A MODIFIED PROCEDURE FOR THE ESTIMATION OF VITAMIN B12 IN NATURAL MATERIALS OF LOW POTENCY

The isotope dilution assay for vitamin B12 depends on the extraction and removal of vitamin B12-like factors prior to the spectrophotometric measurement of vitamin B12. Since it is not suitable for low potency materials, modifications have been made in the method of extraction and purification. Vitamin B12 is measured with Lactobacillus leichmannii 313. The purification procedure was effective in removing pseudovitamin B12 and factor A, but was ineffective in the removal of vitamin B12III. A correction factor was used in place of Co60-labelled vitamin B12 to adjust for loss during purification. Statistical analysis showed this correction factor to be valid and reproducible. Purification did not affect the estimates of vitamin B12 activity of milk, blood plasma, chick mash, or fish meal. True vitamin B12 activity was apparently responsible for 50% of the total vitamin B12 activity of dried cattle faeces and for less than 10% of the total activity of yeast extract. The coefficient of variation of the modified method was 8.7% based on 20 potency estimates.