Loading tests, metabolic methionine

The metabolism of methionine, shown in Figure 11.22, includes two pyridoxal phosphate-dependent steps cystathionine synthetase and cystathionase. Cystathionase activity falls markedly in vitamin deficiency, and as a result there is an increase in the urinary excretion of homocysteine and cystathionine, both after a loading dose of methionine and under basal conditions. However, as discussed below, homocysteine metabolism is affected more by folate status than by vitamin status, and, like the tryptophan load test, the methionine load test is probably not reliable as an index of... 

Early studies of vitamin Be requirements used the development of abnormalities of tryptophan or methionine metabolism during depletion, and normalization during repletion with graded intakes of the vitamin. Although tryptophan and methionine load tests are unreliable as indices of vitamin Be status in epidemiological studies (Section 9.5.4 and Section 9.5.5), under the controlled conditions of depletion/repletion studies they do give a useful indication of the state of vitamin Be nutrition. More recent studies have used more sensitive indices of status, including the plasma concentration of pyridoxal phosphate, urinary excretion of 4-pyridoxic acid, and erythrocyte transaminase activation coefficient. 

Figure 4 Methionine metabolism, the basis of the methionine load test for vitamin Be status.

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