Can the combination of electrochemical regeneration of NAD(+), selectivityof L-alpha-amino-acid dehydrogenase, and reductive amination of alpha-keto-acid be applied to the inversion of configuration of a L-alpha-amino-acid?

The inversion of configuration of L-alanine can be carried out by combining its selective oxidation in the presence of NAD(+) and L-alanine dehydrogen ase, electrochemical regeneration of the NAD(+) at a carbon felt anode, and reductive amination of pyruvate, i.e., reduction of its imino derivative a t a mercury cathode, the reaction mixture being buffered with concentrated ammonium/ammonia (1.28M/1.28M). The dehydrogenase exhibits astonishing acti vity and stability under such extreme conditions of pH and ionic strength. The main drawback of the process is its slowness. At best, the complete inv ersion of a 10 mM solution of L-alanine requires 140 h. A careful and detai led quantitative analysis of each of the key steps involved shows that the enzyme catalyzed oxidation is so thermodynamically uphill that it can be dr iven efficiently to completion only when both the coenzyme regeneration and the pyruvate reduction are very effective. The first condition is easily f ulfilled. Under the best conditions, it is the rate of the chemical reactio n producing the imine which controls the whole process kinetically. (C) 199 9 John Wiley & Sons, Inc.