CIRCULATING METABOLITE HOMEOSTASIS ACHIEVED THROUGH MASS ACTION

Abstract

Circulating metabolites serve as important nutritional supplies for bodily tissues and organs. Failure to maintain their concentrations within the physiological values can lead to severe diseases. This requires homeostatic regulations. For glucose, many glucoregulatory hormones are involved in this process such as 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 many circulating metabolites are cleared via the tricarboxylic acid (TCA) cycle in linear proportionality to their circulating concentration. Abundant circulating metabolites (essential amino acids (valine, leucine, methionine, lysine, and phenylalanine), non-essential amino acids (serine and alanine), citrate, 3-hydroxybutyrate) were administered intravenously in perturbative amounts and their fluxes were measured using stable-isotope labeling. The increased circulating concentrations induced by the perturbative infusions hardly altered their 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-, regular-, and low-protein diets. Protein synthesis rates were insensitive to elevated essential amino acid levels including leucine, nor to different dietary conditions, while amino acid homeostasis during fasting was further supported by enhanced endogenous protein catabolism. Inhibition of the key enzyme of branched-chain amino acid (BCAA) catabolism branched-chain α-keto acid dehydrogenase kinase (BCKDK) accelerated valine clearance flux and enhanced its clearance constant. However, upon protein deprivation, the only flux difference between wild-type and BCKDK whole-body knock-out mice along the BCAA degradation pathway happened in the transamination step, with the reverse flux almost zero. This suggests the BCKDH complex to be well below saturation. It is likely that the rate limiting step is upstream of transamination, and the BCAA transporters work in combination with mass action to avoid BCAA depletion when protein is scarce. Thus, despite the copious regulatory machinery in mammals, circulating metabolite homeostasis is achieved substantially through mass action-driven oxidation.