Identification and characterization of an ecto-(lyso)phosphatidic acid phosphatase in PAM212 keratinocytes.

Intact PAM212 keratinocytes were found to preferentially degrade exogenous phosphatidic acids (PA) containing short fatty acid chains. The product of this degradation was inorganic phosphate (Pi), suggesting that the enzyme was a phosphatase. Systematic studies using enzymatically synthesized PA and lysophosphatidic acid (lysoPA) demonstrated that the sn-2 fatty acid chain length was the determining factor for suitability of PA as substrate for this cell-associated enzyme. Thus 1-acyl-2-lyso-PA provided the best substrate for this enzyme while long-chain PA were poor substrates. The enzyme was effectively inhibited by NaF and Na3VO4, but was insensitive to inhibitors of alkaline phosphatase or other nonspecific phosphatases. The enzyme activity was solubilized from intact cells by proteinases, such as trypsin and papain, and the reaction product Pi was distributed exclusively in the extracellular medium, suggesting that this (lyso)PA phosphatase is an ectoenzyme. These results unequivocally demonstrated the presence of a (lyso)PA phosphatase located at the cell surface. This novel ectoenzyme may provide a mechanism for the inactivation of the potent bioactive phospholipid, lysoPA.