Small, gaseous molecules, known as gasotransmitters (NO, CO, H2S), are produced endogenously in mammalian cells and serve important biological roles. Cyanide (CN) is 35 known as an endogenous mediator in plants and bacteria, while in mammals it has been mainly viewed as a toxin. Here we show that low concentrations of CN are endogenously generated in mouse liver and human hepatocytes from glycine and HOCl-in lysosomes. CN diffuses out of the lysosome to reach various cellular compartments and is detectable in circulating blood. Endogenous CN–via protein cysteine cyanylation and removal of glutathione from proteins 40–exerts pluripotent effects on cell metabolism and gene expression. Endogenous CN production supports cellular ATP production, in part, via increased free fatty acid oxidation. CN also supports cell proliferation and exerts cytoprotective effects. Controlled CN donation exerts cytoprotective effects in vitro and in vivo models of hypoxia and reoxygenation. In this part of the study, we used HepG2 cells samples that were either treated with Amygdalin, Linamarin, Mandelonitrile, 1 uM KCN, 10 nM KCN or left untreated. Amygdalin, Linamarin and Mandelonitrile are known to release cyanide when broken down by metabolic processes. Our objective was to identify the first the targets of cysteine cyanilation. As no enrichment method is available for this modification, we employed direct detection of cyanilated cysteines peptides in the proteome.