Abstract Body (Do not enter title and authors here): Background: Impaired branched-chain amino acid (BCAA) catabolism contributes to the development and progression of heart failure (HF). However, the mechanisms regulating BCAA catabolism under physiological and pathological conditions remain incompletely understood. 3-Mercaptopyruvate sulfurtransferase (3-MST), a mitochondrial hydrogen sulfide (H₂S)-producing enzyme, may play a critical role in this context. We investigated the role of 3-MST-derived mitochondrial H₂S in modulating myocardial BCAA catabolism in two distinct HF models.

Methods: Global 3-MST knockout (KO) and wild-type (WT) mice were investigated. Heart failure with reduced ejection fraction (HFrEF) was induced by transverse aortic constriction, while HF with preserved ejection fraction (HFpEF) was established via L-NAME administration in conjunction with a high-fat diet. Targeted metabolomic analyses were performed to assess BCAA catabolism. Cardiac function and exercise capacity were evaluated using echocardiography, invasive hemodynamics, and treadmill testing.

Results: At baseline, 3-MST KO hearts exhibited reduced mitochondrial H₂S production accompanied by modest impairment in BCAA catabolism. Under HF conditions, BCAA catabolic defects were markedly aggravated in 3-MST KO mice, as evidenced by the accumulation of BCAA metabolic intermediates in both HFrEF and HFpEF hearts compared to WT controls. These metabolic impairments were associated with worsened HF phenotypes, including reduced left ventricular ejection fraction in HFrEF, increased E/e′ ratio in HFpEF, elevated left ventricular end-diastolic pressure, and diminished exercise performance in both HF models. Additionally, skeletal muscle from 3-MST KO mice showed downregulation of BCAA catabolic enzymes. Notably, treatment with exogenous H₂S donors restored BCAA catabolism and ameliorated cardiac dysfunction in 3-MST-deficient mice.

Conclusion: These findings identify mitochondrial H₂S produced by 3-MST as a key regulator of myocardial BCAA catabolism and HF pathophysiology. Loss of 3-MST disrupts BCAA catabolic homeostasis and exacerbates cardiac dysfunction in both HFrEF and HFpEF. Therapeutic replenishment of H₂S may represent an effective strategy to restore metabolic balance and improve outcomes in HF. Ongoing studies aim to further elucidate the molecular interactions between 3-MST and key enzymes of the BCAA catabolic pathway.