Enzymatic synthesis of a CCK-8 tripeptide fragment in organic media

The enzymatic synthesis of the tripeptide derivative Z‐Gly‐Trp‐Met‐OEt is reported. This tripeptide is a fragment of the cholecystokinin C‐terminal octapeptide CCK‐8. Studies on the α‐chymotrypsin catalyzed coupling reaction between Z‐Gly‐Trp‐R1 and Met‐R2 have focused on low water content media, using deposited enzyme on inert supports such as Celite and polyamide. The effect of additives (polar organic solvents), the acyl‐donor ester structure, the C‐α protecting group of the nucleophile, enzyme loading, and substrate concentration were tested. The best reaction medium found was acetonitrile containing buffer (0.5%, v/v) and triethylamine (0.5%, v/v) using the enzyme deposited on Celite as catalyst (8 mg of α‐chymotrypsin/g of Celite). A reaction yield of 81% was obtained with Z‐Gly‐Trp‐OCam as acyl donor, at an initial concentration of 80 mM. The tripeptide synthesis was scaled up to the production of 2 g of pure tripeptide with an overall yield of 71%, including reaction and purification steps. © 1996 John Wiley & Sons, Inc.