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A Radiometric Microbiologic Method for Vitamin B₁₂ Assay

Johns Hopkins Medical Institutions, Baltimore, Maryland

Correspondence: For reprints contact: Marianne F. Chen, Div. of Nuclear Medicine, Johns Hopkins Medical Institutions, 615 N. Wolfe St., Baltimore, MD 21205.

ABSTRACT

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 cyanocobalamin 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.