The ability of deoxyuridine to suppress the incorporation of [3H]thymidine into DNA in rapidly dividing cells (Section 10.3.3.3) can also be used to give an index of functional folate nutritional status. Bone marrow biopsy samples provide the best source, and this has been generally a research tool rather than a screening test; however, transformed lymphocytes can also be used. The dUMP suppression test is probably the most sensitive index of folate depletion; abnormalities are apparent within 5 weeks of initiating folate deprivation, whereas detectably high urinary FIGLU occurs only after 13 weeks depletion, and bone marrow is overtly megaloblastic at 19 weeks (Herbert, 1962, 1987a).
Cells that have been preincubated with deoxyuridine, then exposed to [3H]thymidine, incorporate little or none of the labeled material into DNA. This is because of both dilution of the labeled material in the larger intracellular pool of newly synthesized TMP and also inhibition of thymidylate kinase by thymidine triphosphate.
In normal cells, the incorporation of [3H]thymidine into DNA after preincubation with dUMP is 1.4% to 1.8% of that without preincubation. By contrast, cells that are deficient in folate form little or no thymidine from dUMP, and incorporate nearly as much of the [3H]thymidine after incubation with dUMP as they do without preincubation.
Again, either a primary deficiency of folic acid or functional 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 [3 H]thymidine incorporation. In vitamin B12 deficiency, the addition of vitamin B12 or methylene-tetrahydrofolate, but not methyl-tetrahydrofolate, will normalize dUMP suppression (Killman, 1964; Pelliniemi and Beck, 1980).
10.11 FOLATE AND VITAMIN B,2 REQUIREMENTS AND REFERENCE INTAKES
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