Leucine Aminomutase

Leucine aminomutase catalyzes isomerization of leucine and ^-leucine. As with methylmalonyl CoA mutase, the reaction involves cleavage of the Co-C

bond to form the 5' -deoxyadenosyl radical, which then removes hydrogen from the f -carbon of leucine (or the a - carbon of f -leucine), followed by migration of the amino group and attack by the product radical on deoxyadenosine. In addition to adenosylcobalamin, the reaction is pyridoxal phosphate-dependent (Section 9.3.1); presumably, migration of the amino group involves intermediate formation of pyridoxamine phosphate.

Poston (1984) showed that, in isolated rat tissues, about 5% of the catabolic flux of leucine was by way of aminomutase action to yield f -leucine, and then isobutyryl CoA, with the remainder provided by the more conventional a-transamination pathway leading to the formation of isovaleryl CoA. In patients suffering from vitamin B12 deficiency, there is an elevation of plasma f -leucine, suggesting that the aminomutase may act to metabolize f -leucine arising from intestinal bacteria, rather than as a pathway for leucine catabolism.

10.9 DEFICIENCY OF FOLIC ACID AND VITAMIN B,2

Deficiency of either folic acid or vitamin B12 results in a clinically similar mega-loblastic anemia; because of the neurological damage that accompanies the megaloblastic anemia of vitamin B12 deficiency, the condition is generally known as pernicious anemia. Suboptimal folate status is also associated with increased incidence of neural tube defects (Section 10.9.4), hyperhomocys-teinemia leading to increased risk of cardiovascular disease (Section 10.3.4.2), and undermethylation of DNAleading to increased cancerrisk (Section 10.9.5).

Both vitamin B12 and folate deficiencies are associated with psychiatric illness. Folate deficiency is most commonly associated with depression, whereas cognitive impairment and dementia are seen in about 25% of patients with either deficiency (Bottiglieri, 1996; Green and Miller, 1999). Herbert (1962) noted insomnia, forgetfulness, and irritability during the development of self-imposed folate deficiency, which responded well to the administration of the vitamin.

Folate deficiency is relatively common; 8% to 10% of the population of developed countries have low or marginal folate stores. By contrast, dietary deficiency of vitamin B12 is rare, and deficiency is most often the result of impaired absorption (Section 10.7.1).

Dietary deficiency of vitamin B12 does occur, rarely, in strict vegetarians, because there are no plant foods that are sources of vitamin B12. The small amounts that have been reported in some plants and algae are almost certainly from bacterial contamination.

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