SHORT COMMUNICATION Metabolic acidosis and the importance of balanced equations Andrew N. Lane Teresa W.-M. Fan Richard M. Higashi Received: 6 August 2008 / Accepted: 4 November 2008 / Published online: 20 November 2008 Ó Springer Science+Business Media, LLC 2008 Abstract Balancing biochemical equations for both mass and charge in metabolic networks is critical but unfortu- nately ignored too often. Failure to do so, for example, results in a common misconception about the origin of protons during lactic fermentation. Lactate, rather than lactic acid, is produced by glycolysis, and its production is a mechanism for alleviating intracellular acidosis due to glycolysis. This error is at the core of some recent papers, and is often ambiguous in biochemistry textbooks. Keywords Lactic acidosis  Balanced equation Recurring fundamental errors in the literature concerning the origins of protons in metabolic acidosis compel us to write this note. Intracellular proton homeostasis is critical for cell function. The cytoplasmic pH is typically main- tained with a narrow range, typically about 7.4 under resting conditions (Fan et al. 1988; Hackam et al. 1996; Moolenaar 1986; Schwartz et al. 1989). Thus the rate of production and utilization of protons are carefully balanced. Many cata- bolic pathways are net acid producers (cf. glycolysis, beta oxidation, hydrolysis of proteins and nucleotides, to name a few) whereas anabolic reactions generally consume pro- tons. An imbalance that leads to excess intracellular proton production requires compensatory mechanisms, which is often manifested as extracellular acidification or acidosis. Lactic acid production has been the common ‘‘culprit’’ for acidosis referred to in the literature. In numerous recent articles (Allen and Westerblad 2004; Brahimi-Horn and Pouyssegur 2007; Helmlinger et al. 2002; Mathupala et al. 2007; Pedersen et al. 2004), it is still stated that lactic acid is the source of acidosis that leads to muscle fatigue during exercise. This is a long- standing error that occurs in many (but not all) biochem- istry textbooks, that leads to incorrect biochemical interpretations. The oxygen-independent oxidation of glucose to pyru- vate via glycolysis in tissues is often termed anaerobic, although it also occurs under normoxic conditions. In solid tumors, glycolysis is accelerated (known as the Warburg effect (Warburg 1956)) and leads to extracellular acidosis (Gatenby and Gillies 2004). The following balanced equation shows that the oxidation of glucose to pyruvate is accompanied by the release of protons. The number x depends on the intracellular pH: C 6 H 12 O 6 Glucose þ 2NAD þ þ 2ADPMg  þ 2H x PO 3x 4 ! 2CH 3 COCO  2 pyruvate þ 2NADH þ 2ATPMg 2 þ 2H 2 O þ 2xH þ : ð1Þ Under physiological salt concentrations, the pK of H 2 PO 4 - is about 6.8 (Roberts et al. 1981), hence at pH 7.4 x is 1.2, at pH 6 x is 1.9 and at pH 8 x is 1.06. Thus, glycolysis is a source of protons (acidosis), and under normoxic, resting conditions, where x & 1.3, com- plete oxidation of glucose to pyruvate generates 2.4 H ? . A. N. Lane (&)  T. W.-M. Fan JG Brown Cancer Center, University of Louisville, Sue 431, 529 S Jackson St, Louisville, KY 40202, USA e-mail: anlane01@louisville.edu A. N. Lane  T. W.-M. Fan  R. M. Higashi Center for Regulatory Environmental Analytical Metabolomics, University of Louisville, Louisville, KY, USA T. W.-M. Fan  R. M. Higashi Department of Chemistry, University of Louisville, Louisville, KY, USA 123 Metabolomics (2009) 5:163–165 DOI 10.1007/s11306-008-0142-2