Radioassay of vitamin B12 and other corrinoids

Comparison of three commercial kits and a microbiological assay for the determination of vitamin B12 in serum

Different methods are available for cobalamin determination in serum. Microbiological and radio ligand binding assays are the most commonly used. Kits involving non-isotopic competitive-binding assay have been recently commercialized. In the present work, cobalamins were determined in 146 patient sera, using four methods: a microbiological method, two no boil radio ligand binding assay kits (Magic B12 FOL (NB) from Ciba-Corning and SimulTRAC SNB No Boil from Becton Dickinson) and a non-isotopic kit with acridinium ester labelled cobalamin (Magic Lite from Ciba-Corning). Median (range) cobalamin concentrations in pmol l-1 were 317 (15-1291) using the microbiological method, 355 (25-3469) using the Magic Lite kit, 355 (35-2312) using the Magic B12 FOL (NB) kit and 380 (37-2021) using the SimulTRAC SNB No Boil kit. The ANOVA test indicated that differences between methods were statistically significant (p < 0.01). Competitive-binding methods gave higher results than the microbiological method. Although correlation coefficients were not excellent (0.88 < r < 0.96), the results obtained with the different methods were generally similar and confirmed that competitive methods are useful for detecting low serum concentration of vitamin B12.

Development and application of an alpha-face-specific radioimmunoassay for vitamin B12

The first development of an alpha-face-specific radioimmunoassay for vitamin B12 is described. Sheep, fed a cobalt-deficient diet, and immunized with a conjugate between Co-beta carboxypropyl cobalamin and keyhole limpet hemocyanin, were used to produce antisera. The antisera crossreacted with Co-beta derivatives of vitamin B12, but did not crossreact with the alpha-face vitamin B12 analog cobinamide. The antisera were used to develop a sensitive and reproducible radioimmunoassay that was free from contamination with the nonspecific vitamin B12 binding protein, R-protein. Both the radioimmunoassay and measurements of plasma concentrations of methylmalonic acid were applied to the diagnosis of cobalt/vitamin B12 deficiency in sheep. The assay correlated well with a commercially available radioassay and did not falsely detect normal vitamin B12 concentration in plasma samples containing elevated concentrations of methylmalonic acid.

Development of a specific radioimmunoassay for vitamin B12 and its application in the diagnosis of cobalt deficiency in sheep

A radioimmunoassay (RIA) for vitamin B12 is described. Antisera were raised in rabbits using a conjugate between 5'-O-succinyl cyanocobalamin and chicken serum albumin. The antibody is largely specific for the beta (upper) face of the vitamin B12 molecule. The RIA can detect vitamin B12 down to concentrations of 37 pmol/L and correlated well (r = 0.980, p less than 0.001) with a commercial radioassay. The use of this RIA, together with an assay for plasma methylmalonic acid, in the diagnosis of cobalt/vitamin B12 deficiency in sheep is described.