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chapter 27

Nucleotide Metabolism

TABLE 27-1

Nomenclature o f Nucleosides and Nucleotides

Base

Nucleoside (Base-Sugar)*

Nucleotide^ (Base-Sugar Phosphate)

Purines

Adenine (

6

-aminopurine)

Adenosine

Deoxyadenosine

Adenosine monophosphate (AMP) or adenylic acid

Deoxyadenosine monophosphate (dAMP)

Guanine (2-amino-6-oxypurine)

Guanosine

Deoxyguanosine

Guanosine monophosphate (GMP)

Deoxyguanosine monophosphate (dGMP)

Hypoxanthine (

6

-oxypurine)

Inosine

Deoxyinosine

Inosine monophosphate (IMP)

Deoxyinosine monophosphate (dIMP)

Xanthine (2,6-dioxypurine)

Xanthosine

Xanthosine monophosphate (XMP)

Pyrimidines

Cytosine (2-oxy-4-aminopyrimidine)

Cytidine

Deoxycytidine

Cytidine monophosphate (CMP)

Deoxycytidine monophosphate (dCMP)

Thymine

Thymidine (thymine

Thymidine monophosphate (TMP) (thymine

(2,4-dioxy-5-methylpyrimidine)

deoxyriboside)

deoxyribotide)

Uracil (2,4-dioxypyrimidine)

Uridine

Uridine monophosphate (UMP)

*The sugar residue can be ribose or deoxyribose. If it is deoxyribose, it is identified as such; otherwise it is assumed to be ribose, with the exception

of thymidine, which is deoxyriboside.

IA nucleotide is a nucleoside monophosphate; the monophosphates are sometimes named as acids.

pteroylpolyglutamate hydrolase (also called conjugase)

to pteroylmonoglutamate (folic acid). If the folylpolyg-

lutamates enter the intestinal epithelial cells intact, they

may be converted to folylmonoglutamate within lyso-

somes by lysosomal hydrolase; however, the significance

of this pathway appears to be minor. Folate transport in

the intestine and the choroid plexus is mediated by a

specific carrier, and the disorder

hereditary folate mal-

absorption

is associated with a defective folate carrier.

Individuals affected with hereditary folate malabsorp-

tion exhibit early onset of failure to thrive, megaloblas-

tic anemia, and severe mental retardation. Therapy re-

quires the administration of large doses of oral and

systemic folates. Folate is reduced and converted to

N5-methyltetrahydrofolate in intestines and secreted into

the circulation.

In the plasma, about two-thirds of the folate is protein-

bound. Tissue needs for folate are met by uptake from

Pteroyl residue

— NH-

0

II

_

c —o

1

-C H

I

CH

2

I

CH

2

I

C O — NH -

Additional Glu resid ues

P teroylm onoglutam ate (folate)

FIGURE 27-1

Structure of folic acid showing its components. The numbered part participates in one-carbon transfer reactions. In

nature, folate occurs largely as polyglutamyl derivatives in which the glutamate residues are attached by isopeptide

linkages via the y -carboxyl group. The pteridine ring structure is also present in tetrahydrobiopterin, a coenzyme in the

hydroxylation of phenylalanine, tyrosine, and tryptophan (Chapter 17).