Biochemical Studies on Vitamin B12 Part XI

In Part X, the authors published studies on vitamin B₁₂-like activity of cobalt-porphyrin-derivative prepared from radish-leaves, and the physiological significance of the compound as an active component in the intermediary metabolism of vitamin B₁₂ was described. , Afterward, pure chlorophyll a and b were isolated by column chromatography and the vitamin B₁₂-like activities of Co-porphyrin a and b were repeatedly confirmed.

Recently, effect of Co-porphyrin on the biosynthesis of real vitamin B₁₂ has been tested and the confirmation was also successfully completed by bioautography and paper electrophoresis.

In the present communication, it is discussed as of special importance that cobalt-porphyrin may be the active principle for the formation in vivo of vitamin B₁₂.     

Vitamin B12 absorption from eggs.

The assimilation of 57Co B12 from in vivo labeled eggs was much inferior to that of a comparable amount of crystalline 57Co B12. Furthermore, the absorption varied with the form in which the eggs were served. Judged by the urinary excretion test and the plasma absorption of radioactivity the average absorption from boiled and fried eggs was more than twice that from scrambled whole eggs, but less than half that absorbed from crystalline 57Co B12.     

Transport of Vitamin B12 in Escherichia coli: Genetic Studies

The chromosomal location of two genetic loci involved in the transport of cyanocobalamin (B(12)) in Escherichia coli K-12 was determined. One gene, btuA, is believed to code for the transport protein in the cytoplasmic membrane, because a mutant with an alteration in this gene has lost the ability to accumulate B(12) within the cell although normal levels of the surface receptors for B(12) are present. The other locus, btuB, apparently codes for the surface receptor on the outer membrane. These mutants have lost the ability to bind B(12) and have greatly reduced transport activity, although growth experiments have shown that they can utilize B(12) for growth, but with decreased efficiency. This surface receptor for B(12) also appears to function as the receptor for the E colicins, because btuB mutants are resistant to the E colicins, and mutants selected for resistance to colicin E1 are defective in B(12) binding and transport. The gene order was determined by transduction analysis to be cyc-argH-btuA-btuB-rif-purD. In addition, mutations in metH, the gene for the B(12)-dependent homocysteine methylating enzyme, were obtained in this study. This gene was localized between metA and malB.     

Role of Third Serum Vitamin B12 Binding Protein in Vitamin B12 Transport

A third vitamin B₁₂ binding protein present in normal serum has been shown to participate in transport of labelled vitamin B₁₂ absorbed from the gut. All three vitamin B₁₂ binding proteins in serum were labelled at the same time after oral administration of vitamin B₁₂, implying that “free” vitamin B₁₂ reached the portal blood from the gut mucosa.