Biosynthesis of porphyrins and related, macrocycles. Part 28. Development of a pulse labelling method to determine the C-methylation sequence for vitamin B12

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Abstract
The biosynthesis of vitamin B12 involves enzymic C-methylation of uroporphyrinogen-III at 8 sites. Earlier work had established that the first 3 methyl groups are introduced at positions 2, 7, and 20 in that order. Pulse-labelling experiments with 13C-labelled and unlabelled S-adenosylmethionine show that the remaining 5 C-methyl groups are added in the order 17, 12, 1, 15, 5. The pulse labelling approach depends on assaying the biosynthetic product for extent of 13C-labelling by n.m.r. The results lead to discussion of the late stages of the B12 biosynthetic pathway. This highlights the need for a simple scheme of nomenclature for the various biosynthetic intermediates and a systematic one based on the parent names precorrin and corrin is proposed.