Succinate accumulation and oxidation in cardiac ischemia-reperfusion
Authors: Collins Nicole , Van den Bergh Françoise , Beard Daniel 
Published: May 2023

Ischemia occurs when restricted blood flow prevents oxygen delivery to cells and can result in cell injury or death. The heart is particularly susceptible to the adverse effects of ischemia due to its relatively high oxygen demand and high rate of ATP consumption needed to support contractile function. In acute ischemia, the metabolite succinate accumulates to millimolar concentrations in the myocardium. If and when the myocardium is reperfused following ischemia, rapid oxidation of the accumulated succinate leads to the production of high levels of reactive oxygen species (ROS) putting oxidative stress on cells. Identification of therapeutic targets for ischemia-reperfusion injury requires determining and quantifying the mechanisms of how succinate is produced and oxidized under these conditions. Few studies have attempted to uncover the primary source of carbons contributing to succinate accumulation and the interpretations of these studies are polarized. We hypothesize the reversal of succinate dehydrogenase (SDH) is the primary route of succinate generation. Additionally, when isolated mitochondria respire on high concentrations of succinate, under conditions that mimic reperfusion, excess concentrations of oxaloacetate (OAA) are produced. OAA is a potent inhibitor of SDH and significantly inhibits respiration under these conditions. We hypothesize, following reperfusion, excess OAA must be cleared away before normal energy metabolism can be restored. To test these hypotheses, in vitro experiments on suspensions of purified cardiac mitochondria were used to quantify routes of succinate production during anoxia and succinate oxidation under reperfusion conditions. A computer model of mitochondrial metabolism was developed to aid our research by analyzing data, identifying novel hypotheses, and designing experiments to test identified hypotheses. The combined use of quantitative metabolic data and our mechanistic computational model helped to predict routes of succinate production during ischemia and determine the relative contributions of pathways of OAA clearance under reperfusion conditions. Results suggest during anoxia, succinate is primarily generated by the reversal of SDH. Additionally, by fitting our model to data from mitochondria under reperfusion conditions, we found respiration on succinate causes a rapid accumulation of toxic levels of OAA, which in turn inhibit SDH and impairs ATP production. Potential pathways of OAA clearance include canonical TCA cycle activity via citrate synthase, as well as via malic enzyme (ME), glutamic oxaloacetic transaminase (GOT), and enzymes with oxaloacetate decarboxylase (OD) activity. We determined GOT can readily clear OAA to restore respiration when glutamate is present while ME and OD reactions sustain relatively lower rates of OAA clearance. This research is funded through the National Institute of Health grants HL144657 and F31HL165681 (NC). This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.