Measurement of vitamin B12in the livers and sera of sheep and cattle and an investigation of factors influencing serum vitamin B12levels in sheep

Abstract

Modifications of a radioassay method for the analysis of vitamin B₁₂ using chicken serum as the binder are described. This obviates the need to use individual serum blanks to correct for non-specific binding in vitamin B₁₂ assays of the sera and livers of sheep and cattle. Samples with high vitamin B₁₂ levels can be diluted prior to assay without loss of linearity. Recoveries of added cyanocobalamin or hydroxocobalamin were better than 95% and results correlated significantly with those obtained using a microbiological assay (Poteriochromonas malhamensis). Sera and liver samples stored for four weeks at temperatures ranging from −20° to 22° showed no change in vitamin B₁₂ levels. Withholding food from sheep for 44 hours led to a marked increase in serum vitamin B₁₂. This effect was also evident in sheep eating a limited amount of cut grass. In sheep at pasture there was no evidence of a diurnal variation in serum vitamin B₁₂ levels. Serum vitamin B₁₂ levels in sheep at pasture were shown to be an unreliable indicator of liver vitamin B₁₂.