Risk of neural tube defect-affected pregnancy is associated with a block in maternal one-carbon metabolism at the level of N-5-methyltetrahydrofolate:homocysteine methyltransferase

The disposition of whole blood mono- to hexaglutamyl methylfolate and plasma homocysteine (HCY) was used to evaluate potential lesion sites in one-carbon metabolism which could be responsible for neural tube defect(NTD)-affected pregnancies. An isocratic high-performance liquid chromatographic system (HPLC) with photodiode array detection was used to quantify and speciate whole-blood methylfolate into mono-, di-, tri-, tetra-, penta-, and hexaglutamate forms. This technique was also used with off-line radioassay to identify nonmethyl whole-blood folates. Isocratic HPLC with fluorescence detection was used to quantify SBDF derivatized homocysteine in plasma. The study investigated blood from 11 women who had experienced a previous NTD- affected pregnancy and II controls of similar age and social class, No subjects were pregnant, HCY levels were significantly higher in NTD subjects (P = 0.0486, 95% CI -2.799,0.001 using the Mann-Whitney test), as was the ratio of known intracellular (tri- to hexaglutamyl) methylfolate compared to extracellular (mono- and diglutamyl) methylfolate (P = 0.0062 95% CI -0.543, 3.862 using the Mann-Whitney test). Vitamin B-12, red cell folate, circulating total methylfolate, and circulating mono- to hexaglutamyl methylfolates showed no difference between population groups. The disposition between individual and cumulative glutamate chain lengths of methylfolate showed significant trends which differed between population groups: (i) total blood methylfolate (Glu(1-6)) appears to be utilized by N-5-methyltetrahydrofolate:homocysteine methyltransferase (MS) in control blood but not NTD blood, where it appears to accumulate following a 45-min incubation; (ii) whole-blood hexaglutamyl methylfolate (5CH(3)-H(4)PteGlu(6)) becomes a larger proportion of the total blood methylfolate in NTD than in control populations; and (iii) the intermediate glutamate chains of methylfolate (Glu(1-5)) remain relatively constant as 5CH(3)-H(4)PteGlu(6) accumulates in NTD but appear to increase linearly with 5CH(3)-H(4)PteGlu(6) in controls. The significant elevation of HCY in the NTD population is associated with the increasing proportion of 5CH(3)-H(4)PteGlu(6) relative to the total methylfolate, since, when corrected for HCY level, the proportion of 5CH(3)-H(4)PteGlu(6) to total methylfolate is similar in NTD and control populations. These trends are consistent with a defect at the level of vitamin B-12 dependent; MS which ''traps'' folate at the 5CH(3)-H(4)PteGlu(6) level. (C) Academic Press.