Expression, purification, and kinetic characterization of a recombinant 80-kDa intracellular calcium-independent phospholipase A₂

A CHO cell-derived 80-kDa recombinant polypeptide (GenBank number I15470) putatively encoding a calcium-independent phospholipase A₂ was expressed in S. frugiperda cells resulting in over a 15-fold increase in a calcium-independent phospholipase A₁/A₂ activity which was entirely inhibitable by (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran- 2-one. The recombinant polypeptide was purified from cytosol by sequential tandem affinity chromatographies employing ATP-agarose and calmodulin- Sepharose stationary phases. This strategy resulted in the rapid purification (36 h) of recombinant phospholipase A₂ activity in 56% overall yield to a single intense 80-kDa protein band on SDS-polyacrylamide gel electrophoresis after silver staining. The purified protein possessed phospholipase A₁, phospholipase A₂, and lysophospholipase activities. Microbore anion exchange chromatography demonstrated that the 80-kDa protein band was comprised of multiple distinct isoforms including an anionic isoform which possessed over a 5-fold higher specific activity (5 μmol/mg · min) than earlier eluting isoforms. Collectively, these results unambiguously demonstrate that: 1) the 80-kDa polypeptide catalyzes phospholipase A₁/A₂ and lysophospholipase activities with distinct kinetic parameters; 2) calmodulin and ATP both interact with the catalytic polypeptide independent of regulatory proteins; and 3) distinct isoforms of this polypeptide exist which possess markedly different specific activities.