Vitamin FIGLU

Although the FIGLU test depends on folate nutritional status, the metabolism of histidine wUl also be impaired and a positive result obtained, in vitamin B12 deficiency, because of the secondary deficiency of folate (Section 10.3.4.1). About 60% of vitamin Bi2-deficient subjects show increased FIGLU excretion after a histidine load. 

Transcorrin II bound vitamin B12 Mean cell volume Serum methylmalonic acid Serum homocysteine Urine FIGLU more than 8 h after histidine load — <0.15 <0.22 >100 fL >1 i.mol/L >20 i.mol/L >50 [ig /mL ... 

The data in Table 9.2 concern a study in rats. l he animals w cre raised on nutritionally complete diets or vitamin Bn-deficienl diets for 3 months. Then urine was collect over the course of a day and used for analysis of metabolites related to folate and vitamin Bx2 (FIGLU and methylmalonic acid). The data show that Bu deficiency induces a dramatic increase in urinary FIGLU. Rats do not requite a... 

A. Pernicious anemia occurs when the stomach does not produce adequate intrinsic factor for absorption of vitamin B12, which is required for the conversion of methylmalonyl CoA to succinyl CoA and homocysteine to methionine. A vitamin B12 deficiency results in the excretion of methylmalonic acid and an increased dietary requirement for methionine. The methyl group transferred from vitamin B12 to homocysteine to form methionine comes from 5 -methyl tetrahydrofolate, which accumulates in a vitamin B12 deficiency, causing a decrease in folate levels and symptoms of folate deficiency, including increased levels of FIGLU and decreased purine biosynthesis. 

Folate deficiency leads to accumulation of Figlu, which is excreted in urine. The excretion is very pronounced after a loading dose of histidine, a test used to detect folate deficiency. More sensitive radioisotopic assays use folate binders to the vitamins. High urinary levels of Figlu may coexist with elevated levels of serum folate. Thus, in vitamin Bn (cobalamin) deficiency, since cobalamin participates in the following reaction, FH4 is trapped as... 

FIGURE 53-6 Interrelationships and metabolic roles of vitamin and folic acid. See text for explanation and Figure 53-9 for structures of the various folate coenzymes. FIGLU, formiminoglutamic acid, which arises from die catabolism of histidine Tell, transcobalamin II CH3H4PteGlUj, mediyltetrahydrofolate. 

It is possible that both the gastrointestinal tract and the liver play a more complex role in absorption and metabolism of certain orally administered vitamins than previously recognized. More refined measurements of vitamin adequacy, such as red blood cell transketolase (B] ), glutathiane reductase (B2)> activities or urinary Figlu determinations may give more accurate knowledge of the intravenous vitamin requirements. 

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