This study examines hepatic lipase (HL) mediated phospholipid hydrolysis in
mixtures of apolipoprotein-specific, spherical reconstituted high-density
lipoproteins (rHDL). We have shown previously that apolipoprotein A-I (apoA
-I) and apoA-II have a major influence on the kinetics of HL-mediated phosp
holipid and triacylglycerol hydrolysis in well-characterized, homogeneous p
reparations of spherical rHDL [Hime, N. J., Barter, P. J., and Rye, K.-A. (
1998) J. Biol. Chem. 273, 27191-27198]. In the present study, phospholipid
hydrolysis was assessed in mixtures of rHDL containing either apoA-I only,
(A-I)rHDL, apoA-II only, (A-II)rHDL, or both apoA-I and apoA-II, (A-YA-II)r
HDL. The rHDL contained trace amounts of radiolabeled phospholipid, and hyd
rolysis was measured as the formation of radiolabeled nonesterified fatty a
cids (NEFA). As predicted from our previous kinetic studies, the (A-II)rHDL
acted as competitive inhibitors of HL-mediated phospholipid hydrolysis in
(A-I)rHDL. Less expected was the observation that the rate of phospholipid
hydrolysis in (A II)rHDL was enhanced when (A-I)rHDL were also present in t
he incubation mixture. The rate of phospholipid hydrolysis in (A-YA-II)rHDL
was also greater than in (A-II)rHDL, indicating that apoA I enhances phosp
holipid hydrolysis when it is present as a component of (A-I/A-II)rHDL. It
is concluded that apoA I enhances HL-mediated phospholipid hydrolysis in ap
oA-II containing rHDL, irrespective of whether the apoA-I is present in the
same particle as the apoA-II [as in (A-I/A-II)rHDL] or whether it is prese
nt as a component of a different particle, such as when (A-I)rHDL are added
to incubations of (A-II)rHDL.