I

Inosinate (IMP)

Hypoxanthine

Xanthine

Uric acid

Fig. 4.6. Uric acid synthesis (an abbreviated pathway to show key reactions involving inputs of amino acid N). Enzymes: 1, ribose 5-phosphate pyrophosphokinase; 2, glutamine phosphoribosyl pyrophosphate amidotransferase; 3, phosphoribosyl glycinamide synthetase; 4, amido-li CjclSO ; 5, specific synthetase; 6, xanthine oxi dciso.

Following deamination and molecular cleavage, cysteine and a-ketobutyrate are produced. This pathway underpins an important nutritional relationship and is depicted in Chapter 8. The competing metabolic demands for methionine are considered in some depth in Chapter 17.

Particular note should be made of the roles of the B-complex vitamins that service methionine and homocysteine metabolism. Thus, methionine synthesis from homocysteine requires the tissue presence of FAD, tetrahydrofolate and cobalamin, wheres the synthesis of cystathionine from homocysteine

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