Methionine (Met) is an essential amino acid and the only substrate for synthesis of S-adenosylmethionine (AdoMet), an important biological methyl donor. In liver Met consumed in AdoMet synthesis can be replenished via remethylation (RM) of homocysteine or cleared via transsulfuration (TS) pathway. Our previous modeling studies [1] predicted the existence of two modes of Met metabolism in liver characterized by low metabolic rates and metabolite levels at [Met] equal or below its normal physiological value (~50 µM) and by high metabolic rates and metabolite concentrations at [Met] above this value. According to the model Met metabolism sharply switches from one mode to another and the switch is triggered by [Met]. The switch from “low” to “high” mode is associated with sharp increase in steady-state [AdoMet]. In the present work we experimentally analyzed the dependence of [AdoMet] and Met consumption rate on [Met] in suspension of freshly isolated mouse hepatocytes. Hepatocytes were isolated by the method of Berry et al. [2]. [AdoMet] and [Met] were measured by HPLC as described in [3, 4]. For analysis of experimental data we constructed an extended mathematical model of liver Met metabolism, including detailed description of kinetics of all enzymes involved in Met metabolism and simplified description of folate metabolism. It was found that Met added to hepatocyte suspension is uniformly distributed between the cells and the medium. Thus extracellular and intracellular [Met] are equal. Initial [Met] in hepatocyte suspension was varied from 40 to 400 µM. In this range [AdoMet] increased from 79±37 μmol/l cells (n=24) to 930±350 μmol/l cells (n=12). Corresponding Met consumption rate increased from 0.86±0.40 mmol/h×l cells (n=15) to 6.0±2.4 mmol/h×l cells (n=12). A sharp increase in the steady-state [AdoMet] and Met consumption rate was found within a narrow range of [Met] from 50 to 100 μM (Fig. 1A,B). The dependence of [AdoMet] and Met consumption rate on [Met] below and above this range was weak. Similar results were obtained in rat hepatocytes. Analysis of the model shows that the increase in [Met] above normal physiological value switches Met metabolism from conservation to a disposal mode and is associated with a redistribution of metabolic flux between RM and TS. To our knowledge liver Met metabolism demonstrates the new mechanism of trigger behavior that is based on allosterically regulated switch between two sets of enzymes catalyzing parallel metabolic fluxes.