section 27.3

Biosynthesis of Purine Nucleotides

623

o

■Â

1

1

o

OH

HO

IMP

O

Guanine

MH

Adenine

Hypoxanthine-guanine

'O

OH

HO

GMP

'O

OH

HO

AMP

F IG U R E 2 7 -1 0

Salvage pathways of purine nucleotide synthesis. The preformed purines can be converted to mononucleotides in a

single step, using

PRPP.

of free hypoxanthine and guanine for conversion to uric

acid. Deficiency of adenosine deaminase or purine nucle-

oside phosphorylase results in immunodeficiency diseas

(Chapter 35).

In muscle, a unique nucleotide reutilization pathway,

known as the

purine nucleotide cycle,

uses three enzymes:

myoadenylate deaminase, adenylosuccinate synthetase,

and adenylosuccinate lyase. In this cycle, AMP is con-

verted to IMP with formation of NH

3

, and IMP is

then reconverted to AMP. Myoadenylate deaminase de-

ficiency produces a relatively benign disorder of muscle

(Chapter 21), which is characterized by muscle fatigue

following exercise (see Myoadenylate Deaminase Defi-

ciency).

Nucleoside kinases specific for inosine or guanosine

have been described. Adenosine kinase is widely dis-

tributed in mammalian tissues.

Dietary Purines

Purines derived from food do not participate significantly

in the salvage pathways described above; they are mostly