Serpin Peptidase Inhibitor, Clade A (α-1 Antiproteinase, Antitrypsin), Member 4 (Serpin A4) is a member of the Serpin family. Serpin A4 exists as a monomer and some homodimers. Serpin A4 is expressed by the liver and secreted in plasma. Serpin A4 is a regulator of vascular homeostasis capable of controlling a wide spectrum of biological actions in the cardiovascular and renal systems. It can inhibit intracellular reactive oxygen species formation in cultured cardiac and renal cells. In addition, Serpin A4 has anti-inflammatory effect. Heparin blocks kallistatin's complex formation with tissue kallikrein and abolishes its inhibitory effect on tissue kallikrein's activity.