Methionine load test

Figure 9.5. Methionine load test for vitamin Be status. Methionine synthetase, EC 2.1.1.13 (vitamin Bi2-dependent) 2.1.1.5 (betaine as methyl donor) cystathionine synthetase, EC 4.2.1.22 and cystathionase, EC 4.4.1.1. Relative molecular masses (Mr) methionine, 149.2 homocysteine, 135.2 cystathionine, 222.3 and cysteine, 121.2.

It is apparent that abnormally increased excretion of homocysteine and cystathionine metabolites after a test dose of methionine cannot necessarily be regarded as evidence of vitamin Be deficiency. This means that, like the tryptophan load test, the methionine load test is unreUahle as an index of status in epidemiological studies, although it is (probably) reliable in depletion/ repletion studies to determine requirements. 

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. 

Bremer HJ, Endres W. Primary cystathioninuria. Methionine load tests and response to pyridoxine. Helv Paediatr Acta 1972 27(5) 525-36. 

In Caucasian populations, it has been shown that a normal fasting plasma tHcy concentration is not synonymous with anormal methionine load test (Bostom et al., 1995). Flevated post-methionine-load tHcy concentrations are independently... 

Impairment of remethylation is strongly implicated as the cause of hyper-homocysteinemia in patients with CKD, which has been demonstrated in a radioisotope study in patients with CKD (Van Guldener et al. 1999), although reduced clearance of homocysteine has been suggested as the possible cause in some reports. We reported that supplementation with folic add and methyl-cobalamin normalized the remethylation pathway (Koyama et al. 2002). Compared with the decreases in homocysteine of 17.3 8.4% after supplementation with folic add alone and 18.7 7.5% after that with methylcoba-lamin alone, a combination of foUc acid and methyleobalamin decreased homocysteine by approximately 60% and normalized the findings of the methionine loading test Table 47.1. This result suggest that both coenzymes, folic acid and methyleobalamin, were insufficient due to reduced availability of these coenzymes in patients with CKD. There is also a report that increased MMA in dialysis patients was reduced by the administration of methylcoba-lamin (Nakamura et al. 2002). 

Table 47.3 Methionine loading on patients with CKD Stage V. Group A was treated with folic acid 15 mg/day orally, methylcobamain 500 pg intravenously after each hemodialysis session and vitamin Bg 60 mg/day orally. Group B was treated with folic acid and methyl-cobalamin (without vitamin Bg). All patients were treated for three weeks. A methionine-loading test was conducted before and after supplementation. Amino acid level was measured at fasting and two hours and four hours after methionine load (0.05 g/kg orally). Both groups showed normal findings of homocysteine profile during the methionine loading test after treatment whether with vitamin Bg or not. However, profiles of methionine and cysteine were not normalized. Reproduced with permission from Koyama (2011).

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... 

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

Some 10-25% of the population have a genetic predisposition to hyperhomocysteinemia, which is a risk factor for atherosclerosis and coronary heart disease, as a result of polymorphisms in the gene for methylenetetrahydrofolate reductase. There is no evidence that supplements of vitamin Bg reduce fasting plasma homocysteine in these subjects, and like the tryptophan load test, the methionine load test may be an appropriate index of status in controlled depletion/repletion studies to determine vitamin Bg requirements, but not in population studies. 

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