A colorimetric screening method for microorganisms having methionine γ-lyase activity

This paper reports a convenient screening method for detecting methionine gamma -lyase activity in microorganisms which can grow on DL-methionine as the sole nitrogen source. L-Methionine gamma -lyase has been known to have the most versatile catalytic properties among other pyridoxal-5'-phosphate-dependent enzymes, but only known from a limited number of biological sources. For an efficient screening of the enzyme, we employed 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) to detect the enzyme reaction product methanethiol, which spontaneously reduces DTNB and develops a yellow coloration on and around colonies. The yellow color has emerged and faded away within 72 h, suggesting that the reduced DTNB may be oxidized by spontaneous and/or enzymatic formation of disulfide. This method, despite reduction in specificity compared to the standard assay, allowed us to select candidate microorganisms out of hundreds of other similar colonies merely by observing the yellow coloration.