Orotidylic pyrophosphorylase

Certain therapeutic effects can be attributed to the inhibition of specific enzymic reactions. The inhibition of cholinesterase (Section 1.06.3), orotidylate pyrophosphorylase (Section 1.06.5) and of dihydrofolate reductase (Section 1.06.6.3) have already been discussed. They illustrate two modes of action, chemical alteration of the enzyme and competition with a substrate for the active site. 

Estimates of PRibPP levels from studies with fresh cell lysates indicate that the concentration is about 8 X 10 M which is well below the Km of the enzyme 2 X 10 M (D3, H5). The assay techniques are indirect. In one, C-labeled adenine is added along with excess purified APRT. The amount of AMP- C formed is a measure of the PRibPP present. In a second assay, carboxy-labeled orotic acid is added together with orotidylate pyrophosphorylase and orotidylate decarboxylase. The liberated C02 is a measure of the orotidylic acid which is a measure, in turn, of the PRibPP concentration. 

The yeast orotate phosphoribosyltransferase reaction is unusual in that it is readily reversible, in contrast to the subsequently discovered purine and uracil phosphoribosyltransferase reactions (see Chapters 5 and 12). The formation of orotidylate and the pyrophosphorolysis of orotidylate readily proceed to equilibrium the equilibrium constant for the forward reaction is about 0.1 14)- The reversibility of this reaction was the basis for the earlier name of the enzyme, orotidylate pyrophosphorylase. 

As early as 1949, it was demonstrated that injected or " C-labeled orotic acid was readily incorporated into DNA and RNA of mammalian tissue, indicating that orotic acid is a precursor of nucleic acid pyrimidine. The next step in pyrimidine biosynthesis is the formation of the first nucleotide in the sequence. It involves the reaction between ribosyl pyrophosphate and orotic acid to yield 5 -orotidylic acid the reaction is catalyzed by orotidylic pyrophosphorylase. Thus, the first steps of pyrimidine biosynthesis differ from the early steps of purine biosynthesis in at least two ways. Orotic acid, instead of being synthesized atom by atom as is the case for the purine ring, is made from the condensation of rather large molecules, namely, carbamyl phosphate and aspartic acid. Furthermore, all the steps of purine biosynthesis occur at the level of the nucleotide, but the the pyrimidine ring is closed at the level of the base. 

Orotic aciduria insufficient levels of the enzyme orotidylic pyrophosphorylase orotidylic decarboxylase. Very rare appears first year or so of life severe megobiestic anemia rosisiam to usual treatments mental and physical retardation excreta large quantities of ortic acid in urine needle-shaped crystals in urine. Dietary supplementation with large doses of uridine improves eoemia and mental and physical development. Lady diagnosis essential. 

OROTIDYLATE DECARBOXYLASE Orotidine-5 -phosphate decarboxylase, OROTIDYLATE DECARBOXYLASE Orotidine-5 -phosphate pyrophosphorylase,... 

A metabolic alteration leading to overproduction of either PRPP or glutamine would cause an overproduction of 5 -phosphoribosylamine. Excess PRPP could result from overproduction of its precursor glucose, or from reduced use in other pathways, such as pvrimidine biosynthesis. Increased incorporation of " Qglucose and accelerated turnover of PRPP have been reported. A case of gout with low orotidylic acid pyrophosphorylase and decarboxylase activity has also been described. Whether these metabolic changes constitute the primary injury or are several steps removed from it remains to be established. 

The formation of the first nucleotide in the pyrimidine sequence, orotidylic acid (orotidine 5 -phosphate), was accomplished by the reaction of 5-phosphoribosyl-l-pyrophosphate (PRPP) with orotic acid 83) (Fig. 22). Other pyrimidines and carbamylaspartic acid did not react with PRPP in the presence of the enzyme, which has been named orotidine 5 -phosphate pyrophosphorylase. Several purine analogs, e.g., 6-uracilsulfonic acid, 6-uracil methyl sulfone, which inhibited the growth of several organisms (S78, 379), probably inhibited the formation of orotidylic acid. 

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