In this article we will discuss about the Metabolism of Purine Nucleotides:- 1. Sources of the Various Atoms of the Purine Base 2. Biosynthesis of Purine Nucleotides [DE NOVO] 3. Salvage Reaction 4. Catabolism 5. Formation of Uric Acid.
- Sources of the Various Atoms of the Purine Base
- Biosynthesis of Purine Nucleotides [DE NOVO]
- Salvage Reaction of Purine Nucleotides
- Catabolism of Purines
- Formation of Uric Acid
1. Sources of the Various Atoms of the Purine Base:
a. Glycine is utilized to form the carbon positions 4 and 5 and its α-nitrogen forms the nitrogen in position 7.
b. The amino nitrogen of aspartic acid provides the nitrogen at position 1.
c. The N atoms at positions 3 and 9 are derived from the amide nitrogen glutamine.
d. The carbon atom at position 6 is derived from CO2.
e. The carbons in positions 2 and 8 are supplied from a one carbon (C-I) compound given by the tetrahydrofolate carrier.
This is represented below (Fig. 23.1):
2. Biosynthesis of Purine Nucleotides [DE NOVO]:
a. Ribose-5-phosphate is converted into 1-pyro-phosphoribosyl-5-phosphate (PP ribose P) by PP ribose P synthetase with ATP and Mg++.
b. PP ribose P then reacts with glutamine by the enzyme phosphoribosyl pyrophosphate glutamyl amido-transferase to form 5-phosphoribosylamine by the displacement of pyrophosphate and the formation of glutamate.
c. 5-Phosphoribosylamine reacts with glycine to produce glycinamide ribosylphosphate by glycinamide kinosynthetase in presence of ATP.
d. The N7 of glycinamide ribosylphosphate is formylated by the enzyme glycinamide ribosylphosphate formyl-transferase to transfer the C1 moiety.
e. Amidation from glutamine occurs at the C4 of the formylgycinamide ribosyl-5-phosphate by formalglycinamide ribosyl-5-phosphate synthetase requiring ATP. The amide N becomes position 3 in the purine.
f. Imidazole ring is closed by amino-imidazole ribosyl-5-phosphate synthetase requiring ATP.
g. Respiratory CO2 is utilized requiring biotin to form amino-imidazole carboxylate ribosyl-5-phosphate by amino-imidazole ribosyl-5-phosphate carboxylase.
h. Addition of Aspartate by IX synthetase forms amino-imidazole succinyl carboxamide ribosy 1-5-phosphate which is converted into amino-imidazole carboxamide ribosyl phosphate by adenylosuccinase.
i. Transformylase converts the above products into nucleoside phosphate and then the ring is closed.
j. The major determinant of the overall rate of de novo purine nucleotide biosynthesis is the concentration of PRPP.
The rate of PRPP synthesis depends both on the availability of ribose 5-phosphate and on the activity of PRPP synthetase.
3. Salvage Reaction for Purine Nucleotides:
a. In salvage reaction purines, purine ribonucleosides and purine deoxyribo-nucleosides-are converted to mononucleotides. There is requirement of far less energy than de novo synthesis.
b. The more important mechanisms are:
(a) Phosphoribosylation of a free purine (Pu) by PRPP, forming a purine 5′- mononucleotide (Pu-RP)
Pu + PP-RP → Pu-RP + PPi
This reaction is catalyzed by adenine phosphoribosyltransferase and hypoxanthine guanine phosphoribosyltransferase.
(b) Direct phosphorylation of a purine ribonucleoside (PuR) by ATP
PuR + ATP → PuR-P + ADP
Adenosine kinase catalyzes phosphorylation of adenosine to AMP or of deoxyadenosine to dAMP.
Hepatic Purine Nucleotide Biosynthesis is Stringently Regulated:
a. Mammalian liver is the major site of purine nucleotide biosynthesis. Liver provides purines and their nucleosides for salvage reaction.
b. Human brain has a low level of PRPP amido-transferase and hence depends on exogenous purines.
c. Erythrocytes and polymorphonuclear leukocytes cannot synthesize 5-phosphoribosylamine and so utilize exogenous purines to form nucleotides.
AMP and GMP Feedback regulate their Formation from IMP:
a. Two mechanisms regulate conversion of IMP to GMP and AMP.
b. AMP feedback regulates adenylo succinate synthetase and GMP feedback inhibits IMP dehydrogenase.
c. For the conversion of IMP to AMP requires GTP and conversion of xanthinylate to GMP requires ATP.
d. Cross regulation between the pathways of IMP metabolism thus serves to decrease synthesis of one purine nucleotide when there is a deficiency of the other nucleotide.
4. Catabolism of Purines:
Uric acid is the chief end-product of purine catabolism in man and the higher apes. Other mammals degrade uric acid to allantoin by means of the enzyme, uricase, which is lacking in primates.
Almost all tissues contain enzymes capable of breaking nucleoprotein down to nucleoside which can be oxidized to uric acid. Uric acid is always excreted even on a purine-free diet or in starvation. Urinary uric acid is both endogenous and exogenous in origin.
Organisms that form uric acid as the major nitrogenous waste product are said to be uricotelic. Birds, amphibius and reptiles do not possess uricase activity. These animals excrete uric acid and guanine as the end-products of purine metabolism and nitrogen(protein) metabolism. In man and most of the mammals, urea is the main product of nitrogen metabolism. Hence, they are ureotelic.
In animals other than mammals, uric acid is further degraded to urea and glyoxylic acid.
5. Formation of Uric Acid:
a. Adenine (6-amino-purine) is deaminated by adenylate deaminase to form inosinic acid. Adenylate deaminase is quite abundant in skeletal muscle. Adenosine can also be deaminated to form inosine.
b. Both inosinic acid and inosine give rise to free hypoxanthine, which may be reutilized for nucleic acid synthesis but is most frequently oxidized to xanthine by the enzyme xanthine oxidase present in greatest amount in liver, small intestine and kidney.
c. Xanthine oxidase further oxidizes xanthine to uric acid (2, 6, 8-trioxypurine).
d. Free guanine (2-amino-6-oxy-purine) is deaminated to form xanthine directly by the enzyme guanase, which is very active in most tissues. The liberated xanthine is then converted to uric acid by xanthine oxidase.
The pathway for the formation of uric acid is as follows (Fig. 23.4):
Some uric acid may be produced from nucleic acid by the bacterial flora of the intestinal tract, when it is absorbed and directly excreted. This pathway is a minor contributor to the urinary uric acid of persons on a normal diet.
From recent studies it appears that sodium urate is freely filtered by the mammalian glomerulus. It is reabsorbed and secreted in the proximal tubules and the loop of Henle and partially reabsorbed in the distal convoluted tubules.
The net excretion of total uric acid in normal men is 600-700 mg in 24 hours. Aspirin in high doses competitively inhibits urate excretion as well as reabsorption. Allopurinol competitively inhibits Xanthine oxidase for which uric acid cannot be formed.
Uric acid is mainly excreted in urine, to a lesser extent in digestive fluid, and in small amounts in sweat and saliva. A portion of the uric acid is destroyed by bacterial action in the intestine. This intestinal uricolysis gives rise to urea and ammonia, which are absorbed and excreted by the kidneys. Under conditions of normal production and removal, the body contains a ‘readily miscible uric acid pool’.
The normal uric acid content of serum is 2.5 to 7.0 mg/100 ml for adult males and 1.5 to 6.0 mg/ 100 ml for premenopausal females. One-third of it is loosely bound to plasma proteins, mostly albumin, but some is bound to α1 -α2 globulins. Super- saturation of uric acid causes the disease gout which is much more common in males. Only about 5 per cent of gouty patients are females and most of them are menopausal.
Normal adults excrete less than 450 mg of uric acid daily on a low containing nucleoprotein diet. This indicates that uric acid is formed from the catabolism of endogenous nucleic acids and nucleotides. A high protein and caloric intake causes increased uric acid. The output of uric acid may rise to 1 gram daily on a high purine diet (meat, liver, kidney, sweet breads).
Some uricosuric agents such as salicylates, cinchophen and carinamide increase urinary elimination of uric acid by inhibiting its reabsorption in the renal tubules by blocking the enzymatic transport mechanism. ACTH and adrenocortical oxy-steroids also increase the urinary excretion of uric acid by inhibiting renal tubular reabosrption.
Hyperuricemia is due to overproduction, decreased destruction and decreased renal excretion.
Increased values are observed in all forms of nephritis with nitrogen retention. Values as high as 10 mg /100 ml are frequently observed. Serum uric acid is also increased in eclampsia. In chronic leukemia, blood uric acid level is increased. Uric acid level in blood may also shoot up in sickle cell anemia, thalassemia, heinolyric anemia and macroglobulinemia.