A radiometric microbiologic method for vitamin B12 assay.

A radiometric microbiologic method was developed for assaying vitamin B12 in human serum. In the presence of 14C-arginine and cyanocobalamin, Lactobacillus leichmannii produced significant amounts of 14CO2 within 16-20 hr. The amount of 14CO2 evolved was optimized with the use of guanido-labeled arginine and is proportional to the amount of cyanoco-balamin added. The method gave satisfactory recovery of added vitamin B12 to sera, and it gave values comparable to the standard microbiologic (turbidimetric) method using L. leichmannii. The radiometric microbiologic method allowed elimination of several steps considered essential in the standard microbiologic methods: (A) Extraction of vitamin B12 bound to serum proteins could be accomplished in one autoclaving step with the assay medium at 15 psi for 3-5 min; (B) The precipitated serum proteins did not need to be separated, since they do not interfere with this assay,