1. After fractionation of the vitamin B12-binding proteins of ten normal sera the components containing transcobalamin II and R-type binders of vitamin B12 respectively were studied for endogenous vitamin B12 content by two distinct systems of vitamin B12 assay.
2. The measurements of total serum vitamin B12 by either bioassay with Euglena gracilis or a radioisotope dilution assay agreed closely.
3. The native vitamin B12 carried by transcobalamin II was higher as measured by bioassay than by isotope dilution assay.
4. The presence of the transcobalamin II fraction of human serum altered the key reaction between the binding reagent of the isotope dilution assay, so that this assay failed to measure vitamin B12 quantitatively.
5. Probably, the mean fraction of plasma vitamin B12 carried by transcobalamin II is in the range 20–30%.
The interpretation of serum vitamin B12 concentrations in cattle is not well established but a value <350 ng/1 is commonly used to describe adequacy (MAFF, 1978). Three separate experiments explore this interpretation and the ability of cobalt (Co) bullets to increase serum vitamin B12 concentrations in cattle. In all cases, vitamin B12 has been measured microbiologically, using the organism L. leichmannii. In experiment 3, a number of samples were additionally analyzed using a radio isotope dilution (RID) technique.
Vitamin B12 and folate concentrations in serum and cerebrospinal fluid of neurological patients with special reference to multiple sclerosis and dementia.