['To evaluate the role of matrix metalloproteinase (MMP)-9 deletion on citrate synthase (CS) activity postmyocardial infarction (MI).', 'We fractionated left ventricle (LV) samples using a differential solubility-based approach. The insoluble protein fraction was analyzed by mass spectrometry, and we identified CS as a potential intracellular substrate of MMP-9 in the MI setting. CS protein levels increased in the insoluble fraction at day 1 post-MI in both genotypes (p<0.05) but not in the noninfarcted remote region. The CS activity decreased in the infarcted tissue of wild-type (WT) mice at day 1 post-MI (p<0.05), but this was not observed in the MMP-9 null mice, suggesting that MMP-9 deletion helps to maintain the mitochondrial activity post-MI. Additionally, inflammatory gene transcription was increased post-MI in the WT mice and attenuated in the MMP-9 null mice. MMP-9 cleaved CS in vitro, generating an ∼20\u2009kDa fragment.', 'By applying a sample fractionation and proteomics approach, we were able to identify a novel MMP-9-related altered mitochondrial metabolic activity early post-MI.'] Is citrate synthase a novel in vivo matrix metalloproteinase-9 substrate that regulates mitochondrial function in the postmyocardial infarction left ventricle?