CDNA CLONING AND EXPRESSION OF INTRACELLULAR PLATELET-ACTIVATING-FACTOR (PAF) ACETYLHYDROLASE-II - ITS HOMOLOGY WITH PLASMA PAF ACETYLHYDROLASE

Platelet-activating factor (PAF) acetylhydrolase, which inactivates PA F by removing the acetyl group at the sn-2 position, is widely distrib uted in plasma and tissues, We previously demonstrated that tissue cyt osol contains at least two types of PAF acetylhydrolase, isoforms Ib a nd II, and that isoform Ib is a heterotrimer comprising 45-, 30-, and 29-kDa subunits, whereas isoform II is a 40-kDa monomer. In this study , we isolated cDNA clones of bovine and human PAF acetylhydrolase isof orm II, From the longest open reading frame of the cloned cDNAs, both bovine and human PAF acetylhydrolases II are predicted to contain 392 amino acid residues and to exhibit 88% identity with each other at the amino acid level, Both enzymes contain a Gly-X-Ser-X-Gly motif that i s characteristic of lipases and serine esterases, Expression of isofor m II cDNA in COS7 cells resulted in a marked increase in PAF acetylhyd rolase activity, An immunoblot study using an established monoclonal a ntibody against the bovine enzyme revealed that the recombinant protei n exists in the membranous fraction as well as the soluble fraction, I soform II is expressed most abundantly in the liver and kidney in catt le, but low levels were also observed in other tissues, The amino acid sequence deduced from the cDNA of isoform II had no homology with any subunit of isoform Ib, Interestingly, however, the amino acid sequenc e of isoform II showed 41% identity with that of plasma PAF acetylhydr olase, Combined with previous data demonstrating that isoform II shows similar substrate specificity to plasma PAF acetylhydrolase, these re sults indicate that tissue type isoform II and the plasma enzyme may s hare a common physiologic function.