Dihydroorotate dehydrogenase

The active metabolite of leflunomide, the ring-opened drug A771726, inhibits dihydroorotate dehydrogenase (DHOD) which is the key enzyme of the de novo pyrimidine synthesis. Inhibition of synthesis stops proliferation of activated lymphocytes. The leflunomide derivative FK778 which shows similar therapeutic efficacy but shorter half-life is investigated in clinical trials. 

Steric effects and FMO control have been combined in an elegant way to achieve regiospecific synthesis of pyrazole inhibitors of dihydroorotate dehydrogenase <2006SL901>. When the size of the propargylic acid ester 86 is increased from ethyl to diphenylmethyl, pyrazole 87 is formed from compound 85 regiospecifically (Scheme 3 Table 4) <2006H(68)1007>. 

DIHYDROOROTATE DEHYDROGENASE DIHYDROOROTATE OXIDASE DIHYDROPTERIDINE REDUCTASE DIHYDROPTEROATE SYNTHASE DIHYDROPYRIMIDINASE DIHYDROURACIL DEHYDROGENASE Dihydroxyacetone kinase,... 

Leflunomide is an immunomodulatory dmg inhibiting dihydroorotate dehydrogenase, an enzyme involved in de novo pyrimidine synthesis. It has also anti-inflammatory effects. Leflunomide is able to slow progression of the disease and to cause re-mission/relief of symptoms of rheumatoid arthritis and psoriatic arthritis such as joint tenderness and decreased joint and general mobility in patients. The combined use of leflunomide with methotrexate may... 

Mechanism of Action A DMARD that inhibits dihydroorotate dehydrogenase, the enzyme involved in autoimmune process that leads to rheumatoid arthritis. Therapeutic Effect Reduces signs and symptoms of rheumatoid arthritis and slows structural damage. 

Leflunomide undergoes rapid conversion, both in the intestine and in the plasma, to its active metabolite, A77-1726. This metabolite inhibits dihydroorotate dehydrogenase, leading to a decrease in ribonucleotide synthesis and the arrest of stimulated cells in the Gi phase of cell growth. Consequently, leflunomide inhibits T-cell proliferation and production of autoantibodies by cells. Secondary effects include increases of interleukin-10 receptor mRNA, decreased interleukin-8 receptor type A mRNA, and decreased TNF-a-dependent nuclear factor kappa (NF- ) activation. 

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.]... 

In the third step, the pyrimidine ring is closed by dihy-droorotase to form 1-dihydroorotate. Dihydroorotate is then oxidized to orotate by dihydroorotate dehydrogenase. This flavoprotein in some organisms contains FMN and in others both FMN and FAD. It also contains nonheme iron and sulfur. In eukaryotes it is a lipoprotein associated with the inner membrane of the mitochondria. In the final two steps of the pathway, orotate phosphoribosyltransferase yields orotidine-5 -phosphate (OMP), and a specific decarboxylase then produces UMP. 

The third reaction is the oxidation of the ring to form a carbon-carbon bond. The reducing equivalents are transferred to a flavin cofactor of the enzyme dihydroorotate dehydrogenase. The product is orotic acid. 

Leflunomide (le FLEW nom eyed) is an isoxazole immunomodulatory agent that preferentially causes cell arrest of the autoimmune lymphocytes through its action on dihydroorotate dehydrogenase (DHODH). Stimulation of a T cell by an antigen-presenting cell drives the lymphocyte into its replicative cycle. Many enzymes, including those required for de novo purine, pyrimidine, RNA, and membrane synthesis, are upregulated in the G1 phase of the cycle. 

Dihydroorotate dehydrogenase, the enzyme catalyzing the dehydrogenation of dihydroorotate to orotate (reaction 4 of the pathway Fig. 15-15), is located on the outer side of the inner mitochondrial membrane. This enzyme has FAD as a prosthetic group and in mammals electrons are passed to ubiquinone. The de novo pyrimidine pathway is thus compartmentalized dihydroorotate synthesized by trifunctional DHO synthetase in the cytosol must pass across the outer mitochondrial membrane to be oxidized to orotate, which in turn passes back to the cytosol to be a substrate for bifunctional UMP synthase. Mammalian cells contain two carbamoyl phosphate synthetases the glutamine-dependent enzyme (CPSase II) which is part of CAD, and an ammonia-dependent enzyme (CPSase /) which is found in the mitochondrial matrix, and which is used for urea and arginine biosynthesis. Under certain conditions (e.g., hyperammonemia), carbamoyl phosphate synthesized in the matrix by CPSase I may enter pyrimidine biosynthesis in the cytosol. 

Fig. 15-15 The de novo pyrimidine biosynthetic pathway. CAP, carbamoyl phosphate CA-asp, /V-carbamoyl-L-aspartate DHO, L-dihydroorotate Oro, orotate OMP, orotidine 5 -monophosphate. Enzymes (1) carbamoyl phosphate synthetase II (2) aspartate transcarbamoylase (3) dihydroorotase, (4) dihydroorotate dehydrogenase (5) orotate phosphoribosyltransferase (6) OMP decarboxylase (7) nucleoside monophosphate kinase (8) nucleoside diphosphate kinase (9) CTP synthetase.

The third structure of labile sulfur is a persulfide linkage which has been proposed by Massey and his coworkers (39) for dihydroorotate dehydrogenase. This enzyme has 2 atoms of iron, and 2 moles of labile sulfur besides its flavin moiety. They suggested that in catalysis the iron sulfide functions as an iron-sulfur radical, and the iron is not reduced by substrate. 

Knecht, W., Bergjohann, U., Gonski, S., Kirschbaum, B., Loefeler, M. (1996). Functional expression of a fragment of human dihydroorotate dehydrogenase by means of the baculovirus expression vector system, and kinetic investigation of the purified recombinant enzyme. 

Thomson, T. A., Spinella-Jaegle, S., Morand, T., CouRTiN, O., Sautes, C., Westwood, R., Hercend,T., Kuo, E. A., Ruuth, E. (1995). Dihydroorotate dehydrogenase is a high-afHnity binding protein of A77 1726 and mediator of a range of biological effects of the immunomodulatory compound. 

FIGURE 5. (A) Interaction of atovaquone with the Fe-S center (on left) in the cytochrome bc complex via H-bonding with the coordinated His (Plate VIII). (B) The active center of a class 2 dihydroorotate dehydrogenase (PDB ID 1UUM) with a hound inhibitor atovaquone (top baU-and-stick stmcture), orotic acid (green) and riboflavin 5 -(dihydrogen phosphate) (FMN, pink) (Plate IX)... 

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