Homocysteine is a key intermediate in sulphur amino acid (SAA) metabolism, yielding cysteine (Chapter 8) and taurine. It is established that taurine is an essential amino acid for felines (Chapter 1). The importance of homocysteine in human health is now well recognized in that plasma levels are higher than normal in patients with coronary, cerebrovascular or peripheral arterial occlusive disease. In addition, evidence is accumulating of a relationship between homocysteine and a number of B-complex vitamins (Table 26.7). In particular, vitamin B12 is required for methionine synthase which methylates homocysteine to form methionine. In the conversion of homocysteine into cystathionine another B-complex vitamin, pyridoxal phosphate, serves as a vital co-factor. The association of homocysteine with folate and riboflavin has also been described (Table 26.7). In pigs, prolonged vitamin B12 deficiency is associated with hyperho-mocysteinaemia, and in cattle a similar effect has been reported in long-term moderate deficiency of Co (see Chapter 1). In addition,

Table 26.7. Homocysteine metabolism: selected research titles illustrating relationships with vitamins and nitric oxide.



Homocysteine, vitamins and arterial occlusive disease: an overview Plasma homocysteine: a risk factor for arterial occlusive diseases Relationship among homocysteine, vitamin B12 and cardi SO diS63S6 in the elderly

Relationship between plasma homocysteine, vitamin status and extracranial carotid-artery stenosis

The use of homocysteine and other metabolites in the specific diagnosis of vitamin B12 deficiency

Vitamins as homocysteine-lowering agents

Homocysteine, EDRFa and endothelial function (physiological reaction between homocysteine and NO to form the stable adduct S-nitroso-homocysteine)

Plasma homocysteine levels in Taiwanese vegetarians (associated with folate and vitamin B12 status)

Relationship between riboflavin and plasma homocysteine is influenced by folate status

Rosenberg (1996) Malinow (1996) Herzlich et ai. (1996)

Stabler etat. (1996)

Brattstrom (1996) Upchurch et al. (1996)

aEndothelium-derived relaxing factor.

Upchurch et al. (1996) identified a pathway for the physiological reaction between homocysteine and NO to form the stable adduct S-nitrosohomocysteine.

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