This article, by Han et al. (1999) [Tong-Hae Han et al., 1999: (http://ajpendo.physiology.org/cgi/content/full/276/3/E580#B1) (http://www.ncbi.nlm.nih.gov/pubmed/10070027?dopt=Abstract)] is interesting and shows that the kidneys, in particular, and liver are the major organs involved in the clearance of intravenously-infused 5-methyltetrahydrofolate (MTHF). Given the major role that the renal reabsorption of MTHF or folic acid (FA), by various transporters, is likely to play in determining the serum MTHF concentration, it is interesting to know the Km values for folic acid (0.67 uM) and MTHF (slightly less than 1 uM) reabsorption across the rat renal brush border membrane. Han et al. (1999) also found that the reabsorption of MTHF+FA, via the collective activities of the various transporters, is saturable, and this is to be expected.
A number of interesting pieces of information have come out of research on the metabolism and clearance of folates from the liver. The liver would be expected to accumulate the majority of orally-administered folic acid [and also a sizeable portion of intravenously-administered MTHF (Han et al., 1999)], in particular, and also MTHF, and my statements
(http://hardcorephysiologyfun.blogspot.com/2009/01/estimating-interstitial-fluid.html) about using the serum MTHF+FA concentration to approximate the interstitial fluid MTHF+FA concentration, outside the brain, is meant to apply only to tissues outside the liver and outside parts of the kidneys. Following the oral administration of MTHF or FA, the concentration of MTHF or FA in the portal vein will be, transiently, considerably higher than the Cmax, the peak serum folate (MTHF+FA) concentration in the systemic circulation. Wright et al. (2005) estimated that the first-pass effect for oral FA, which is generally assumed to be the result of the uptake and presystemic metabolism of FA or of MTHF (derived from the intestinal metabolism of FA) by the liver, would cause about 73 percent of a dose to be taken up, from the portal vein, by the liver, assuming that 90 percent of an oral dose is absorbed [Anthony Wright et al., 2005: (http://jn.nutrition.org/cgi/content/full/135/3/619) (http://www.ncbi.nlm.nih.gov/pubmed/15735104?dopt=Abstract)]. In contrast, Wright et al. (2005) estimated that 58 percent of a dose of folinic acid (5-formyl-THF) is likely to be presystemically sequestered and metabolized by the liver. More importantly, the cells in the intestinal tract are only able to convert about 260 ug of FA into MTHF, and most of a large oral dose is likely to enter the liver and systemic circulation as unmetabolized FA and not, as is usually assumed, as MTHF (Wright et al., 2005). Wright et al. (2005) also noted that researchers had, in fact, shown that the concentrations of unmetabolized FA in the portal vein do increase considerably in response to dosages of between 500 and 1,000 ug of FA. Given that FA has been shown to be less potent than reduced folates at producing cell proliferation in cell culture experiments [Balk et al., 1978: (http://www.ncbi.nlm.nih.gov/pubmed/212184); Watkins and Cooper, 1983: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1152268)], the assumption that the serum folate is predominantly MTHF may overestimate the effects of a given concentration of serum folate on extrahepatic cells. Another implication is that even if the serum folate is assayed and found to consist of predominantly MTHF, this is likely to be only the steady-state value. For example, an increase in the serum FA concentration in the systemic circulation could cause increases in intracellular FA in extrahepatic cells and interfere with the folate cycle in those cells.
The intracellular concentration of total folates in the liver has been shown to be relatively insensitive to increases in supplementary intakes of folates, but this "saturation" is unlikely to extend to many extrahepatic tissues and actually would be expected to increase the flexibility with which extrahepatic tissues can be targeted for folate repletion. Researchers have sometimes attempted to derive quantitative pharmacokinetic models that might seem to provide evidence of saturation of tissue stores, but most of these studies have only used folic acid and have only used tiny ranges of doses, even in animal studies [Keagy, 1982: (http://jn.nutrition.org/cgi/content/abstract/112/2/377) (http://www.ncbi.nlm.nih.gov/pubmed/7057273); Lin et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15321809); Clifford et al., 1984: (http://www.ncbi.nlm.nih.gov/pubmed/2262809?dopt=Abstract)]. Also, some of these studies have used the apparent saturability of intracellar total folates in the liver as evidence of saturation in extrahepatic cell types. These types of articles are interesting but tend to use tiny dosages, and the models are not consistent with any of the myriad data from cell culture experiments. For example, Lin et al. (2004) showed that the folate uptake into the bone marrow appeared to be saturated across a tiny dosage range, in the neighborhood of 1 mg/d of folic acid. This does not mean the bone marrow is saturated with folic acid but just means that, at that narrow range of serum folate levels, some sort of dynamic equilibration process has become established. For example, folates bind loosely to albumin [Birn et al., 2006: (http://ajprenal.physiology.org/cgi/content/full/291/1/F22) (http://www.ncbi.nlm.nih.gov/pubmed/16760376?dopt=Abstract)], and that binding could have influenced the uptake measurement that Lin et al. (2004) relied upon. The idea that the cells in the bone marrow medulla would become saturated across a 20-nM increment in serum folate would contradict every cell culture experiment performed over the last 30 years and would be inconsistent with the results of countless in vivo studies. Even though Lin et al. (2004) varied the intakes of folic acid to attempt to get dose-response data for the "marrow uptake" measurement, the intake-vs-uptake data are consistent with the type of data one would get at a fixed, small dose of folic acid in the presence of inter-individual variation.
There is also extreme variability in the responses to dosages of folic acid between studies (http://hardcorephysiologyfun.blogspot.com/2008/12/heres-my-old-paper-on-folate-and-more.html). Han et al. (1999) showed that the plasma membrane uptake transporters and transporters on the hepatic canalicular membrane that export folates into the bile have large capacities for responding to changes in intracellular folate levels. The evidence from countless articles in humans have shown that extrahepatic cells, such as those in the central nervous system, do not have access to the kinds of superfluous concentrations of extracellular folates that the liver has access to. But the evidence from cell culture studies, even those using very large concentrations of extracellular folates, indicate that the theoretical, cytostatic effects that might be expected to result from the intracellular accumulation of folates do not readily occur, even at extracellular folate concentrations as high as 9,300 nM (9.3 uM) (Brown et al., 2006: http://www.ncbi.nlm.nih.gov/pubmed/16469322). Even if researchers had not shown that the proliferation rates of cultured cells increase as the extracellular folate concentrations are increased to between 3-9 uM, the minimization of uracil incorporation into DNA requires extracellular folate concentrations up to 3 uM [Mashiyama et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15183762); Courtemanche et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15322179?dopt=Abstract)]. Additionally, Brown et al. (2004) found that the cell morphologies, even apart from the proliferation rates, of endothelial cells were substantially different at the higher extracellular folate concentration (9.3 uM) and were consistent with improvements in barrier function and with aspects of the cytoskeletal organization. Thus, the proliferation rate is not the only aspect of cellular functions that is responsive to extracellular concentrations of folates in the micromolar range.
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