ONE-STEP PROCESS FOR INCORPORATION OF L-METHIONINE INTO SOY PROTEIN BY TREATMENT WITH PAPAIN

The plastein reaction is known to be applicable to covalently incorporating essential amino acids into proteins. However, this reaction requires two enzymatic processes: protein degradation and resynthesis. In the present study we have simplified the process and made it possible to incorporate L-methionine ethyl ester (L-Met-OEt) into soy protein isolate (SPI) by its one-step treatment with papain (EC 3.4.4.10). A most preferable set of the conditions for the one-step process was established as follows: concentration of SPI (protein content: 92.0%) in a reaction medium, 20 wt %; initial pH of the medium, 10.0; ratio of papain [3.24 units mg'1 min1 (25 °C, pH 6.2)] to SPI, 1: 100 (w/w); concentration of L-cysteine (activator), 1 mM; and incubation time, 8 h (at 37 °C). When a 20: 1 (w/w) mixture of SPI and L-Met-OEt-HCl was incubated under this set of conditions, it was observed that 81.5 molar % of the original L-Met-OEt was utilized to be incorporated. DL-Met-OEt could be used for the incorporation of L-methionine since the D isomer remained intact during the enzymatic process. © 1979, American Chemical Society. All rights reserved.