PAF-DEGRADING ACETYLHYDROLASE IS PREFERENTIALLY ASSOCIATED WITH DENSELDL AND VHDL-1 IN HUMAN PLASMA - CATALYTIC CHARACTERISTICS AND RELATION TO THE MONOCYTE-DERIVED ENZYME

In human plasma, platelet activating factor (PAF)-degrading acetylhydr olase (acetylhydrolase) is principally transported in association with LDLs and HDLs; this enzyme hydrolyzes PAF and short-chain forms of ox idized phosphatidylcholine, transforming them into lyso-PAF and lysoph osphatidylcholine, respectively. We have examined the distribution, ca talytic characteristics, and transfer of acetylhydrolase activity amon g plasma lipoprotein subspecies separated by isopycnic density gradien t ultracentrifugation; the possibility that the plasma enzyme may be p artially derived from adherent monocytes has also been evaluated. In n ormolipidemic subjects with Lp(a) levels <0.1 mg/mL, acetylhydrolase w as associated preferentially with small, dense LDL particles (LDL-5; d =1.050 to 1.063 g/mL) and with the very-high-density lipoprotein-1 sub fraction (VHDL-1; d=1.156 to 1.179 g/mL), representing 23.9+/-1.7% and 20.6+/-3.2%, respectively, of total plasma activity. The apparent K-m values for PAF of the enzyme associated with such lipoproteins were 8 9.7+/-23.4 and 34.8+/-4.5 mu mol/L for LDL-5 and VHDL-1, respectively: indeed, the K-m value for LDL-5 was some 10-fold higher than that of the light LDL-1, LDL-2, and LDL-3 subspecies, whereas the K-m of VHDL- 1 was some twofold greater than those of the HDL-2 and HDL-3 subspecie s. Furthermore, when expressed on the basis of unit plasma volume, the V-max of the acetylhydrolase associated with LDL-5 was some 150-fold greater than that in LDL-1 (d=1.019 to 1.023 g/mL). No significant dif ferences in the pH dependence of enzyme activity or in sensitivity to protease inactivation, sulfydryl reagents, the serine protease inhibit or Pefabloc, or the PAF antagonist CV 3988 could be. detected between apo B-containing and apo A-I-containing lipoprotein particle subspecie s. Incubation of LDL-1 (K-m=8.4+/-2.6 mu mol/L) and LDL-2 (d=1.023 to 1.029 g/mL; K-m=8.4+/-3.3 mu mol/L) subspecies with LDL-5, in which ac etylhydrolase had been inactivated by pretreatment with Pefabloc, demo nstrated preferential transfer of acetylhydrolase to LDL-5. Acetylhydr olase transferred to LDL-5 from the light LDL subspecies exhibited a K -m of 9.4+/-2.2 mu mol/L, a value characteristic of the particle donor s. Finally, acetylhydrolase (K-m=(2)3.4+/-7.6 mu mol/L) released by ad herent human monocytes in culture was found to bind preferentially to small, dense LDL subspecies upon incubation of Pefabloc-inactivated pl asma with monocyte supernatant. We conclude that a form of acetylhydro lase with distinct catalytic properties is preferentially associated w ith small, dense LDL-5 and VHDL-1 particles in human plasma, suggestin g that the surface properties of LDL-5 and VHDL-1 particles are distin ct from those of other lipoproteins containing either apo B or apo A-I . This hypothesis is consistent with the transfer of active enzyme fro m the light LDL subspecies to LDL-5, as well as with the preferential binding of monocyte-derived acetylhydrolase to LDL-5. Finally, because the penetration of arterial intima by lipoproteins is inversely propo rtional to particle size, our data suggest that both small, dense apo B-containing lipoproteins tie, LDL-5) and small apo A-I-rich lipoprote ins tie, VHDL-1) may play a key anti-inflammatory role in arterial tis sue.