Estimating the Interstitial Fluid Folate+Methyltetrahydrofolate Concentrations

This is a discussion of some articles that lend credence to the concept that, outside the brain, the serum concentrations of 5-methyltetrahydrofolate (MTHF) and folic acid (FA) can be regarded as being comparable to their respective concentrations in the extracellular, interstitial fluid (ISF). Outside of the brain, endothelial cells lack the intercellular tight junctions that would limit passive diffusion from the blood to the extravascular, extracellular fluid. The articles whose authors have used serum folate concentrations as approximations for the ISF folate concentrations, around cells outside the central nervous system (CNS), are discussed at the end of this posting.

In the CNS, however, it is not clear if there is any difference between the concentrations of MTHF+FA in the ISF of the CNS parenchyma and the concentrations of MTHF+FA in the CSF. The pathways by which CSF circulates and drains from the brain suggest that there is not likely to be a substantial concentration gradient, for MTHF+FA, across the ependymal cells that line the ventricles and central canal in the CNS. CSF in the ventricles can diffuse out of the ventricles, between epithelial, ependymal cells lining the cerebral ventricles, and thereby carry constituents of the CSF into the ISF of the periventricular gray matter and white matter (Weller, 1998). A number of authors have discussed the way in which the movements, by bulk flow or volume transmission, of labeled growth factors and tracers from the CSF into the ISF does not limit the bulk transport of compounds or growth factors or other signalling proteins from the CSF to the ISF [(Thorne et al., 2004: http://www.ncbi.nlm.nih.gov/pubmed/15262337) (Proescholdt et al., 2000: http://www.ncbi.nlm.nih.gov/pubmed/10658638)]. Similarly, some of the ISF from the gray matter of the brain drains directly into the periarterial/perivascular ISF and, via those routes, into the ISF of the cervical lymphatic vessels and cervical lymph nodes [Thorne et al., 2004; Weller, 1998: http://www.ncbi.nlm.nih.gov/pubmed/9786239]. Presumably there would not be drastic differences between the concentrations of MTHF+FA in the ISF in those parts of the brain and the concentration of MTHF+FA in the ISF of the gray matter or other portions of the CNS.

Large differences between the serum concentrations of MTHF+FA, cobalamins, and homocysteine and their respective concentrations in the the CSF can, however, exist. Hansen and Blau (2004) found that the CSF MTHF concentrations in 137 control subjects ranged from 42.0 to 119.6 nM, and the mean concentration was 67.0 [Flemming Hansen and Nenad Blau, 2004: (http://www.biopku.org/pdf/blau_hansen.pdf) (http://www.ncbi.nlm.nih.gov/pubmed/15781200)]. The CSF concentration of MTHF in a person with cerebral folate deficiency and neurological symptoms had been 34.4 nM before treatment with folinic acid and was 127.1 during treatment with 15 mg/d folinic acid. Tanji et al. (2000) [Kurenai Tanji et al., 2000: (http://www.ncbi.nlm.nih.gov/pubmed/11018246)] noted that the concentrations of total folates in the CSF tend to normally be about four times the serum concentrations, and the authors also mentioned that the blood-CSF barrier, consisting of the epithelial cells of the choroid plexuses, is a major site of blood-to-CSF transport of folates, particularly MTHF. The authors cited references (references 39, 43, and 44) to support those statements. Ormazabal et al. (2006) [Aida Ormazabal et al., 2006: (http://www.bh4.org/pdf/ormazabal_cca.pdf) (http://www.ncbi.nlm.nih.gov/pubmed/16624264)] found that, among people between the ages of 4 and 18, in a control group, the average CSF MTHF concentration was 56 nM. The average serum folate concentration, which is generally assumed, in the absence of high-dose folic acid supplementation, to consist of predominantly MTHF, among people in this control group was 18.1 nM, and the ratio of CSF/serum folate was 3.2. The CSF MTHF concentration was also found to correlate inversely with age among controls, across the range of ages, from birth to 18, that the authors had sampled (Ormazabal et al., 2006). Finally, Blom et al. (1993) [Blom et al., 1993: (http://www.ncbi.nlm.nih.gov/pubmed/7609440)] found that the ratio of CSF/serum homocysteine from six controls was about 1,000. The CSF homocysteine concentration was therefore very low and ranged from 0.007-0.020 uM, and the serum homocysteine concentrations ranged from 5-18 uM (Blom et al., 1993). Blom et al. (1993) also found that the CSF cobalamin concentrations ranged from 2.1-22.9 pM (0.0021-0.0229 nM) in controls, and the serum cobalamin concentrations ranged from 142-541 pM (0.142-0.541 nM).

There is reason to think that the ratio of CSF/serum folate may frequently deviate from the usual value of 3-4. In a person with subacute combined degeneration due to cobalamin deficiency, the CSF/serum folate ratio was 21.8 nM/16.6 nM, or ~1.3 (Blom et al., 1993). Pineda et al. (2006) found that a patient with cerebral folate deficiency displayed a CSF MTHF/serum MTHF+FA ratio of 8 nM/14.2 nM (~0.56), and the CSF MTHF/serum MTHF+FA ratio a control subject was 48 nM/66 nM (~0.73) [Merce Pineda et al., 2006: (http://www.bh4.org/pdf/pineda.pdf) (http://www.ncbi.nlm.nih.gov/pubmed/16365882)].

In the absence of some data on a significant concentration gradient, for MTHF+FA, between the CSF and the cerebral ISF, the CSF concentrations of folate, homocysteine, and cobalamin can be regarded as being loosely comparable to the concentrations of those compounds, either free or bound to their individual binding proteins, in the ISF of the CNS parenchyma. Outside of the CNS, the serum MTHF+FA and ISF MTHF+FA concentrations can also be viewed as being loosely comparable also.

These articles, below, also support the concept that the ISF concentrations of MTHF+FA, outside the CNS, can be viewed as being generally comparable to the serum MTHF+FA concentrations. By extension, it is possible to regard the ISF concentrations of MTHF or FA, in either the CNS or in extravascular cells (or in vascular endothelial or smooth muscle) outside the CNS, as being loosely analogous to the extracellular concentrations of folates in the culture media of cell cultures:

Mashiyama et al. (2004) considered the serum folate concentration to be comparable to the concentration of folate in the extracellular fluid, and this would allow for the use of the serum folate concentration, at least outside the CNS, as a proxy for the extracellular fluid folate concentration (Mashiyama et al., 2004: http://www.ncbi.nlm.nih.gov/pubmed/15183762).

This article discusses the general equivalence between serum and extracellular fluid MTHF+FA concentrations outside the brain [Balk et al., 1978: (http://cancerres.aacrjournals.org/cgi/reprint/38/11_Part_1/3966.pdf) (http://www.ncbi.nlm.nih.gov/pubmed/212184)]. That article also shows that extracellular, reduced folates (MTHF or folinic acid) can be considerably more potent than extracellular folic acid in promoting cell proliferation in cultured cells (up to 100 times as potent in this article).