Studies on enzymic peptide synthesis: I. Synthesis of tryptophane analogue of Leu-enkephalin

A tryptophane analogue of Leu-enkephalin, Tyr-Gly-Gly-Trp-Leu-NH₂, was synthesized by stepwise elongation from C-terminal Leucylamide towards the N-terminal tyrosine using α-chymotrypsin and papain catalysis. The selections of appropriate enzymes and suitable solvents were studied. It was found that an N-blocked amino acid ester served better as a donor than the corresponding free amino acid in the peptide formation, irrespective of whether α-chymotrypsin or papain was used, while, on the other hand, an N-blocked amino acid was needed as the donor in case of thermolysin. In the last step of pentapeptide synthesis catalyzed by α-chymotrypsin, the phase transfer process seems to be preferable to the homogeneous solution method. The results of this work showed some advantages by the use of an immobilized enzyme. The tryptophane analogue of Leu-enkephalin was also synthesized by the conventional organic method. Preliminary in vitro bioassay indicated that the synthetic Try-Gly-Gly-Try-Leu-NH₂ can inhibit the contraction of the guinea Pig's ileum caused by electric stimulation to nearly the same extent as the natural Leu-enkephalin does.