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.     

The biosynthesis of vitamin B12.

The use of 13C-Fourier transform nuclear magnetic resonance (F.t.-n.m.r.) has led to the observation that while 8 molecules of [2-13C]ALA are incorporated into vitamin B12 in P. shermanii, [5-13C]ALA labels only seven of the carbon atoms of cyanocobalamin, i.e. one of the amino methyl groups of ALA is "lost" in the process. It has also been confirmed that seven of the methyl groups of B12 are derived from 13CH3-enriched methionine and further that the chirality of the gemdimethyl grouping at C12 labelled with [13CH3]methionine is R. A soluble enzyme mixture from the 37000 or 100000 g supernatant of disrupted cells of P. shermanii converts both 14 C-labelled ALA and [14C]uro'gen III to cobyrinic acid, the simplest corrinoid material on the pathway to vitamin B12 and the coenzyme, in presence of NADPH, Co2+, Mg2+, S-adenosyl-methionine and glutathione. Multiply-labelled uro'gens (13C, 14C and 3H) have been used to show that incorporation takes place without randomization. A sequence for corrin synthesis from uro'gen III is presented.