(A) Simulated methionine (Metin, black line) and homocysteine (Hcyin, blue line) influx perturbations used to generate the output shown in the other panels. (B) Metabolite concentration responses. (C, D) Homocysteine methylation reaction flux (VMS + VBHMT) response. In panels (B) – (D), blue lines show the output of the reference model; grey lines (N = 200) show the output of the models containing the newly identified parameter vectors. The parameter vectors used in (B) and (C) were identified by inversion of concentration data only, and in (D) by inversion of both concentration and flux data.