Vitamin assay, plasma levels

Some hydroxy metabolites of coplanar PCBs, such as 4-OH and 3,3 4,5 -tet-rachlorobiphenyl, act as antagonists of thyroxin (Chapter 6, Section 6.2.4). They have high affinity for the thyroxin-binding site on transthyretin (TTR) in plasma. Toxic effects include vitamin A deficiency. Biomarker assays for this toxic mechanism include percentage of thyroxin-binding sites to which rodenticide is bound, plasma levels of thyroxin, and plasma levels of vitamin A. 

Recently a multiple assay procedure for determination of all the metabolites of vitamin D has been described ° A flow sheet of the analysis is shown in Fig. 7. Plasma vitamin D metabolite levels in normal and anephric human subjects are shown in Table, 1. 

Vitamin C status can be assessed by measuring plasma levels or urinary excretion. However, due to practical disadvantages, e.g., quantitative sampling of urine and instability of vitamin C, determination of vitamin C in urine has been mainly replaced by determination of plasma levels. Methods include direct determination by FIPLC, or automated assays based on a derivatization of ascorbic acid forming colored or fluorescent derivatives. While a plasma concentration <11.4p.molH is widely used to characterize deficiency, literature data signifying adequate status vary from >17 to 28.4p.moll. ... 

Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin Bs), which is a cofactor for the cystathionine synthase reaction. Some patients respond to the administration of pharmacological doses of pyridoxine (25-100 mg daily) with a reduction of plasma homocysteine and methionine. Pyridoxine responsiveness appears to be hereditary, with sibs tending to show a concordant pattern and a milder clinical syndrome. Pyridoxine sensitivity can be documented by enzyme assay in skin fibroblasts. The precise biochemical mechanism of the pyridoxine effect is not well understood but it may not reflect a mutation resulting in diminished affinity of the enzyme for cofactor, because even high concentrations of pyridoxal phosphate do not restore mutant enzyme activity to a control level. 

Therapy, in the short term, is with intravenous unfractionated or subcutaneous low molecular weight heparin. Aspirin, given in low doses between 50 and 100 mg per day, is sufficient to diminish platelet-vessel interaction. Alternatives include 100-200 mg of sulphinpyrazone once or twice a day or dipyridamole where 100 mg four times a day can be used on its own or between 25 and 75 mg combined with aspirin three times a day. More recently thiopy-ridines, as a class, has been shown to have equivalence at 250 mg twice a day. In hyperhomocysteinaemia the risk is reduced by 5 mg of folate and 100 mg of vitamin Bg daily, with addition of oral vitamin Bi2 of less clearly defined benefit. The effect of this intervention requires re-assay at 3-monthly intervals, following standard methionine challenge, to ensure that suitable suppression has been achieved in the plasma amino acid level (Table 5). 

Serum Levels. This probably is the most reliable, but it does require very sensi-ti e assay methods for those vitamins required in microgram (10 ) amounts. It also requires knowledge as to how the vitamin is transported free or bound to a plasma protein or cn a specific transport protein. Examples of the latter are transport proteins for vitamin A and vitamin D,. The tocopherols will be found in the lipoproteins (VLDL, LDL, etc.). 

Because the serum/plasma concentration of the active form of vitamin D3, l,25-(OH)2-D3, is extremely low (30-70 pg/ml), it has been conventionally measured by a radioreceptor assay technique using the vitamin D receptor in the chnical field. Although a method has been reported for the LC-MS assay of l,25-(OH)2-its applicabihty was proved only for the rat serum assay this method is of less practical use in the clinical field. Even if an up-to-date mass spectrometer model is employed, it may be difficult to measure the serum/plasma l,25-(OH)2-D3 levels with a clinically available sample volume (<1 ml). 

Folate can be measured in plasma or serum by microbiological assay using L. casei as the test organism, but this test can be confounded if the subject is on antibiotic treatment. Serum folate values reflect recent dietary intake and a vitamin deficiency is ascribed only where serum folate remains low over a period of time. Plasma folate levels are thought to reflect the day-to-day variations in dietary folate levels while red blood cell folate is a better indicator of long-term tissue storage levels. 

Jorgensen, C. S., Christiansen, M., Norgaard-Pedersen, B., et al. 2004, Gc globulin (vitamin D-binding protein) levels An inhibition ELISA assay for determination of the total concentration of Gc globulin in plasma and serum. Scand J Clin Lab Invest, 64 157-66. 

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