Rebuttal from Vinnakota and Kushmerick

Abstract

Point:CounterpointMuscle lactate and H+ production do/do not have a 1:1 association in skeletal muscleRebuttal from Vinnakota and KushmerickPublished Online:01 May 2011https://doi.org/10.1152/japplphysiol.01506.2010bMoreSectionsPDF (37 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations ShareShare onFacebookTwitterLinkedInEmail Net proton release from glycolysis or glycogenolysis and ATP hydrolysis and proton buffering. Dr. Robergs correctly states it is necessary to “understand the organic chemistry of the reactions,” but his list is incomplete both in the reactions occurring and in his omission of Mg2+ and K+ binding to phosphate moieties that also bind H+. We specifically account for all known cation binding in calculating proton stoichiometric coefficients (3) (Dr. Robergs' α) and show that at physiological free Mg2+ and K+ concentrations lactate-proton ratio from anaerobic glycogenolysis is close to 1:1 not 3:1 (4).Dr. Robergs' analysis demonstrates how easy it is to confuse reactions that generate or take up H+ and buffers and to lose track of the stoichiometric accounting. A logical framework for defining H+ uptake for each reaction in a given direction and a formula for the pH dependence of the proton coefficients is needed (3, 4). This method gives a coherent framework for metabolic proton generation, proton transport, and proton buffering as shown in Eqs. 2 and 3 of our “Point.” Thus his use of “absolute” and “gross” release of H+ has no meaning. Robergs' view of lactate dehydrogenase flux as a buffer is wrong; when pyruvate is transported into the mitochondria along with H+, the reaction has the same effect as H+ consumption due to lactate dehydrogenase reaction. LDH flux is necessarily equal to overall glycolytic or glycogenolytic flux in the steady state, in contrast to Dr. Robergs' view that “separate enzyme catalyzed reactions are not coupled reactions.” During the transient and steady states we analyzed there was no mismatch detectable in fluxes of component glycolytic reactions (see supplementary material of Ref. 4) although buffer capacity changed substantially during the experiment. Experimental evidence also shows that most of the glycogen consumed during intense exercise appears as lactate (2).Dr. Robergs' characterization of Marcinek et al.'s work as “errors of science.”Equation 11 in Marcinek et al.'s study describes the observed pH transient as the sum of protons generated from glycogenolysis and the hydrolysis of the ATP generated from glycogenolysis (ΔH+) minus the protons consumed during the breakdown of PCr to form ATP and the hydrolysis of that ATP, i.e., the Lohman reaction (γΔPCre) divided by the total buffer capacity. This is a valid calculation, although simpler than used in our “Point.” Marcinek et al.'s analysis recognizes concurrent ATP hydrolysis and synthesis. The analyses were made precisely to test whether net ATP breakdown is necessary to generate H+.REFERENCES1. Marcinek DJ , Kushmerick MJ , Conley KE. Lactic acidosis in vivo: testing the link between lactate generation and H+ accumulation in ischemic mouse muscle. J Appl Physiol 108: 1479–1486, 2010.Link | ISI | Google Scholar2. Medbo JI. Glycogen breakdown and lactate accumulation during high-intensity cycling. Acta Physiologica Scand 149: 85–89, 1993.Crossref | PubMed | Google Scholar3. Vinnakota K , Kemp ML , Kushmerick MJ. Dynamics of muscle glycogenolysis modeled with pH time course computation and pH-dependent reaction equilibria and enzyme kinetics. Biophys J 91: 1264–1287, 2006.Crossref | PubMed | ISI | Google Scholar4. Vinnakota KC , Rusk J , Palmer L , Shankland E , Kushmerick MJ. Common phenotype of resting mouse extensor digitorum longus and soleus muscles: equal ATPase and glycolytic flux during transient anoxia. J Physiol 588: 1961–1983, 2010.Crossref | PubMed | ISI | Google Scholar Previous Back to Top Next FiguresReferencesRelatedInformation More from this issue > Volume 110Issue 5May 2011Pages 1491-1491 Copyright & PermissionsCopyright © 2011 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.01506.2010bHistory Published online 1 May 2011 Published in print 1 May 2011 PDF download Metrics Downloaded 207 times