysaccharides (17B45). Pyrimidine biosynthesis is very complicated and involves formic acid, glutamate, and aspartate as starting materials in a series of enzymatic reactions to eventually form orotic acid. Orotic acid, or uracil-... 

When ornithine transcarbamoylase (OTC) is deficient, the carbamoyl phosphate that normally would enter the urea cycle accumulates and floods the pathway for pyrimidine biosynthesis. Under these conditions, excess orotic acid (orotate), an intermediate in pyrimidine biosynthesis, is excreted in the urine. It produces no ill effects but is indicative of a problem in the urea cycle. 

In the next step of pyrimidine biosynthesis, the entire aspartate molecule adds to carbamoyl phosphate in a reaction catalyzed by aspartate transcarbamoylase. The molecule subsequently closes to produce a ring (catalyzed by dihydroorotase), which is oxidized to form orotic acid (or its anion, orotate) through the actions of dihydroorotate dehydrogenase. The enzyme orotate phosphoribosyl transferase catalyzes the transfer of ribose 5-phosphate from PRPP to orotate, producing orotidine 5 -phosphate, which is decarboxylated by orotidylic acid dehydrogenase to form... 

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