editor:

Roh’rt

comments

Pyrimidine George

E. Shambaugh,

H.

Hc’rinan,

M.D.

in biochemistry

2 III

ABSTRACT Pyrimidines can be synthesized through either a de novo pathway from glutamine and bicarbonate or from uracil via a salvage pathway to form a common product, uridine 5’ monophosphate. The six enzymes in the de novo pathway are carbamyl phosphate synthetase II, aspartate transcarbamylase, dihydroorotase, dihydroorotic acid dehydrogenase, orotate phosphoribosyl transferase, and orotidine 5’ phosphate decarboxylase. The first three (carbamyl phosphate synthetase, aspartate transcarbamylase, dihydrcorotase) and last two enzymes (orotate phosphoribosyl transferase, orotidine 5’ phosphate decarboxylase) exist in separate multifunctional complexes. Both complexes tend largely to be cytoplasmic but may be linked to the membrane-bound enzyme (dihydroorotic acid dehydrogenase). The de novo biosynthesis of pyrimidines may be rate-limited at carbamyl phosphate synthetase and orotate phosphoribosyl transferase. Each complex is controlled by an activator phosphoribosyl pyrophosphate, in addition to product inhibitors, uridine 5’ monophosphate, or uridine 5’ triphosphate. Under normal conditions control is maintained by channeling carbamyl phosphate within the enzyme complex during its subsequent biochemical transformations. Such channeling is altered in hyperammonemic states wherein large quantities of carbamyl phosphate generated from the intramitochondrial ammonia-utilizing carbamyl phosphate synthetase I leak through the mitochondrial membrane to be converted to carbamyl aspartate via aspartate transcarbamylase and through sequential steps to form increased quantities of uridine 5’ monophosphate. The de novo pathway for pyrimidine biosynthesis is closely related to the de novo pathway for purine biosynthesis via common utilization of phosphoribosyl pyrophosphate as a substrate. Underutilization of phosphoribosyl pyrophosphate by the purine pathways as in the Lesch-Nyhan syndrome is accompanied by increased availability of this activator/substrate with consequent enhancement of de novo pyrimidine synthesis. The salvage pathway for pyrimidine biosynthesis converts uracil to uridine 5’ monophosphate, and is rate limited by uridine kinase which is inhibited by cytidine triphosphate. Pyrimidine biosynthesis in adult tissues is accomplished largely through the salvage pathway, while in tissues of the conceptus the de novo pathway predominates. Both pathways are increased in regenerating tissue or in the human lymphocyte undergoing blast transformation. Congenital disorders ofpyrimidine biosynthesis are limited to the second enzyme complex in the de novo pathway. Hereditary orotic aciduna type I is consequent to a deficiency in orotate phosphoribosyl transferase and orotidine 5’ phosphate decarboxylase, and type II to a deficiency in orotidine 5’ phosphate decarboxylase. Both types respond to exogenous uridine. Studies in healthy subjects, treated with allopurinol to block orotidine 5’ phosphate decarboxylase, have indicated that purines and pyrimidines of dietary origin may have potential relevance for homeostatic regulation of pyrimidine biosynthesis. Am. J. Clin. Nutr. 32: 12901297, 1979.

Pyrimidines are ubiquitously distributed throughout the plant and animal kingdoms to serve combined roles of structure and function. The biosynthesis of pyrimidines represents one of the oldest pathways in the living cell, and the sequential steps have remained intact through the evolutional hierarchy of living things. Thus mechanisms studied in 1290

The American

Journal

of Clinical

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From the Center for and Nutrition, Northwestern and Veterans Administration

Chicago,

Illinois. in part

2Supported

theVherans 05071 and tional Nutrition

Endocrinology, Metabohsm University Medical School Lakeside Medical Center,

AM-Ol

Institutes 32: JUNE

by the Medical Research nttion bHe1thSee 169, the Kroc Foundation,

of Health

Grant

l979,pp.l290-1297.

MRP

Service and

Na-

HD-llO2l. Printed

in U.S.A.

PYRIMIDINE

lower

BIOSYNTHESIS

organisms,

such as the eucaryote Neuhave found application in man. Several detailed reviews are currently available on the subject ofpyrimidine metabolism (1-5). This report will therefore attempt to highlight new information and focus on our present knowledge of control mechanisms and their relevance for nutrition. Pyrimidine biosynthesis is closely interrelated with purine metabolism and urea biosynthesis, but unlike the crystalline end prodrospora

(DHOD), orotate phosphoribosyl transferase (OPRT), and orotidine-5’ phosphate decarboxylase (ODC). The de novo pathway begins with the synthesis ofcarbamyl phosphate

crassa,

uct of purine soluble product

products

metabolism, of urea

or the biosynthesis,

stable the

from

but end

of pyrimidine

nucleotides

r I

to

CARBAMYL PHOSHATE SYNTHETASE

ASPARTATE TRA$SCAR8AMYLASE

I GLUTAAVHCAO#,

‘4RB4MYL PHOSMIATE

-s

glutamine,

DIHY

I

I

cps n

on

OOROTA$E

K DIHYDROOROT ACID I

DHO 5

CLUSTER

I

H20

1[HYDROOROTIC ACID DEHYDROGENAS DHOD / /

I I

L4W(INE 5’ MONOPHOSPHATE

OROTATE OROTIDINE 5’ PHOSPHATE DECARBCOCYLASE OROTIDINE #{149} S’MONOPHOSPHATE

ODC

,

//

I

______________

I

5

/

,_..

OPRT

6

/

I,

PHOSPHORIBOSYL TRANSFERASE

(UMP)I I

Mg-aden-

I

CARBAMYL A.ARTATE

2 ENZYME

and

#{149}

ASPARTATE

L

bicarbonate,

osine triphosphate (ATP), a series of reactions catalyzed by CPS II (6). Carbamyl phosphate is now combined with aspartate to form carbamyl aspartate a reaction catalyzed by ATC. Carbamyl aspartate is then converted to dihydroorotic acid by DHO. Since CPS II, ATC, and DHO can be copurified on column chromatography of tissue fractions (7-10), it has been suggested that these three enzymes may exist in cells as a multifunctional complex. This complex is depicted as enzyme cluster i in Figure 1. Clusters confer not only a degree of stability for their constituent enzymes, but provide for a maximally effective channeling of substrates. That CPS ii is likely the rate limiting enzyme in cluster I is suggested not only by low levels of this enzyme relative to ATC and DHO (2, 11) but by a Km for glutamine that is at least two magnitudes lower than tissue concentrations of this amino acid (2, 11). The Km of CPS II for bicarbonate (11 mM) and ATP (3.2 mM) indicates that this enzyme may be saturated with the former substrate but may be allosterically controlled by physiological concentrations of the latter (9). Recent studies have identified two subunits for CPS II in Escherichia coli: a heavy subunit and a light subunit

biosynthesis, varying RNA and DNA, are degraded to CO2 and ammonia leaving little recognizable traces of their passing. Pyrimidines can be synthesized by two pathways, a de novo and a salvage pathway. As uridine 5’ monophosphate represents a common pyrimidine nucleotide formed by both pathways and is the parent compound for the other major pyrimidines, i.e., cytidine 5’ monophosphate, deoxycytidine 5’ monophosphate, and thymidine 5’ monophosphate, the present discussion will focus on those steps culminating in the synthesis of uridine 5’ monophosphate. The de novo pathway for pyrimidine biosynthesis (Fig. 1), contains six enzymes: carbamyl phosphate synthetase (CPS II), aspartate transcarbamylase (ATC), dihydroorotase (DHO), dihydroorotic acid dehydrogenase from

1291

I

iii:

J4

OROTIC ACID

‘PRPP

I I J

ENZYME

CLUSTER

II

FIG. 1. The de novo pathway for pyrimidine biosynthesis. The six enzymes are numbered consecutively appear in the pathway. Names of enzymes are above the line and abbreviations below. The enzyme clusters relationship to the membrane-bound DHOD have not been proven to exist in all tissues. Similar qualifications to the subunit concepts of CPS II.

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as they and the apply

SHAMBAUGH

1292

designated as a and /1, respectively (12). The subunit has a glutaminase action and apparently operates to cleave the glutamine to generate ammonia. The a subunit utilizes ammonia and bicarbonate to form carbamyl phosphate (12). A sister enzyme, the mitochondrial n-acetyl L-glutamate dependent CPS I utilizes ATP, ammonia, and bicarbonate to catalyze the formation of carbamyl phosphate destined to form urea (13). The heavy subunit of the extramitochondrial CPS II is therefore similar to the mitochondrial CPS I. The properties ofCPS II conferred by a combination of a and 8 subunits not only permit a high affinity for glutamine but this subunit complex could provide an additional means for channeling substrates. The second enzyme ATC catalyzes the first reaction unique to pyrimidine biosynthesis, the formation of carbamyl aspartate from carbamyl phosphate and aspartate. Carbamyl aspartate is then converted to the cyclic dihydroorotic acid by loss of water, a reaction catalyzed by the third enzyme in this cluster, DHO. The subcellular localization of both ATC and DHO varies in different species and these enzymes have been found in cytoplasm of rat liver, but in Ehrlich ascites tumor cells 30% of total DHO activity has been found in the nucleus (7). CPS II and ATC activity have a similar distribution in these cells. In contrast to the cluster of the first three enzymes, DHOD the fourth enzyme, which catalyzes the reversible oxidation of dihydroorotic acid to orotic acid, is localized in the nucleus in Ehrlich ascites tumor cells and in the mitochondria of rat liver (7, 14). The membrane association is depicted in Figure 1 by the extension of DHOD from the double layered cross-hatched arc shown on the right. The fifth and sixth enzymes, OPRT and ODC, catalyze respectively the formation of oroti-

dine 5’ phosphate from orotic acid, and the decarboxylation of orotidine 5’ phosphate to form undine 5’ monophosphate. These enzymes exist as a complex located in the cytoplasm and depicted in Figure 1 as enzyme cluster II. Although the complex is relatively stable, attempts to separate these two enzymes has resulted in a rapid loss of activity, particularly that of OPRT (7, 15) which has prevented structural evaluation. It has been suggested that all six enzymes may operate as a unit and that the two cytoplasmic clusters may be associated with the membrane bound enzyme DHOD (7), shown on the right by the broken lines extending from DHOD to enzyme clusters I and II (Fig. 1). The regulation of the de novo pathway is complex. Several years ago it was shown that uridine 5’ triphosphate (UTP) inhibited CPS II thereby providing a built-in feedback regulation of pyrimidine biosynthesis (16). Later it was shown that phosphoribosyl pyrophosphate (PRPP), a substrate for OPRT, was also capable of enhancing CPS II activity by

/I

increasing

PRPP

AMP ‘ -

L’L-I2E1-”#{176}

the affinity

of this

enzyme

for ATP

(17). These relationships are depicted in Figure 2. PRPP is shared in common by purine metabolism and is a substrate for amidophosphoribosyl transferase in the de novo pathway for purine biosynthesis, for hypoxanthineguanine phosphoribosyl transferase in a purine salvage pathway, and for the synthesis of adenosine monophosphate via adenine phosphoribosyl transferase (1, 18). An increase or a decline in synthesis of PRPP may therefore effect concordant changes in purine and pyrimidine biosynthesis (19) while an enhanced utilization by either pathway might result in reciprocal changes in the other (20, 21). The activator, PRPP, and inhibitor, UTP, provide for tight regulation of product formation by the de novo pathway via alter-

I PRPP

_j

PURINE

BIOSYNTHESIS

1

-

(JTP

FIG.

2. Control

of the de novo pathway for pyrimidine biosynthesis. PRPP is utilized for the formation of ATP from AMP or for purine biosynthesis in addition to participating as a substrate for the formation of orotidine 5’ monophosphate via OPRT. PRPP activates not only CPS II but may activate OPRT as shown by the lines labeled with plus signs. UMP inhibits ODC and UTP inhibits CPS II, as shown by minus signs. The two enzyme clusters are enclosed by the broken lines. It is evident from this Figure that each cluster has an activator and an inhibitor.

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PYRIMIDINE __________________ CARBAMYL iOj

I TE

NH3 OTC-

AlP

A

1293

4 _______________ N113 PHOSPHA

I

CPS

BIOSYNTHESIS

C/TROLL/NE

ARG/N/NOSL/CCIN4TE

MITOCHONORIAL7

/

LA

--.G

______________________________________

OWI1THVAE

hIEMBRANE

SWTAMINE

cPS5

ASP4RTATE

ATC -

Transcarbamylase

of Omithine

Deficiency ____________________

-

L

5

W1P(4TE

or Arginine

4 NH3

CAR8A*flt HCO

prSPHATE

cPSI+

MA7P

ii;

u/OcHoNDL,2MEMBRANE,

GWTAMWE HCOj Mg

--

e--

‘#{188}

CARAMYL

s acRlosp#w

aic

ASPARTATE

ATP

FIG. 3. The relationship between carbamyl phosphate, pyrimidine biosynthesis, and urea biosynthesis. Under physiological conditions depicted at the top of Figure, carbamyl phosphate derived from the intramitochondrial CPS i, is channeled through OTC, and combined with ornithine to form citrulline. Citrulline is converted to arginine via an ammonia-capturing step catalyzed by argininosuccinate synthetase. Arginine is cleaved by arginase to form ornithine and urea. The ornithine is now available to combine with additional carbamyl phosphate. Carbamyl phosphate generated by the extramitochondrial CPS II is channeled through ATC, and combined with aspartate to form carbamyl aspartate which is converted through several steps to uridine monophosphate. At the bottom of the Figure is shown what happens in congenital deficiency of OTC or in animals fed arginine-deficient diets. If arginine is deficient, ornithine is limited and carbamyl phosphate can not be utilized to generate citrulline. Similarly, if OTC is deficient citrulline is not generated and ammonia is not captured by the arginine succinate synthetase reaction. This ammonia becomes available for enhanced carbamyl phosphate generation by CPS I. The carbamyl phosphate excess leaks through the mitochondrial membrane to participate in the extramitochondrial ATC reaction and the result is enhanced uridine formation.

ation of CPS II levels. More recently it has been shown in Ehrlich ascites cells that an increase of PRPP from 0. 1 to 1 mM results in a 9- to 13-fold increase in CPS II activity and only a 2-fold increase in OPRT (7). Since the Km of OPRT for PRPP in Ehrlich ascites cells was 1.25 x i0 M and levels of PRPP vary between 2.8 to 6 x l0 M (22), it was suggested that a rate-limiting step in the de novo pathway for pyrimidine biosynthesis in this tissue might not be CPS II but OPRT. An additional potential site of regulation was suggested at ODC since uridine 5’ monophosphate, UMP, and cytidine 5’ monophosphate, CMP, inhibit the activity of this enzyme (15, 23, 24). The controlling mechanisms for the enzyme cluster OPRT and ODC in Figure 2 are represented by PRPP as a potential activator and UMP as a product inhibitor. Normal

regulation

of the

de novo

pathway

been shown to be altered by specific pathological states. The first includes alterations in function of specific urea cycle enzymes. Under physiological conditions wherein both urea-synthesizing CPS I and ornithine transcarbamylase (OTC) and pyhas

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CPS II and ATC carbamyl phosphate

rimidine-synthesizing

functioning

normally,

tightly channeled and within the

within

the

are is

mitochondrial

cytoplasmic enzyme cluster, respectively, as depicted at the top of Figure 3. In N. crassa mutants a single deficiency of CPS I or CPS II results in an arginine or uridine requirement, respectively, since the presence of OTC and ATC prevent accumulation ofcarbamyl phosphate (25). It has been shown,

however,

that

mutants

of Neurospora

lacking CPS I and ATC can synthesize urea via carbamyl phosphate generated by CPS II, while mutants lacking CPS II and OTC can generate

pyrimidines

via carbamyl

phosphate

generated by CPS I, indicated at the bottom of Figure 3. In the mammal similar mechanisms may operate. Rats fed arginine-deficient diets excrete low quantities of urea and increased orotic acid (26). Since arginase cleaves arginine to form ornithine and urea, diminished arginine availability would result in a deficiency of ornithine preventing effective conversion of carbamyl phosphate to citrulline via OTC. Carbamyl phosphate accumulates. Lacking substrate citrulline, the synthesis of arginine succinate with the at-

1294

SHAMBAUGH

tendant

capture

ammonia

ofammonia

accumulates.

I for ammonia lated ammonia to generate

This

even

more

barrier

through

carbamyl

leaks

hereditary

and

CPS

I

phosphate.

through

to increase

acid

the mito-

the

extramito-

leading

to

accumulation

of

syndrome

PRPP idine

for purine biosynthesis

cytes from such to have elevated

where

ammonia

underutilization

biosynthesis is enhanced patients

been

while

acid

excretion

agents

af-

to normal

de novo pathway predominates

in the adult

is more important cyte transformation,

for pyrim-

brain the salvage (39). During adult a marked

(37, 38) pathway

lymphoenhancement

of both CPS II and the salvage pathway for pyrimidine biosynthesis have accompanied heightened thymidine incorporation into DNA. The latter was diminished only by salvage pathway inhibitors that did not affect elevated levels of CPS 11(40). Such findings suggested that salvage pathway function may predominate in the mature transformed lymphocyte. In regenerating adult intestine, enzymes of both the de novo and salvage pathway increase concordantly (41). Disorders of pyrimidine biosynthesis can be congenital or acquired. To date, known

of

reported

CTP, UDP, and UTP levels (21). A third means for altering control is to lower the production of UTP thereby releasing the inhibitory regulation of the pathway. This is seen in pyrimidine starvation (31), in DEGRADATION

Pylimidrns 5’4

URIDI1ES’

and ODC allopurinol

deficiency of orotic by the restitution

is evidenced orotic

neonatal rat the idine biosynthesis

obtains, pyrimand lympho-

have

of OPRT

after uridine supplementation. In addition to the de novo pathway pyrimidines can be generated by a salvage pathway (Fig. 4). Here uracil derived from dietary or endogenous pryimidine degradation is converted to the riboside uridine, then phosphorylated by uridine kinase to form uridine monophosphate. The latter reaction is rate limiting and is inhibited by cytidine triphosphate (36). The relative importance of these pathways in mammalian tissues may change with developmental age. In the brain of the fetal and

and/or carbamyl phosphate. These disorders include congenital and acquired deficiencies of ornithine transcarbamylase (29, 30). Citrullinemia and argininosuccinic aciduria have also been accompanied by an increased excretion of orotic acid (1). The second situation wherein control mechanisms impinging on CPS II are bypassed is the feed-forward from enhanced PRPP levels. In the LeschNyhan

synthesis

of excessive

chondrial pool. Here excess carbamyl phosphate combines with aspartate to generate increasing quantities of carbamyl aspartate which is converted in turn to orotic acid. A similar sequence of events has been postulated to explain orotic aciduria in rats given diets deficient in ornithine and citrulline (27). Excessive carbamyl phosphate production may also underlie the observed enhancement of orotic acid excretion in rats given ammonia (28). In man, increased orotic acid excretion has been reported in disorders of the urea cycle

deficiencies

(32), and in patients treated with or 6-azauridine, pharmacological fecting ODC (33-35). That UTP plays a key role in the enhancement

the Km of CPS M), the accumu-

is high (l0 can be cycled

overabundance

chondrial

is obtunded, Since

Nuclsosids

Phos#{216}oryIoss

UmAsW

MONOPAESDHATE

#{149} LIRACIL

#{149}

(LIMP)

-

DIII YDROIIRACIL UR(IDOPRt)P#MAfAT(

C02,

N#,

8 ALANII’E

SALVAGE tkids,s

Phosphorylssi

LIRACIL

Urldine Xmas. #{149} URIDINE

Ribose

#{149} LEIDIPE

PI

-

MgATP

I

FIG. 4. The salvage pathway is shown at the top. Degradation

for pyrimidine

5’ MONOPHOSPPIATE (LIMP)

-P

2

biosynthesis.

occurs in an ordered

fashion

The degradation

of the ribose. In the salvage pathway shown below events are reversed. uracil with the concomitant loss of inorganic phosphorus to form phosphorylase and shown in the Figure as reaction 1. The uridine is now step catalyzed by uridine kinase, reaction 2.

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of UMP

with loss of the phosphate

to the pyrimidine

base uracil

first followed

by removal

Phosphorylated ribose is combined with uridine, a reaction catalyzed by uridine phosphorylated to UMP via a rate-limiting

PYRIMIDINE

congenital disorders are limited to the second enzyme cluster of the de novo pathway (Fig. 1) and include either a combined deficiency of OPRT and ODC (type I hereditary orotic aciduria) or a deficiency of a single enzyme ODC, (type II hereditary orotic aciduria). The double enzyme deficiency in type I hereditary orotic aciduria has prompted the suggestion that type I may represent a deficit in a genetic regulatory mechanism controlling synthesis of the two enzymes (32). From considerations of the instability of OPRT when the complex of OPRT and ODC is disrupted (7, 1 5) it is also possible that a genetic deficiency of a single enzyme, ODC, could result in a marked instability ofOPRT and produce the picture of a combined enzyme deficiency. An excessive excretion of orotic acid is seen in type I and excretion of orotic acid plus orotidine in type II. Hereditary orotic aciduria is transmitted as an autosomal recessive without phenotypic expression or with growth

retardation,

anemia,

and

leucopenia

in the

heterozygote. A homozygous defect resulting in deficiency of these enzymes may not be compatible with life (1). Auxotrophism is the term used when an enzyme defect in microbial mutants results in a normal end product becoming an essential requirement for growth (35,42). Hereditary orotic aciduria has been proposed as an example of auxotrophism in man since therapy with a preformed pyrimidine, uridine, effects normalization of orotic aciduria and restitution of

anemia, leucopenia, to infection, and

increased susceptibility retarded neonatal growth

(1).

The need for clinical information regarding pyrimidme metabolism has never been greater. With the gradual widening of the gap between world wide protein demands and protein availability, the utilization of single cell protein derived from microbes has begun to receive serious consideration as a protein source for man (43). One of the problems facing workers in this field is the large quantity of RNA in proliferating microbes. To answer some of the questions posed, potential effects of RNA supplementation have been examined in the rat. It was observed that plasma levels of uric acid and uridine were doubled when RNA (100 g/kg diet) was ingested, but that such est when compared

changes to the

were very mod20-fold rise in

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BIOSYNTHESIS

plasma uracil

1295

levels (44). Since uric acid and in the urine mirrored plasma composition in these animals, it is evident that studies are needed to examine whether excretory mechanisms in man are capable of dealing with large quantities of exogenous pyrimidines as a lifelong dietary commitment. In approaching this question long term follow-up of uridine treated patients with hereditary orotic aciduria should prove invaluable. Because these rare defects are the only known congenital deficiency in pyrimidine biosynthetic enzymes, and acquired enzyme defects have been limited to patients with pernicious anemia in relapse (3 1) or with acute leukemia treated with the carcinostatic agent 6-azauridine (35), inquiry into the dietary man

uracil levels

regulation and the

of pyrimidine biosynthesis in experimental animal had not until it was observed a relatively

been feasible nontoxic drug, allopurinol, resulted in orotic aciduria and orotidinuria in gouty patients (33, 34). It was subsequently shown in an in vitro system that mononucleotides of allopumo! and oxypurinol acted by inhibiting ODC (45). Despite the observation that patients treated with allopurinol excrete only 1 to

5%

in the drug

of the

amount

hereditary has

permitted

of orotic

orotic

reported this assessment of

acidurias

systematic

acid

(46),

exogenous purines and pyrimidines on pyrimidine homeostasis in man. Since purine and pyrimidine biosynthesis are interrelated (19, 21), and rates of pyrimidine production in man have been calculated to be similar in magnitude to purine production (47,48), clinical studies have examined the impact of RNA supplements on metabolism of pui-ines and pyrimidines concurrently (49). In these studies healthy volunteers treated with allopurinol 400 mg/day and maintained on purine and pyrimidine-free diets manifested a low uric acid excretion and an orotic aciduria. When RNA hydrolyzates 4 g/day were administered as a combined source of purines and pyrimidines, uric acid excretion rose markedly. When purme nucleotides alone were given, i.e., guanosine monophosphate ig/day, or AMP 3g/day, excretion of uric acid increased proportionately to account for 80% of the purines administered. It was concluded that dietary purines were predominately excreted and played at best a minor role in the regulation of purine synthe-

SHAMBAUGH

1296

sis in man (49). In contrast to enhanced uric acid excretion, RNA hydrolysates resulted in a 70% fall in orotic acid excretion. Pyrimidine nucleotides, uridine monophosphate 3 g/day and cytidine monophosphate 3 g/day, effected a 65 and 45% fall, respectively. In addition the purine nucleotides AMP and guanosine

monophosphate

were also shown by 50%. That the

to inhibit orotic aciduria effect of purine and pyrimidine replacements was not at the site of allopurinol inhibition was

evidenced

by

a maintenance

of height-

ened ODC levels (50). It was concluded that the inhibition of pyrimidine metabolism might be exercised at a site proximal to ODC. In this regard it has been shown that the increase in bicarbonate incorporation into acid-soluble uridine nucleotides in the phytohemagglutinin stimulated human lymphocyte is inhibited by uridine without affecting enhanced levels of CPS II (5 1). Thus alterations in allosteric control mechanisms demonstrated in mouse spleen (9) could be a potential mechanism in man. Unlike uridine, the purine nucleosides, guanine and adenosine, diminish induction of CPS II and ATC in the phytohemagglutinin-stimulated lymphocyte (52). As PRPP synthesis can be inhibited by adenosine, inosine, or guanosine it is possible

(53)

that

purines

may

act

by

lowering PRPP levels. This could prevent activation of CPS II and delete a required substrate for the formation of orotidylic acid. Adenine in addition inhibits uridine kinase (52). Thus excessive quantities of this purine could theoretically shut off pyrimidine biosynthesis resulting in pyrimidine starvation and interference with induction of new proteins. Although the specific mechanisms have yet to be examined, it is evident that dietary purines and pyrimidines may both play an important role in regulating pyrimidine biosynthesis in man.

2.

author acknowledges the expert provided by June Pedersen.

secretarial

as-

3. SWEENEY, Enzymes

1.

W. N., AND L. H. SMITH, JR. Disorders of purine and pyrimidine metabolism. In: The Metabolic Basis of Inherited Disease (4th ed), edited by J. B. Stanbury, J. B. Wyngaarden and D. S. Frederickson. New York: McGraw-Hill Bcok Co., 1978, p. 1045. KELLEY,

Downloaded from https://academic.oup.com/ajcn/article-abstract/32/6/1290/4666284 by University of Wyoming Libraries user on 18 June 2018

Adv.

and arginine

Enzyme

Reg.

9: 19,

M. J., D. H. HOFFMAN

in pyrimidine

G. A. POORE.

AND

biosynthesis.

Adv.

Enzyme

Reg. 9: 51, 1970. 4.

0.

HITCHINGS,

nisms

in purine

H.

Indications

and

for

pyrimidine

mechaas re-

control

biosynthesis

vealed by studies with inhibitors.

Adv. Enzyme

Reg.

12: 121, 1974. 5.

HENDERSON,

J. K.

J. F.,

LOWE

AND

J.

BARANKIEW-

Icz. Purine and pyrimidine metabolism: pathways, pitfalls and perturbations. CIBA Found. Symp. 48: 3, 1976. 6. HAGER, S. E., AND M. E. JoNas. A glutamine-dependent enzyme for the synthesis ofcarbamyl phosphate for pyrimidine biosynthesis in fetal rat liver. J. Biol.

Chem.

242: 5674, 1967.

7. SHOAF, W. T., AND M. E. JONES. Uridylic acid synthesis in Ehrlich ascites carcinoma. Properties, subcellular distribution and nature of enzyme cornplexes of the six biosynthetic enzymes. Biochemistry 12: 4039, 1973. 8. M0RI, M., H. ISHIDA AND M. TATIBANA. Aggregation states and catalytic properties of the multienzyme complex catalyzing the initial steps of pyrimidine biosynthesis in rat liver. Biochemistry 14: 2622, 1975. 9. LEVINE, R. L., N. J. HOOGENRAAD AND N. KRETCHMER. Regulation

synthetase from 3694, 1971.

of activity

mouse

of carbamoylphosphate

spleen.

Biochemistry

10:

10. LUE, P. F., AND V. 0. KAPLAN. The aspartate transcarbamylase and carbamoyl phosphate synthetase of yeast: a multi-functional enzyme complex. Biochem. Biophys. Res. Commun. 34: 426, 1969. 11. SHAMBAUGH, G. E., III, S. C. MROZAK, B. E. MmGER AND N. FREINKEL. Glutamine-dependent carbamyl phosphate synthetase during fetal and neonatal life in the rat. Develop. Biol. 37: 171, 1974. 12. TROTrA, P. P., L. M. PINKOS, R. H. HASCHEMEYER AND A. Maisma.

omer synthetase

subunits. 13.

Reversible

of glutamine-dependent

RATNER,

into

catalytically

dissociation

ofthe

carbamyl active

mon-

phosphate

heavy

J. Biol. Chem. 249: 492, 1974. S. Enzymes of arginine and urea

and

light

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Pyrimidine biosynthesis.

editor: Roh’rt comments Pyrimidine George E. Shambaugh, H. Hc’rinan, M.D. in biochemistry 2 III ABSTRACT Pyrimidines can be synthesized thro...
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