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We studied the susceptibility of the pyrrolidone moiety and the pyroglutamyl-peptide bond at pGlu-X-Ala-Phe-OH (X = Gly, Ala, Tyr, Ile, Pro, His, Lys, Arg, Thr, Ser, Asp, Glu and Trp) to 1 N HCl or 2 M trifluoromethanesulfonic acid at 60 degrees C. Here we describe the rates of the cleavage reaction of the pGlu-X bond, the pyrrolidone ring-opening reaction of the pGlu moiety and the hydrolysate accumulation. The rank order of the susceptibility rates of the cleavage reactions was Ser &gt; Pro, Gly &gt; Arg, Ala, Glu, Thr, Asp &gt; His, Lys &gt; Trp, Tyr, Ile, and that of the ring-opening reaction was Ile &gt; Tyr, Trp &gt; Arg, His, Lys, Asp &gt; Glu &gt; Ala &gt; Pro, Gly &gt; Ser &gt; Thr. The rank order of the half-lives of the model peptides was Pro &gt; Arg, Lys, Ile &gt; His, Glu &gt; Ala, Tyr &gt; Asp &gt; Gly &gt; Ser &gt; Thr. The results indicated that a bulky and sterically hindered side chain of the amino acid residue neighboring the pGlu moiety favors the ring-opening reaction, and retards the decomposition on acid hydrolysis and the cleavage reaction. Thus, the ring-opening and the cleavage reactions were greatly affected by the amino acid residue neighboring the pGlu moiety in the hydrolysis of pGlu-peptides.

Hydrolytic Cleavage of Pyroglutamyl-Peptide Bond. II. Effects of Amino Acid Residue Neighboring the pGlu Moiety.