Human serum paraoxonase/arylesterase's retained hydrophobic N-terminal leader sequence associates with HDLs by binding phospholipids - ApolipoproteinA-I stabilizes activity

In serum, human paraoxonase/arylesterase (PON1) is found exclusively associ ated with high density lipoprotein (HDL) and contributes to its antiatherog enic properties by inhibiting low density lipoprotein (LDL) oxidation. Diff iculties in purifying PON1 from apolipoprotein A-I (apoA-I) suggested that PON1's association with HDL may occur through a direct binding between thes e 2 proteins. An unusual property of PON1 is that the mature protein retain s its hydrophobic N-terminal signal sequence. By expressing in vitro a muta nt PON1 with a cleavable N-terminus, we demonstrate that PON1 associates wi th lipoproteins through its N-terminus by binding phospholipids directly ra ther than binding apoA-I. Nonetheless, apoA-I stabilized arylesterase activ ity more than did phospholipid alone, apoA-II, or apoE. Consequently, we st udied the role of apoA-I in PON1 expression and HDL association in mice gen etically deficient in apoA-I. Though present in HDL fractions at decreased levels, PON1 arylesterase activity was less stable than in control mice. Fu rthermore, PON1 could be competitively removed from HDL by phospholipids, s uggesting that PON1's retained N-terminal peptide allows transfer of the en zyme between phospholipid surfaces, Thus, our data suggest that PON1 is sta bilized by apoA-I, and its binding to HDL and physiological distribution ar e dependent on the direct binding of the retained hydrophobic N-terminus to phospholipids optimally presented in association with apoA-I.