The initial step in the fermentation of leucine to acetate, isobutyrate, and ammonia by Clostridium sporogenes is the B12 coenzyme-dependent conversion of alpha-leucine to beta-leucine (3-amino-4-methylpentanoate). The amino group migration reaction, catalyzed by leucine 2,3-aminomutase, is reversible and is inhibited by intrinsic factor. The enzyme activity has been found in several clostridia, in rat, sheep, rhesus, and African green monkey livers, and in human leukocytes.