Original Contribution COMPARISON OF LOW-DENSITY LIPOPROTEIN MODIFICATION BY MYELOPEROXIDASE-DERIVED HYPOCHLOROUS AND HYPOBROMOUS ACIDS ANITRA C. CARR,* ERIC A. DECKER, † YOUNGJOON PARK, † and BALZ FREI* *Linus Pauling Institute, Oregon State University, Corvallis, OR, USA; and † Department of Food Science, Chenoweth Laboratory, University of Massachusetts, Amherst, MA, USA (Received 7 December 2000; Accepted 27 March 2001) Abstract—Myeloperoxidase (MPO), a heme enzyme secreted by activated phagocytes, catalyzes the oxidation of halides to hypohalous acids. At plasma concentrations of halides, hypochlorous acid (HOCl) is the major strong oxidant produced. In contrast, the related enzyme eosinophil peroxidase preferentially generates hypobromous acid (HOBr). Since reagent and MPO-derived HOCl converts low-density lipoprotein (LDL) to a potentially atherogenic form, we investigated the effects of HOBr on LDL modification. Compared to HOCl, HOBr caused 2–3-fold greater oxidation of tryptophan and cysteine residues of the protein moiety (apoB) of LDL and 4-fold greater formation of fatty acid halohydrins from the lipids in LDL. In contrast, HOBr was 2-fold less reactive than HOCl with lysine residues and caused little formation of N-bromamines. Nevertheless, HOBr caused an equivalent increase in the relative electro- phoretic mobility of LDL as HOCl, which was not reversed upon subsequent incubation with ascorbate, in contrast to the shift in mobility caused by HOCl. Similar apoB modifications were observed with HOBr generated by MPO/H 2 O 2 / Br - . In the presence of equivalent concentrations of Cl - and Br - , modifications of LDL by MPO resembled those seen in the presence of Br - alone. Interestingly, even at physiological concentrations of the two halides (100 mM Cl - , 100 M Br - ), MPO utilized a portion of the Br - to oxidize apoB cysteine residues. MPO also utilized the pseudohalide thiocyanate to oxidize apoB cysteine residues. Our data show that even though HOBr has different reactivities than HOCl with apoB, it is able to alter the charge of LDL, converting it into a potentially atherogenic particle. © 2001 Elsevier Science Inc. Keywords—Ascorbate, Hypobromous acid, Hypochlorous acid, Low-density lipoprotein, Myeloperoxidase, Thiocya- nate, Free radicals INTRODUCTION Development of atherosclerotic lesions is a complicated, multi-step process [1]. Although the initiating steps in- volved are not fully understood, oxidative modification of low-density lipoprotein (LDL) is thought to be an early event in lesion development [2]. LDL trapped within the subendothelial space can be oxidized by leu- kocytes and vascular cells, including neutrophils, mono- cytes, macrophages, endothelial cells, and smooth mus- cle cells [2]. Oxidatively modified LDL has a number of potentially atherogenic effects on these cells, including enhanced recruitment and retention of leukocytes [2], enhanced uptake of the modified LDL by macrophages, resulting in the formation of lipid-laden “foam” cells [2], and cytotoxic and proliferative effects towards endothe- lial and smooth muscle cells, respectively [3]. All of these effects can potentially contribute to the develop- ment and progression of atherosclerotic lesions. Leukocytes and vascular cells contain a number of enzymes capable of generating reactive species that can oxidatively modify LDL [4]. These enzymes include NAD(P)H oxidases, which generate superoxide and, via spontaneous dismutation, hydrogen peroxide [5] and ni- tric oxide synthase isozymes, which generate nitric oxide that can react with superoxide to form peroxynitrite [6]. In addition, leukocytes release heme peroxidases such as myeloperoxidase (MPO) from neutrophils and mono- Address correspondence to: Anitra Carr, Ph.D., Linus Pauling Insti- tute, 571 Weniger Hall, Oregon State University, Corvallis, OR 97331- 6512, USA; Tel: (541) 737-5085; Fax: (541) 737-5077; E-Mail: anitra.carr@orst.edu. Free Radical Biology & Medicine, Vol. 31, No. 1, pp. 62–72, 2001 Copyright © 2001 Elsevier Science Inc. Printed in the USA. All rights reserved 0891-5849/01/$–see front matter PII S0891-5849(01)00552-4 62