Binding assays folates

W. Huang, A. Feltus, A. Witkowski, and S. Daunert, Homogeneous bioluminescence competitive binding assay for folate based on a coupled glucose-6-phosphate dehydrogenase-bacterial luciferase enzyme system. Anal. Chem. 68, 1646-1650 (1996). 

Free folate, released by conjugase action, is absorbed by a carrier-mediated mechanism in the jejunum. However, the folate in mUk is mainly bound to a specific binding protein (which has been used in radioligand binding assays for folate) the protein-folate complex is absorbed intact, mainly in the ileum, by a mechanism that is distinct from the j ejunal transport system for free folate. The biological availability of folate from miLk, or of folate from diets to which mUk has been added, is considerably greater than that of unbound folate, whereas that of folate from cereal foods, or of free folic acid taken with cereal foods, is lower. 

A3. Archibald, E. L., Mincey, E. K., and Morrison, R. T., Estimation of serum folate levels by competitive protein binding assay. Clin. Biochem. (Ottawa) 5, 232-241 (1972). 

Shaw, W., Slade, B. A., Harrison, J. W., and Nino, V., Assay of serum folate Difference in serum folate values obtained by L. casei bioassay and competitive protein binding assay. Clin. Biochem. (CHtawa) 7, 165-178 (1973). 

Stralsjo, L., Ahlin, H., Witthoft, C., and Jastrebova, J. 2003. Folate determination in berries by radioprotein-binding assay (RPBA) and high performance liquid chromatography (HPLC). Fur. Food Res. Technol. 216 264-269. 

Enzyme protein binding assay Based on competitive reaction between folate isomers and folate binding protein. 

Finglas, P.M., Faulks, R.M., and Morgan, M.R.A., 1988. The development and characterization of a protein-binding assay for the determination of folate—potential use in food analysis. Journal of Micronutrient Analysis. 4 295-308. 

Parker, N., Turk, M. J., Westrick, E., Lewis, J. D., Low, P. S., Leamon, C. P. 2005. Folate receptor expression in carcinomas and normal tissues determined by a quantitative radioligand binding assay. Anal Biochem, 338, 284-93. 

Measurement of the serum or red blood cell concentration of folate is the method of choice, and a number of simple and reliable radioligand-binding assays have been developed. There are a number of problems involved in radioligand-binding assays for folate, and in some centres microbiological determination of plasma or whole-blood folates is the preferred technique. 

The microbiological method for total folate determination is still the most widely used and accepted procedure, and competitive-binding assays are most typically used for blood folate analyses. Separation, identification, and quantification of natural folates using chromatographic procedures, particularly HPLC, can be achieved but is not straightforward and requires careful selection of chromatographic conditions for the required separations (7). 

K Wigertz, M Jagerstad. Comparison of a HPLC and radioprotein-binding assay for the determination of folates in milk and blood samples. Food Chem 54(4) 429-436, 1995. 

Protein-binding assays Several enzyme protein-binding assays and radio-protein-binding assays have also been developed and commercialized as kits for rapid evaluation of this vitamin in blood, plasma, and different foodstuffs. Despite their rapidity, these methods lack adequate discrimination between different forms of folates. 

Recently, radioassay methods have been refined to measure folates in biological samples. These techniques use radioactively labeled folates and competitive protein binding.80 Johnson et al.81 compare this method with traditional microbiological assay with L. casei. 

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