CAT and cancer: Moreover, combined treatment with the highly preferred substrate L-Met, L-Trp, or L-Leu, but not with the moderately preferred L-Arg or the poorly catalyzed L-Glu, heightened the cytotoxic activities of NK-LAAO on cancer cells, whereas these effects could be counteracted by the removal of NK-LAAO-produced H2O2 with CAT (Fig. 3A).