Vitamin B-12 biosynthesis. A model system for isopropanolamine formation by reaction between reduced corrinoid and...

When a solution of 14C-labeled L-threonine is incubated with aquocobalamin in the presence of excess of a reductant such as 2-mercaptoethanol or sodium borohydride, a labeled corrinoid adduct is formed. The corrinoid participates stoichiometrically, and not catalytically. Since 35S-labeled 2-mercaptoethanol is not part of the adduct, the reduction of the corrinoid from the Co(III) to the Co(II) form is a prerequisite for the reaction. Various independent tests show the formation of isopropanolamine. The reaction system is an attractive model for the so far unexplained decarboxylation of threonine to isopropanolamine in the course of corrinoid biosynthesis.