Interaction of lecithin : cholesterol acyltransferase (LCAT)center dot alpha(2)-macroglobulin complex with low density lipoprotein receptor-related protein (LRP) - Evidence for an alpha(2)-macroglobulin/LRP receptor-mediatedsystem participating in LCAT clearance

The reaction of lecithin:cholesterol acyltransferase (LCAT) with high densi ty lipoproteins (RDL) is of critical importance in reverse cholesterol tran sport, but the structural and functional pathways involved in the regulatio n of LCAT have not been established. We present evidence for the direct bin ding of LCAT to alpha (2)-macroglobulin (alpha M-2) in human plasma to form a complex 18.5 nm in diameter. Forty percent of plasma LCAT-HDL was associ ated with a2M; moreover, most of the LCAT in cerebrospinal fluid and in the medium of cultured human hepatoma cell line was associated with alpha M-2. Purified recombinant human LCAT (rLCAT) labeled with I-125 bound to native and methylamine-activated alpha M-2 (alpha M-2-MA) in vitro in a time- and concentration-dependent manner, and this binding did not depend on the pre sence of lipid. rLCAT bound to alpha 2M-MA with greater affinity than to al pha 2M. Furthermore, rLCAT did not activate alpha 2M as phosphatidylcholine -specific phospholipase C does. Reconstituted HDL particles (LpA-I) inhibit ed the binding of rLCAT to alpha M-2 more efficiently than native HDL3 did. LCAT associated with alpha M-2 was enzymatically inactive under both endog enous and exogenous assay conditions. Purified rLCAT alone did not bind to low density lipoprotein receptor-related protein (LRP) as lipoprotein lipas e (LPL) does; however, when rLCAT was combined with alpha 2M-MA to form a c omplex, binding, internalization, and degradation of rLCAT took place in LR Pexpressing cells (LRP (+/+)) but not in cells deficient in LRP (LRP (-/-)) . It is concluded that the binding of LCAT to alpha 2M inhibits its enzymat ic activity. Furthermore, the finding supports the possibility that the LRP receptor can act in vivo to mediate clearance of the LCAT-alpha M-2 comple x and may significantly influence the bioavailability of LCAT.