Orotate reductase

A major class of enzymes that catalyze oxidation-reduction reactions. This class includes dehydrogenases, reductases, oxygenases, peroxidases, and a few synthases. Examples include alcohol dehydrogenase (EC 1.1.1.1), aldehyde oxidase (EC 1.2.3.1), orotate reductase (EC 1.3.1.14), glutamate synthase (EC 1.4.1.14), NAD(P) transhydrogenase (EC 1.6.1.1), and glutathione peroxidase (EC 1.11.1.9). 

Reaction of aspartic acid (14) with carbamoyl phosphoric acid (17) in the presence of the allosteric enzyme aspartate carbamoyltransferase (aspartate transcar-bamoylase) gives N-carbamoyl aspartic acid (18), which is cyclised to L-dihy-droorotic acid (19) by dihydroorotase. Oxidation of L-dihydroorotic acid by flavoprotein, orotate reductase gives orotic acid (20), which reacts with 5-phosphori-bosy 1-1-pyrophosphate (PRPP) in the presence of orotate phosphoribosyl transferase to form orotidine 5 -monophosphate (OMP, 21). Decarboxylation of OMP by orotid-ine 5 -phosphate decarboxylase yields uridine 5 -monophosphate (UMP, 22), which acts as precursor for the cytidine nucleotides (CTP) (Chart 6). 

Aspartate carbamyltransferase 2 dihydroorotase 3 orotate reductase 4 orotate phosphoribosyl-transferase 5 orotidine-5 -phosphate decarboxylase 6 cytidylate kinase, nucleotide diphosphate kinase 7 cytidine triphosphate synthetase 8 nucleoside monophosphate kinase, ribonucleoside diphosphate reductase, phosphatase 9 thymidylate synthase... 

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]. 

Fig. lA. Anabolic and catabolic pathways of 5-FU. DPD dihydropyrimidine dehydrogenase, DP di-hydropyrimidinase, pUP beta-ureidopropionase, UP uridine phosphorylase, OPRT orotate phospho-ribosyl transferase, UK uridine kinase, TP thymidine phosphorylase, TK thymidine kinase, RNR ribonucleotide reductase. The three active metabolites (shown in rectangles) are FdUMP (5-fluoro-2 -deoxyuridine 5 -monophosphate) inhibiting TS (thymidylate synthase), and FUTP (5-fluorouridine 5 -triphosphate) and FdUTP (5-fluoro 2 -deoxyuridine 5 -triphosphate) interfering with RNA and DNA, respectively. 

Figure 7-20. De novo synthesis of purines and pyrimidines. Ribonucleotide reductase (R.R.) catalyzes the reduction of the ribose moiety in ADP, GDP, and CDP to deoxyribose. The source of each of the atoms is indicated in the boxes at the bottom of the figure. In hereditary orotic aciduria, the enzymes converting orotate to UMP are defective ( ).

Entamoeba histolytica. There are several lines of evidence that E. histolytica is capable of pyrimidine de novo biosynthesis. It will incorporate orotate into nucleic acids (82), it has aspartate transcarbamoylase (83), and it will grow in medium that has been depleted of pyrimidines (84). It apparently can also meet its pyrimidine requirements by salvage (85). Its salvage pathways are similar to those of T. vaginalis, with the exception of the ability to salvage thymine (Fig. 6.15). Ribonucleotide reductase and thymidylate synthase activities are apparently present, since this parasite will grow in a pyrimidine deficient medium (84). 

The following formally a,p-unsaturated acids are not substrates of enoate reductase benzoic, orotic, nicotinic, and shikimic acid. 

Lysinuric protein intolerance Hereditary orotic aciduria Pyrimidine-5-nucleotidase def. Familial LCAT def. Wilson disease Acid lipase def. (Wolman disease) 3-Phosphoglycerate dehydrogenase deL Methylentetrahydrofolate reductase def. Methioninsynthase def. 

The key steps in Fluorouracil metabolism are shown in Scheme 1. Up to 80 % of 1 administered as injection is transformed to dihydrofluorouracil (DHFU, 13) by dihydropyrimidine dihydrogenase (mostly in liver tissues). However, this metabolite is not involved into antineoplastic activity instead, 13 itself and its further metabolites are responsible for most of the toxic effects of 1. The main mechanism of activation of Fluorouracil is conversion to fluorouridine monophosphate (FUMP, 14), either directly by orotate phosphoribosyltransferase, or via fluorouridine (FUR, 15) through the sequential action of uridine phosphorylase and uridine kinase. 14 is then phosphorylated to give fluorouridine diphosphate (FUDP, 16), which can be either phosphorylated again to the active metabolite fluorouridine triphosphate (ITJTP, 19), or reduced to fluorodeoxyuridine diphosphate (FdUDP, 18) by ribonucleotide reductase. In turn, 18 can either be dephosphorylated or phosphorylated to generate... 

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