Exclusive Association of Paraoxonase 1 with High-Density Lipoprotein Particles in Apolipoprotein A-I Deficiency Hiroshi Noto,* Yoshiaki Hashimoto,† Hiroaki Satoh,* Masumi Hara,* Naoyuki Iso-o,* Masako Togo,* Satoshi Kimura,* and Kazuhisa Tsukamoto* ,1 *Department of Metabolic Diseases and †Department of Clinical Laboratory Medicine, Graduate School of Medicine, University of Tokyo, Tokyo, Japan Received October 19, 2001 Paraoxonase1 (PON1) is a high-density lipoprotein (HDL)-associated protein which removes peroxidized lipids from lipoproteins. It has been proposed that apolipoprotein A-I (apoA-I) is an important determi- nant for its stabilization on HDL. However, little is known about its existence and activity in an apoA-I- deficient state in humans. To characterize the nature of PON1 in apoA-I deficiency, we investigated PON1 in an apoA-I-deficient patient. When serum was analyzed on fast protein liquid chromatography, PON1 protein was distributed almost exclusively on HDL despite the absence of apoA-I; on the other hand, 38.5% of PON1 protein was found in the lipoprotein-free fraction when the lipoproteins were fractionated through ul- tracentrifugation. The stability of PON1 activity in the patient serum was almost the same as in the normal control sera throughout incubation at 14°C for 7 days. However, when the sera were incubated at 37°C for 24 h, its activity declined more than those in the nor- mal controls (19% versus 4% reduction of the initial values). Our results demonstrated that PON1 protein possesses a preferential association with HDL even in the absence of apoA-I, although apoA-I is a crucial factor for the maximal activity and stabilization of PON1. © 2001 Elsevier Science Key Words: paraoxonase; apolipoprotein A-I; high- density lipoprotein. Coronary heart disease (CHD) is the leading cause of death in Western countries. Among many independent risk factors for CHD, several epidemiological studies have identified a low plasma high-density lipoprotein (HDL) cholesterol level is the strongest one (1– 4). The reverse cholesterol transport, the mechanism that recruits excess cholesterol from the peripheral tissues and transports it to the liver, is an established antiatherogenic property of HDL. Another potential antiatherogenic property of HDL is its capacity to pro- tect low-density lipoprotein (LDL) from lipid peroxida- tion, which is the crucial first step for atherogenesis in the arterial wall (5). Paraoxonase1 (PON1) is one of the enzymes which have been proposed to exert an antiatherogenic effect. PON1 is an enzyme that was originally found to cata- lyze paraoxon and other poisons (6). However, recent studies have clarified it hydrolyses phospholipid and cholesteryl ester hydroperoxides derived from arachi- donic and linoleic acid at the sn-2 position thereby protecting LDL from oxidation (7). PON1 has two genomic polymorphisms with amino acid substitutions at positions 55 and 192. The position-192 polymorphism (Q vs R alleles) modulates the PON1 activity and determines the capacity of HDL to protect LDL against oxidative modification in vitro (8). Although there have been some epidemiological studies showing the relation between the PON1 alleles and CHD (6), its clinical relevance is still controversial. In an animal model, however, the relationship between PON activity and atherosclerosis has recently been demonstrated in PON knockout mice (9). PON1 protein is exclusively localized on the surface of HDL particles in normal human and mouse plasma, and apolipoprotein A-I (apoA-I) has been proposed to be an important factor for its preferential localization on HDL (10). Furthermore, Sorenson et al. showed that PON1 protein exists on HDL even in a complete ab- sence of apoA-I in mice, and its activity is less stable in such a condition (11). However, James et al. reported Abbreviations used: PON1, paraoxonase 1; HDL, high-density li- poprotein; apo, apolipoprotein; CHD, coronary heart disease; LDL, low-density lipoprotein; FPLC, fast protein liquid chromatography. 1 To whom correspondence and reprint requests should be ad- dressed at Department of Metabolic Diseases, Graduate School of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. Fax: +81-3-5800-8806. E-mail: kazuhisa-tky@ umin.ac.jp. Biochemical and Biophysical Research Communications 289, 395– 401 (2001) doi:10.1006/bbrc.2001.5985, available online at http://www.idealibrary.com on 395 0006-291X/01 $35.00 © 2001 Elsevier Science All rights reserved.