Plasma and Red Cell Reference Intervals of 5-Methyltetrahydrofolate of Healthy Adults in Whom Biochemical Functional Deficiencies of Folate and Vitamin B12 Had Been Excluded
5-Methyltetrahydrofolate (5-MTHF) is the predominant form of folate and a strong determinant of homocysteine concentrations. There is evidence that suboptimal 5-MTHF availability is a risk factor for cardiovascular disease independent of homocysteine. The analysis of folates remains challenging and is almost exclusively limited to the reporting of “total” folate rather than individual molecular forms. The purpose of this study was to establish the reference intervals of 5-MTHF in plasma and red cells of healthy adults who had been prescreened to exclude biochemical evidence of functional deficiency of folate and/or vitamin B12. Functional folate and vitamin B12 status was assessed by respective plasma measurements of homocysteine and methylmalonic acid in 144 healthy volunteers, aged 19–64 years. After the exclusion of 10 individuals, values for 134 subjects were used to establish the upper reference limits for homocysteine (13？μmol/L females and 15？μmol/L males) and methylmalonic acid (430？nmol/L). Subjects with values below these cutoffs were designated as folate and vitamin B12 replete and their plasma and red cell 5-MTHF reference intervals determined, : 6.6–39.9？nmol/L and 223–1041？nmol/L, respectively. The application of these intervals will assist in the evaluation of folate status and facilitate studies to evaluate the relationship of 5-MTHF to disease. 1. Introduction 5-MTHF, the predominant form of folate (vitamin B9) in plasma and red cells, is a substrate for the methionine synthase and vitamin B12 (methylcobalamin form—methyl-Cbl) mediated conversion of homocysteine (tHcy) to methionine (Figure 1). Suboptimal 5-MTHF availability leads to an increase in circulating homocysteine (hyperhomocysteinaemia) which has been associated with many diseases and health complications including cardiovascular disease [1, 2]. There is also evidence to suggest that 5-MTHF deficiency may be a cardiovascular risk factor independent of homocysteine [3, 4]. Figure 1: Homocysteine, folate, and vitamin B 12 metabolism. THF (tetrahydrofolate), 5-MTHF (5-methyltetrahydrofolate), MTHFR (methylene tetrahydrofolate reductase), MS (methionine synthase), CBS (cystathionine beta-synthase), SAM ( S-adenosyl methionine), Cbl (cobalamin), TC II (transcobalamin), holo TC (holotrascobalamin), OH-Cbl (hydroxocobalamin), MMA-CoA (methylmalonyl-CoA), and MMA (methylmalonic acid). In the plasma of healthy humans, 5-MTHF typically constitutes 80–90% of total folate [5, 6]. Circulatory concentrations of 5-MTHF are partly dependant on methylenetetrahydrofolate reductase (MTHFR)
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