Authors : Bhat DS, Gruca LL, Bennett CD, Katre P, Kurpad AV, Yajnik CS, Kalhan SC
Publication Year : 2018
Methionine loading test (MLT) has been used primarily to identify defects in transsulfuration of homocysteine in cystathionine beta synthase deficiency. It may not be as useful to evaluate remethylation pathway, in vitamin B-12 and folate deficiencies.
We used tracer isotope labelled MLT to interrogate transsulfuration and remethylation independently in vitamin B-12 deficiency.
We studied vitamin B-12 deficient women with a tracer labelled MLT before and eleven months after treatment with vitamin B-12. The fractional contribution of [13C]homocysteine to breath CO2 was used as a measure of transsulfuration, and difference in the intracellular enrichment of [13C]methionine and that of [C2H3]methionine as a measure of remethylation of homocysteine. Combined pre- and post-treatment results were analyzed to investigate the association between plasma vitamin B-12 concentrations and measures of homocysteine metabolism.
The subjects were 17 years old, with a BMI of 19.4 kg/m2. Treatment with vitamin B-12, 2µg/day increased plasma B-12 from 93 (78.7, 106.2) [median (25th, 75th centiles)] to 161.5 (125.5, 226.2) pmol/L; 44% were below less than 150pmol/L after treatment. Fasting homocysteine concentration was significantly lower and that of cysteine higher in subjects with B-12 levels greater than 150pmol/L. The tracer estimated transsulfuration of homocysteine was lower and remethylation higher with B-12 levels greater than 150pmol/L when compared with those less than 150pmol/L.
The tracer labelled MLT in combination with fasting parameters is a robust way to estimate parameters of methionine metabolism and can be used in the field where prime-constant rate infusion studies cannot be done efficiently.