Shigeaki Irino

[46] Production of aspartame by immobilized thermoase

Abstract The enzymatic method for the production of aspartame has many advantages over chemical methods, e.g., cheap racemic phenylalanine can be used and no β-aspartame is produced. The former leads to a lowering in the cost of raw materials and the latter to a simplified purification procedure. The disadvantages as compared with chemical methods are that the racemization of unreacted d -Phe-OMe is necessary and that an expensive catalyst is used. The racemization of d -Phe-OMe can be done easily as stated before, but the recovery and recycling of the delicate biocatalyst are not easy, although thermoase is a very stable enzyme. Here we have presented two possibilities for using immobilized thermoase, i.e., continuous column operation and batchwise operation. In both procedures, the use of an organic cosolvent is required, which leads to deactivation of the catalyst. Therefore it is most important to clarify the mechanism of deactivation by organic solvents and to find strategies to overcome it, perhaps by development of more sophisticated immobilization methods. In an industrial operation using the batchwise mode, deterioration of the immobilized enzyme occurs by the friction caused by stirring. Furthermore, separation of the immobilized enzyme is necessary after batchwise reaction, which is troublesome and costly. In the continuous column operation, channeling of the organic and aqueous layers was observed in the packed column, thus rendering the efficiency of the catalyst low, and in addition, deactivation tended to occur more rapidly than during batchwise operation. Nevertheless, the continuous column operation seems to be more advantageous than the batchwise operation from the viewpoint of industrial production needs.

Enzymatic Production of Aspartame

Aspartame (1.-aspartyl-1.-phenylalanine methyl ester, APM) is the methyl ester of the terminal dipeptide of gastrin, the hormone that is released from the antral mucosa during digcstion. I t i s about 200 times as sweet as sucrose, and has a pleasant sweetness without a bitter aftertaste. Recently, i t was approved in many countries as a food additive. and is receiving much attention as a low-calorie sweetener. Proteinases catalyze the hydrolysis of peptide bonds. This is an equilibrium reaction. and thus a peptide bond may be formed in a significant yield if one can choose the proper enzymc and conditions. W e thought that aspartame might be synthesized enzymatically. After screening many enzymes, we found that some metalloproteinases that hydrolyze an amine or hydrophobic amino acids can catalyze the following react ions: