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Nitroglycerin is a potent vasodilator in clinical use since the late 1800s. It functions as a prodrug that is bioactivated by formation of an enzyme-based thionitrate, E-Cys-NO₂. This intermediate reportedly decomposes to release NO and NO₂^- but their relative yields remain controversial. Hence, we determined barriers for NO and NO₂^- production from the model thionitrate, CH₃SNO₂, using comprehensive high-level quantum chemistry calculations [CCSD(T)//MP2/aug-cc-pVTZ]. We find that the sulfenyl nitrite, CH₃SONO, readily releases NO on (S)O-N bond homolysis but CH₃SONO formation from CH₃SNO₂ either by S-NO₂ bond homolysis or concerted rearrangement faces prohibitively high barriers (?H_calc/?H^‡_calc > 42 kcal/mol). Dramatically lower barriers (?H^‡_calc ~ 17-21 kcal/mol) control NO₂^- release from CH₃SNO₂ by gas-phase hydrolysis or nucleophilic attack by OH^- or CH₃S^- on the sulfur atom within the C-S-NO₂ molecular plane. Moreover, attack by either anion along the S-NO₂ bond results in barrierless NO₂^- release (?H^‡_calc ~ 0 kcal/mol) since a s-hole (i.e., area of positive electrostatic potential) extends from this bond. Consistent with our high-level calculations, ALDH2 and GAPDH, enzymes implicated in nitroglycerin bioactivation via an E-Cys-NO₂ intermediate, catalyze mainly or exclusively NO₂^- release from the prodrug.