Radioisotope Dilution

Radioisotope dilution assays are based on the principle of competition between radioactive labeled ( Co) vitamin B 2 and cobalamins extracted from matrices for binding sites on the intrinsic factor (a glycoprotein). Binding is in proportion to the concentration of the radioactive and nonradio active B 2 with the concentration of intrinsic factor as the limiting factor. Free cobalamins are separated from those bound on the intrinsic factor by absorption... 

Radioisotope dilution using the chelating agent bathophenanthroline has been used to determine down to 5 xg/l iron in seawater [357]. 

Sharama et al. [479] compared results obtained in the determination of cobal-amins in ocean waters by radioisotope dilution and bioassay techniques. These workers showed that the isotopic methods measured both biologically active and inactive cobalamins indiscriminately when porcine factor was used as the B12-specific binder. 

Chemical Pathway Studies. Radioisotope Dilution Methods. 

A mixture is being assayed by radioisotope dilution analysis. 10 mg of the labelled analyte (0.51 pCi mg-1) was added. 1.5 mg of the pure analyte was separated and its specific activity measured and found to be 0.042 pCi mg1. What was the amount of analyte in the original sample ... 

MTHF was initially measured by microbiological and radioisotope dilution assay [12, 13], and later by high-pressure liquid chromatography (HPLC) using electrochemical (EC), ultraviolet, or fluorescence detection [14-16]. Compared to other methods, EC detection is more sensitive. 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

I Andersson, R Lundqvist, R Oste. Analysis of vitamin B12 in milk by a radioisotope dilution assay. Milchwissenschaft 45 507-509, 1990. 

B-G Osterdahl, K Jann6, E Johansson, H Johnsson. Determination of vitamin B,2 in gruel by a radioisotope dilution assay. Int J Vit Nutr Res 56 95-99,1986. 

Measurements using radioisotope dilution techniques are rapid and the more recently designed kits appear to be reliable in skilled hands. 

G3. Green, R., Newmark, P. A., Musso, A. M., and Mollin, D, L., The use of chicken serum for measurement of serum vitamin B12 concentration by radioisotope dilution Description of method and comparison with microbiological assay results. Br. J. Haematol. 27, 507-527 (1974). 

Kl. Kolhouse, J. F., Kondo, H., Allen, N. C., Podell, E., and Allen, R. H., Cobalamin analogues are present in human plasma and can mask cobalamin deficiency because current radioisotope dilution assays are not specific for true cobalamin. N. Engl. J. Med. 299, 785-792 (1978). 

L2. Lau, K. S., Gottleib, C., Wasserman, L. R., and Herbert, V., Measurement of serum vitamin B12 level using radioisotope dilution and coated charcoal. Blood 26, 202-214 (1965). 

C]Tryptophan gave inactive alkaloids but tritiated 2,4-dihydroxy-quinoline (34) and its N-methyl derivative were incorporated into (47) (0.009 % and 0.020% respectively) an early route had suggested the derivation of what was essentially (34) from tryptophan. Radioisotope dilution showed the presence of both these quinoline precursors together with iV-acetyl- and N-methyl-anthranilic acid in A. baueri. A satisfactory incorporation of N-methylanthranilic acid into (47) was found in Evodia xanthoxyloides, and this, together with its natural occurrence, indicates that early methylation may be important in the biosynthesis of acridone alkaloids. 

An attempt to emulate this pathway in vitro followed, unfortunately hampered by a dismal MnC>2 promoted oxidative coupling. However, a radioisotope dilution study did indeed suggest a 0.012% conversion to radioactive salutaridine 79, Scheme 6. 

Ruzicka, J. and Lamm, C.G. (1969) Automated determination of traces of mercury in biological materials by substoichiometric radioisotope dilution. Talanta, 16,157-168. 

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