Inhibition of Plm enzyme activity by TXA and lysine. High concentration of TXA inhibits Plm activity. The enzyme assay was performed on both μPlm and Plm using the chromogenic substrate S2251. (Top panel) Activities of μPlm and Plm are shown as normalized residual activity of the initial rate and are attenuated in the presence of increasing concentration of TXA (0-125 mM), suggesting that TXA is also a weak active site inhibitor. Similar response was observed when lysine was tested but not with alanine as expected. Interestingly, at 100 mM, methionine inhibits up to 20% of the Plm activity. (Bottom panel) Also shown are the IC₅₀ of TXA and lysine (Lys) on Plm activity derived from the experiment; that of methionine (Met) and alanine (Ala) is too low to be determined. See supplemental Data for details. N.D., not determined.