Measurement of Serum Binding Capacity for Vitamin B 12

Abstract

Summary

Four different methods for determination of “free” vit B₁₂ and the fraction “bound” by the serum proteins have been compared. The serum binding capacity for vit B₁₂. at varying concentrations of total vit B₁₂ has been determined with the same methods. None of the methods showed correlation between original serum vit B₁₂ concentrations and binding of added vit B₁₂. When binding capacity was determined by dialysis and by using uptake of vit B₁₂ by wild type E. coli, there was good agreement. This showed a mean binding capacity of 1350 pg vit B₁₂/ml serum when total vit B₁₂ concentration was below 10,000 pg/ml. Amount of vit B₁₂ unavailable for L. leichmannii increased almost proportionally with total concentration of vit B₁₂. Amount of vit B₁₂ unavailable for E. gracilis was almost constant, irrespective of total concentration of vit B₁₂.

We therefore propose that the term “microbiologically unavailable vit B¹²” should always include the name of the organism and the initial total concentration of vit B₁₂, and that the serum binding capacity for vit B₁₂ should be defined as the amount of vit B₁₂ per ml serum which is non-dialyzable at such initial total concentration of vit B₁₂ that more than one-fourth and less than half of the initial total amount of vit B₁₂ remains undialyzed.