**4. Chymase**

Chymases [EC 3. 4.21.39] comprise a family of serine peptidases that may generate Ang II by hydrolysis of the Phe8 -His9 bond of Ang I and other peptide precursors [Ang-(1-12), Ang- (1-25)] (α-chymases) or metabolize Ang II at Tyr4 -Ile5 to form Ang-(1-4) and Ang-(5-8) (βchymases) [34–39]. Humans express α-chymase while rodents express primarily β-chymases, as well as other isoforms (mouse MCP-4 and rat MCP-5) that more closely resemble α-chymase in regard to the processing of Ang I to Ang II [35]. The human and mouse enzymes may also play a role in the conversion of the endothelin precursor to the active peptide, as well as the activation of various inflammatory cytokines [40]. Chymases (35 kDa) are synthesized and stored in an inactive proform within mast cells and neutrophils that are released with other proteases (cathepsin G, tryptases, and renin) upon degranulation following injury or inflammation [41]. Although chymases are soluble enzymes, they associate with the cell membrane and may locate to the extracellular surface of tissues following release. The serine protease inhibitor chymostatin is typically used to demonstrate specificity; however, chymostatin inhibits other Ang II-generating enzymes (cathepsin G and elastase-2). Thus, chymostatin sensitivity for Ang II generation does not necessarily demonstrate the involvement of chymase and more selective approaches should be considered [42–45]. The extent that chymase or other peptidases participate in the formation of circulating or tissue Ang II through an ACEindependent pathway remains equivocal [46].
