Accumulation of Oxidized Folates (Dihydrofolate and Folic Acid) in Cobalamin Deficiency: Association With Mitochondrial Dysfunction

The authors of this article [Smith et al., 1973: (http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=4204322) (http://www.ncbi.nlm.nih.gov/pubmed/4204322)] suggested that the development of fatty liver in sheep, in response to prolonged cobalamin (vitamin B12) deficiency, might eventually impair the capacity of cells in the liver to maintain intracellular folates in a reduced state. The authors found that prolonged cobalamin deficiency both depleted intracellular total folates and decreased the percentages of folates that were polyglutamylated, as would be expected. But they also found increases in the percentages of unmetabolized folic acid (a fully-oxidized folate) and dihydrofolate (DHF) in response to the more long-term cobalamin depletion.

This explanation by Smith et al. (1973) is consistent with the research showing mitochondrial dysfunction in response to cobalamin depletion [Leeds and Brass, 1994: (http://www.jbc.org/cgi/content/abstract/269/6/3947) (http://www.ncbi.nlm.nih.gov/pubmed/7508436?dopt=Abstract); Toyoshima et al., 1996: (http://www.ncbi.nlm.nih.gov/pubmed/8774237); Nakai et al., 1991: (http://www.ncbi.nlm.nih.gov/pubmed/1679919)], and fatty liver is essentially the result of mitochondrial dysfunction and injury in hepatocytes [Pessayre et al., 2004: (http://www.ncbi.nlm.nih.gov/pubmed/15489566)]. Given that cobalamin depletion caused the intracellar folates to become depleted, the mitochondrial dysfunction would be expected to have resulted from the adverse metabolic effects of both folate depletion and cobalamin depletion. But that's not even the issue with this. The core point of that article, by Smith et al. (1973), is that there's some disturbance in the cellular redox state that decreases the activities of the cytosolic NADPH-dependent enzymes and the mitochondrial, NAD+-dependent enzymes of the folate cycle. The increases in oxidized folates (Smith et al., 1973) would be expected to result from a decrease in the normal, cytosolic NADPH/NADP+ ratio, and a decrease in that ratio could occur in response to inhibition of the tricarboxylic acid cycle enzymes by methylmalonic acid and propionic acid (Toyoshima et al., 1996; Nakai et al., 1991). The NADPH/NADP+ ratio (in the cytosol) is normally about 4 [Horne, 2004: (http://jn.nutrition.org/cgi/content/full/133/2/476) (http://www.ncbi.nlm.nih.gov/pubmed/12566486?dopt=Abstract)], and the intramitochondrial NAD+/NADH ratio is normally really high. Methylmalonic and propionic acid also inhibit respiratory chain enzymes directly, and the inhibition of the TCA cycle enzymes in the context of methylmalonic and propionic acid accumulation can be viewed as being secondary to or as occurring in concert with those inhibitory effects. The result would be a decrease in the intramitochondrial NAD+/NADH ratio, and this could be expected to eventually decrease the cytosolic NADPH/NADP+ ratio. Ceconi et al. (2000) [Ceconi et al., 2000: (http://cardiovascres.oxfordjournals.org/cgi/content/full/47/3/586) (http://www.ncbi.nlm.nih.gov/pubmed/10963731?dopt=Abstract)] found, for example, that the assumptions about cellular redox couples can become invalid in response to reperfusion-induced oxidative stress. A similar situation would be expected to occur in the context of mitochondrial dysfunction in general.

I don't have time to get into the research now, but the methyl trap and, to a lesser extent, the formate starvation hypotheses, models that attempt to account for the effects of cobalamin deficiency on the folate cycle, are based on the implicit assumption that cobalamin deficiency produces no disturbances in mitochondrial functioning or in the cellular redox state. The assumption is that the metabolic effects can be explained in terms of changes in allosteric effects, such as by the allosteric inhibition of methylenetetrahydrofolate reductase by S-adenosylmethionine (in response to methionine infusion in cobalamin deficiency, etc.). But I think that the article by Smith et al. (1973) (and the other articles I cited in relation to mitochondrial function in cobalamin deficiency) are important and could help explain the greater effectiveness of reduced folates in neurodegenerative disorders or advanced disease states (in which mitochondrial dysfunction could decrease the capacities of cells to maintain intracellular folates in a reduced state).