Circulating metabolite homeostasis achieved through mass action

While glucose homeostasis in the circulation is tightly controlled by insulin and other hormones, dedicated hormonal regulators do not exist for most other circulating metabolites. Using perturbative metabolite infusions with isotope labelling in mice, Li et al. show that homeostasis of many circulating metabolites is considerably regulated through mass action-driven oxidation. Homeostasis maintains serum metabolites within physiological ranges. For glucose, this requires insulin, which suppresses glucose production while accelerating its consumption. For other circulating metabolites, a comparable master regulator has yet to be discovered. Here we show that, in mice, many circulating metabolites are cleared via the tricarboxylic acid cycle (TCA) cycle in linear proportionality to their circulating concentration. Abundant circulating metabolites (essential amino acids, serine, alanine, citrate, 3-hydroxybutyrate) were administered intravenously in perturbative amounts and their fluxes were measured using isotope labelling. The increased circulating concentrations induced by the perturbative infusions hardly altered production fluxes while linearly enhancing consumption fluxes and TCA contributions. The same mass action relationship between concentration and consumption flux largely held across feeding, fasting and high- and low-protein diets, with amino acid homeostasis during fasting further supported by enhanced endogenous protein catabolism. Thus, despite the copious regulatory machinery in mammals, circulating metabolite homeostasis is achieved substantially through mass action-driven oxidation.

Automated summary

The study reveals that the homeostasis of most circulating metabolites is regulated through mass action-driven oxidation. While glucose homeostasis is controlled by insulin, the master regulator for other metabolites remains unknown. This finding highlights the importance of mass action-driven oxidation in maintaining circulating metabolite homeostasis.