Regulation of Purine Biosynthesis

Regeneration of the ribonucleotide reductase is accomplished in Escherichia coli and in mammals by thioredoxin, a dithiol polypeptide (M.W. 12,000) coenzyme, which also plays a role in other protein disulfide reductase reactions. In thioredoxin, two cysteine residues in the sequence -Cys-Gly-Pro-Cys are converted to cystine. Reduced thioredoxin is regenerated by thioredoxin reductase, a flavoprotein enzyme that uses NADPH + H+. 

coli mutants unable to synthesize thioredoxin are still able to form deoxyribonucleotides. In these bacteria, a related substance, glutaredoxin, and two molecules of glutathione carry out the reduction. In Lactobacillus, the triphosphate is reduced and vitamin is an essential coenzyme. Another example of this use of vitamin Bi2 is in Euglena, where the diphosphates are reduced. The mammalian system is nearly identical to that of E. coli. 

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The lac repressor monomer, a chain of 360 amino acids, associates into a functionally active homotetramer. It is the classic member of a large family of bacterial repressors with homologous amino acid sequences. PurR, which functions as the master regulator of purine biosynthesis, is another member of this family. In contrast to the lac repressor, the functional state of PurR is a dimer. The crystal structures of these two members of the Lac I family, in their complexes with DNA fragments, are known. The structure of the tetrameric lac repressor-DNA complex was determined by the group of Mitchell Lewis, University of Pennsylvania, Philadelphia, and the dimeric PurR-DNA complex by the group of Richard Brennan, Oregon Health Sciences University, Portland. 

Patients with Lesch-Nyhan syndrome have hyperuricemia, indicating an increased biosynthesis of purine nucleotides, and markedly decreased levels of hypoxanthine phbs-phoribosyl transferase (HPRT). The hyperuricemia can be explained on the basis of a decrease in which regulator of purine biosynthesis ... 

Regulation of purine biosynthesis. Red arrows show points of inhibition 0 or activation . In addition to the feedback inhibition, GTP stimulates ATP synthesis, and ATP stimulates GTP synthesis, thus helping to ensure a balance between the pools of the two nucleoside triphosphates. The full biosynthetic pathways are shown in figures 23.10 and 23.11. 

K12 Kelley, W. N., Fox, J. and Wyngaarden, J. B. Regulation of purine biosynthesis in cultured human cells. I. Effects of orotic acid. Biochim. Biophys. Acta, 215, 512-516 (1970)... 

The relevance of these studies of the partially purified enzjrme to the regulation of purine biosynthesis in vivo is uncertain. 

The hormonal regulation of purine biosynthesis is not known and there are more indications taken from clinical data based on experimental ground. 

Henderson, J.F. (1972) Regulation of Purine Biosynthesis, American Chemical Society Monograph 170, Washington, D.C. 

Glucagon increases DNA synthesis in regenerating rat liver or even in nonoperated rat liver and an elevated level of plasma glucagon has been documented after hepatectomy. Other investigations have demonstrated that the concentration of 5-phosphoribosyl 1-pyrophosphate(PP-ribose-P), which is an important regulator of purine biosynthesis de novo, and the rate of purine biosynthesis de novo itself are increased by glucagon administered in mice and in isolated rat hepatocytes,... 

Studies vivo as well as in cultured human fibroblasts suggest that the intracellular concentration of PP-ribose-P plays an essential role in the regulation of purine biosynthesis de novo (Kelley, et al., 1970 Kelley, Fox and Wyngaarden,... 

In a large percentage of patients with gout, hyperuricemia is the result of an increase in the rate of purine biosynthesis de novo (Wyngaarden and Kelley, 1972). Consequently, it is important to understand the molecular basis for the regulation of purine biosynthesis in man. 

In the last few years, phosphoribosylpyrophosphate (PRPP) has been viewed as a most important intermediate metabolite and studies have indicated that its concentration and rate of synthesis may play a critical role in the regulation of purine biosynthesis da novo. Hershko et al (l) found increased in vitvo formation of PRPP in the erythrocytes of some gouty subjects. They further suggested that the increase in the rate of purine uptake found in these patients was attributable to enhanced PRPP formation. 

Recent advances in the understanding of human purine metabolism have been stimulated by the discovery of specific inborn errors of this pathway in man. In particular, the demonstration of the deficiency of hypoxanthine-guanine phosphoribosyltransferase (HGPRT) in the Lesch-Nyhan syndrome and in some patients with gout has contributed essential information on the regulation of purine biosynthesis novo and on the critical role of this reutilization pathway in central nervous system function in man. The search for other disorders led to the description of a partial deficiency of adenine phosphoribosyltransferase (APRT) in four members in three generations of one family. Each of the subjects partially deficient in APRT exhibited a normal serum urate concentration and the propositus had a normal excretion of uric acid (Kelley, et al., 1968). We have investigated a second family partially deficient in APRT (Fox and Kelley, in press). 

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