Purine metabolic regulation

The biosynthesis of purines and pyrimidines is stringently regulated and coordinated by feedback mechanisms that ensure their production in quantities and at times appropriate to varying physiologic demand. Genetic diseases of purine metabolism include gout, Lesch-Nyhan syndrome, adenosine deaminase deficiency, and purine nucleoside phosphorylase deficiency. By contrast, apart from the orotic acidurias, there are few clinically significant disorders of pyrimidine catabolism. 

Adenosine deaminase (ADA) was the first therapeutic enzyme coupled to PEG with the aim of reducing clearance and thereby overcoming the short half-life of ADA. Patients deficient in ADA are unable to regulate purine metabolism. As a result purine metabolites (e.g., adenosine monophosphate) accumulate to cytotoxic levels in B-lymphocytes and lead to severe B-cell depletion that presents clinically as severe combined immunodeficiency syndrome (SCIDS). While intramuscular injection of unmodified ADA provides some relief, antibodies develop rapidly against the protein and prevent it from being useful as replacement therapy. Even in the absence of antibodies, unmodified ADA s plasma half-life is only a few minutes. 

Several enzyme systems regulate purine metabolism. Abnormalities in these regulatory systems can result in overproduction of uric acid. Uric acid also may be overproduced as a consequence of increased breakdown of tissue nucleic acids, as with myeloproliferative and lymphoproliferative disorders. Dietary purines play an unimportant role in the generation of hyperuricemia in the absence of some derangement in purine metabolism or elimination. 

Cydic purine nucleoside monophosphates (cAMP and cGMP) are important second messengers in metabolic regulation (i.e. they carry messages within the cell, triggered by extracellular hormones). 

Purine and pyrimidine nucleotides are fundamental to life as they are involved in nearly all biochemical processes. Purine and pyrimidine nucleotides are the monomeric units of both DNA and RNA, ATP serves as the universal cellular energy source, adenine nucleotides are components of three key coenzymes (NAC", FAD and Co A), they are used to form activated intermediates, such as UDP-glucose, and they serve as metabolic regulators. 

S.K. Wadman, P.K. de Bree, A.H. van Gennip, J.W. Stoop, B.J.M. Zegers and G.E.J. Staal, Urinary purines in a patient with a severely defective T cell immunity and a purine nucleoside phos-phorylase deficiency, "Purine Metabolism in Man-II regulation of pathways and enzyme defects", M.M. Muller, E. Kaiser and J.E. Seegmiller, Plenum Publishing Corporation, New York (1977) pp 471-477. 

Our own work (3) and that of others (2) with E. coll have shown that the de novo purine biosynthetic pathway is regulated by both a repressor molecule (pur R gene product) and by feedback inhibition. However, Chinese hamster cells are much more sensitive to feedback inhibition by adenine than E, coli and, unlike the situation in E. coli, no repression of PRPP amidotransferase or formyglycinamide biosynthesis could be detected. If repression did occur, it would have to be by a mechanism not normally associated with the purine biosynthetic pathways or at a site late in the purine bios3mthetic pathway. Moreover, the nucleotide pools of cells treated for 2 h with with actinomycin D or cycloheximide showed a substantial increase in nucleotide levels. This Increase in nucleotide concentration is probably sufficient in itself to inhibit de novo purine biosynthesis by feedback inhibition without recourse to a repression mechanism, Snyder and Henderson (10) have also reported an effect of actinomycin D on purine metabolism in Ehrlich ascites cells. In this case, there was no large effect (11% inhibition) on de novo purine biosynthesis, Snyder and Henderson (10) proposed that this decrease was due to a 29% reduction in PRPP levels as a result of increased (1,3-fold increase in ATP and 2,8-fold Increase in GTP) nucleotide pools. These observations are consistent with our data in which a 58% decrease in PRPP level is found over a 2-h period in Chinese hamster cells grown in actinomycin D, The extent of inhibition in Chinese hamster cells is much greater than that reported for Ehrlich ascites cells and may reflect a difference between cells,... 

Figure 3.1 shows the nucleotides formed from the purine adenine - the adenine nucleotides, adenosine monophosphate (AMP), adenosine diphosphate (ADP) and adenosine triphosphate (ATP) - as well as the nucleotide triphosphates formed from the purine guanine and the pyrimidine uracil (see also section 10.3.2 for a discussion of the role of cyclic AMP in metabolic regulation and hormone action). 

It has been established in earlier investigations >2 that the synthesis of purine nucleotides in mammalian red blood cells (RBC) is governed by the extent of endogenous supply of 5-phosphoribosyl-1-pyrophosphate (PRPP), as an essential intermediary. Consequently, the elucidation of the mechanisms controlling the formation of PRPP within the cell appears to be of crucial importance for the understanding of the overall metabolic regulation of purine nucleotide biosynthesis. 

Intracellular PP-ribose-P, a ribose sugar with phosphate at the five position and pyrophosphate at the 1 position (Fig. 1), is complexly regulated by a balance between synthesis and degradation. Any alteration in PP-ribose-P concentration may potentially alter the rate of purine biosynthesis novo since PP-ribose-P is rate limiting for this pathway. Drug induced changes of intracellular PP-ribose-P may in this way substantially alter human purine metabolism. 

Much of recent progress in our understanding of the regulation of purine metabolism in man has resulted from in vitro studies on cells from patients with inborn errors of metabolism (1, 2). 

Gots, J S. Regulation of purine and pyrimidine metabolism. In Metabolic Pathways, 3rd Ed., Vol. 5, Metabolic Regulation, Ed. H. J. Vogel, New York, Academic Press, 1971, p. 225. 

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). 

This fortuitous observation of a case of gout with a deficiency of an enzyme concerned in the regulation of uric acid synthesis by feed back, raises the question whether this deficiency is responsible for the abnormal purine metabolism and uric acid hyper-synthesis. 

The role of the end products of a metabolic pathway in regulating their own biosynthesis was first demonstrated by Roberts et al. (1955). Working with E. co/z, they showed that amino acid synthesis from glucose is inhibited by the addition of amino acids to the incubation medium. Umbarger (1956) demonstrated that end products may inhibit the activity of enzymes mediating end-product synthesis. Often this inhibition is exerted on the first enzyme of the metabolic sequence. End products may also inhibit enzyme synthesis itself, as is frequently observed in anabolic pathways for amino acids, purines, and pyrimidines. This latter mode of metabolic regulation is termed repression and may occur independently of feedback inhibition. Both mechanisms may be involved in regulation of the same biosynthetic pathway. However, unlike feedback inhibition, which provides very rapid control, repression is a relatively slow process which permits adjustment of metabolism over an extended period of time. 

Nucleotide levels apparently regulate CPS activity (O Neal and Naylor, 1968,1976 Ong and Jackson, 1972a,b Parker and Jackson, 1981). Pyrimidine nucleotides inhibit CPS while purine nucleotides (GMP, IMP) stimulate activity. Ornithine activated CPS and could overcome inhibition by pyrimidines whereas arginine did not affect the activity of CPS from any of these sources. Metabolic regulation of plant OCT has not been reported. Compartmentaliza-tion at the subcellular and tissue levels as well as developmental control are certainly important in the regulation of this key pathway. Molecular, biochemical, and immunological approaches will all be necessary to unravel the regulatory features of these key enzymes and metabolic pathways. 

Goordinated regulation of purine and pyrimidine nucleotide biosynthesis ensures their presence in proportions appropriate for nucleic acid biosynthesis and other metabolic needs. 

This seventh edition includes discussions of neurotransmitters ranging from acetylcholine through other amines, amino acids, purines, peptides, steroids and lipids Whereas in most cases their metabolism and receptor interactions are known, much current research involves questions of identification of effector pathways, their regulation and control. 

---