Assessment of folate status
Measurement of the serum or red blood cell concen-tration of folate is the method of choice, and several simple and reliable radioligand binding assays have been developed. There are problems involved in radioligand binding assays for folate, and in some centers microbiological determination of plasma or whole blood folates is the preferred technique. Serum folate below 7 nmol/l or erythrocyte folate below 320 nmol/l indicates negative folate balance and early depletion of body reserves. At this stage the first bone marrow changes are detectable.
The ability to metabolize a test dose of histidine pro-vides a sensitive functional test of folate nutritional status; formiminoglutamate (FIGLU) is an interme-diate in histidine catabolism, and is metabolized by the folate-dependent enzyme formiminoglutamate formiminotransferase. In folate deficiency the activity of this enzyme is impaired, and FIGLU accumulates and is excreted in the urine, especially after a test dose of histidine: the so-called FIGLU test.
Although the FIGLU test depends on folate nutri-tional status, the metabolism of histidine will also be impaired, and hence a positive result obtained, in vitamin B12deficiency, because of the secondary deficiency of free folate. About 60% of vitamin B12-deficient subjects show increased FIGLU excretion after a histidine load.
Rapidly dividing cells can either use preformed TMP for DNA synthesis, or synthesize it de novo from dUMP. Stimulated lymphocytes incubated with [3H]-TMP will incorporate the label into DNA. In the presence of adequate amounts of methylene-tetrahy-drofolate, the addition of dUMP as a substrate for thymidylate synthetase reduces the incorporation of [3H]-TMP as a result of dilution of the pool of labeled material by newly synthesized TMP and inhibition of thymidylate kinase by thymidine triphosphate.
In normal cells the incorporation of [3H]-thymi-dine into DNA after preincubation with dUMP is 1.4–1.8% of that without preincubation. By contrast, cells that are deficient in folate form little or no thy-midine from dUMP, and hence incorporate nearly as much of the [3H]-thymidine after incubation with dUMP as they do without preincubation.
Either a primary deficiency of folic acid or func-tional deficiency secondary to vitamin B12 deficiency will have the same effect. In folate deficiency, addition of any biologically active form of folate, but not vitamin B12, will normalize the dUMP suppression of [3H]-thymidine incorporation. In vitamin B12 defi-ciency, addition of vitamin B12 or methylene-tetrahydrofolate, but not methyl-tetrahydrofolate, will normalize dUMP suppression.