Ethylene Production from Peptides and Protein Containing Methionine

It has been reported that the C0-C4 carbons of methionine are converted to ethylene in FMN-light (23) and Cu"!-ascorbate (10) model systems. Yang (22) has shown that C-C, carbons of 2-keto-4-methylthiobutyrate and the Cr, carbons of 3-methylthiopropionaldehyde (i.e., methional) are converted to ethylene in a model peroxidase system. When methionine was fed to apple tissue, the C-C4 carbons were apparently converted to ethylene (1, 11). These reports demonstrate that the CrCa carbons of methionine or its related derivatives are converted to ethylene in model systems and in plant tissue. It was thus of interest to demonstrate whether or not methionine residues in protein and peptides could be converted to ethylene. Using a peroxidase system described by Yang (21), it was found that peptides containing a C-terminal methionine residue, but not an N-terminal or internal residue, produced ethylene at initial rates two to four times greater than the small but significant rate from methionine. Purified egg albumin did not produce any ethylene; however, after limited proteolysis of egg albumin, significant ethylene was formed. Of particular importance was the finding that N-acetyl-D,L-methionine produced ethylene at a rate about equal to methional. The methods used were similar to those described by Yang (21). The incubation mixture contained the following in a total volume of 1 ml: 0.05 ,mole of MnSO4, 0.02 ,umole of resorcinol, 50 imoles of KH,PO, (pH 7.8), 3.0 ,ug of peroxidase, 2.0 umoles of NaHSO., and 0.5, 1.0, or 2.0 ,umoles of substrate. The NaHSO. was added to start the reaction, and the ethylene concentration was determined with a gas chromatograph after 5-min incubation at room temperature. In experiments with crystallized egg albumin, 12 umoles of H110, was substituted for the MnSO,. The peptides and N-formyl-D,Lmethionine were purchased from Mann, New York, New York. Horseradish peroxidase (Rz = 3) was obtained from Worthington, Freehold, New Jersey. Crystallized egg albumin was obtained from Pierce, Rockford, Illinois. Methional was purchased from Eastman, Rochester, New York. The results of ethylene production from peptides and other derivatives of methionine in the peroxidase system are shown in Figure 1. Peptides which contained an N-terminal (L-MetGly-Gly) or internal residue (Gly-L-Met-Gly and N-F-L-MetGly-Gly)' of methionine produced little or no detectable ethylene, respectively. This finding was unchanged even for