Ecto‐diadenosine 5',5'''‐P^{1},P^{4}‐tetraphosphate (Ap_{4}A)‐hydrolase is expressed as an ectoenzyme in a variety of mammalian and human cells and adds new aspects to the turnover of Ap_{4}A
Affiliations: The Blood Center of Southeastern Wisconsin, Blood Research Institute, 8727 Watertown Plank Rd, Milwaukee, WI 53226, USA | Institut für Biochemie der Friedrich‐Alexander‐Universität Erlangen‐Nürnberg, Medizinische Fakultät, Fahrstr. 17, 91054 Erlangen, Germany
Note: [] Correspondence to: Annette von Drygalski, MD, The Blood Center of Southeastern Wisconsin, Blood Research Institute, P.O.Box 2178, Milwaukee, WI 53226‐2178, USA. Tel.: +1 414 937 3833; Fax: +1 414 937 6284; E‐mail: avdrygalski@bcsew.edu.
Abstract: Ap_{4}A and other dinucleotides participate in the regulation of hemostasis and blood pressure control. With the exception of two previously reported surface anchored ectoAp_{4}A‐hydrolases on bovine aortic endothelial and chromaffine cells, all Ap_{4}A‐hydrolases reported are intracellular or freely soluble. We demonstrated that ectoAp_{4}A‐hydrolases are present on a broad variety of cell types of different species: rat mesangial, bovine corneal epithelial, human Hep‐G2 and peridontal cells. Ectoenzyme properties were evaluated on rat mesangium cells. Chromatography of purified plasma membranes on Sephacel 300 resulted in enrichment of ectoAp_{4}A‐hydrolase and in separation from ectoATPase. In contrast to ATPase, Ap_{4}A‐hydrolase was stable at room temperature. EctoAp_{4}A‐hydrolase also recognized ATP as substrate, and therefore is not highly specific. The molecular weight was 180 kD. Unlike ectoAMPase ectoAp_{4}A‐hydrolase was not attached via a glycosyl‐phosphatidylinositol (GPI)‐moiety. Concentrations of PI‐PLC 10–100‐fold higher than effective for ectoAMPase cleavage (10–100 mU/ml) plus extensively extended incubation times up to eight hours did not result in cleavage of ectoAp_{4}A‐hydrolase. The enzyme ectoAp_{4}A‐hydrolase might presage a direction for pharmaceutical manipulation in the control of blood pressure and hemostasis.
