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J. Biol. Chem., Vol. 278, Issue 52, 52071-52074, December 26, 2003
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**
From the
Genome Research Center for Birth Defects and Genetic Diseases,
Asan Institute for Life Sciences, University of Ulsan College of Medicine, Asan Medical Center, Seoul 138-736, Korea, ¶Cardiovascular Research Center, John A. Burns School of Medicine, University of Hawaii, Honolulu, Hawaii 96822, and ||School of Life Sciences and Biotechnology, Korea University, Seoul 136-701, Korea
Received for publication, August 11, 2003 , and in revised form, October 6, 2003.
| ABSTRACT |
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-aminopropionitrile-inhibitable activity of 0.022-0.032 units/mg toward a nonpeptidyl substrate, benzylamine. These results indicate that LOXL4, with the four scavenger receptor cysteine rich domains, may also function as an active amine oxidase. Availability of the pure and active forms of LOXL4 will be significantly helpful in functional studies related to substrate specificity and crystal structure of this amine oxidase, which should provide significant insights into functional differences within the LOX family members. | INTRODUCTION |
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In recent years, four novel LOX-like proteins, LOXL, LOXL2, LOXL3, and LOXL4, have been identified, each containing the characteristic domains of LOX in the carboxyl (C) terminus, such as the copper-binding domain, the lysyl-tyrosyl-quinone residues and the cytokine receptor-like domain (10-14). The presence of a LOX family suggests that the multiple diverse functions currently attributed to a single enzyme, LOX, may be because of different LOX-like proteins. In addition to the conserved C-terminal domains, LOXL2, LOXL3 and LOXL4, unlike LOX and LOXL, contain four scavenger receptor cysteine rich (SRCR) domains in the amino (N) terminus, thus forming a subfamily within the LOX family. Based on the conservation of the C-terminal LOX domains, these SRCR domain-containing members were predicted to contain the amine oxidase function, but it has yet to be determined. SRCR domains are known to mediate ligand binding in a number of secreted and receptor proteins (15, 16). The presence of SRCR domains within LOXL2, LOXL3, and LOXL4 indicates that these three proteins may play novel functions, possibly involving components of the extracellular matrix.
The murine homologue of LOXL4, reported as LOXC, was initially identified as a transcript specifically expressed in the cartilage (17). However, Northern analyses of the human LOXL4 gene showed more ubiquitous expression in many tissues, but it was different from the other members of the LOX family (10, 14), suggesting distinct tissue and substrate specificity of this protein. In this study, we report for the first time that LOXL4 functions as an active amine oxidase. The LOXL4 protein was expressed as His-tagged recombinant forms in inclusion bodies by using an Escherichia coli expression vector, extracted with 8 M urea, and renatured into enzymatically active forms toward a nonpeptidyl substrate, benzylamine. The amine oxidase activity of LOXL4 was sensitive to
-aminopropionitrile (BAPN), a well known specific inhibitor of LOX.
| EXPERIMENTAL PROCEDURES |
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-D-galactopyranoside (IPTG) was added. After 4 h of induction, the transformants were harvested by centrifugation at 7,000 x g for 20 min and resuspended in 60 ml of lysis buffer (50 mM Tris, pH 8.0, 1 mM EDTA, 100 mM NaCl, 1 mM PMSF, and 1 mg/ml of lysozyme). Triton X-100 (1%, w/v) and DNase (0.1 mg/ml) were added sequentially to the lysates at intervals of 30 min during inverted mixing, and then sonication was repeated twice at 70% efficiency with a sonicator (Sonics & Materials, Inc). The inclusion bodies were precipitated by centrifugation at 8,000 x g for 20 min and homogenized in 60 ml of 8 M urea, 10 mM K2HPO4, pH 8.2, and 3 mM
-mercaptoethanol. The solubilized inclusion bodies were purified with the nickel-nitrilotriacetic acid agarose resins (Qiagen) according to the manufacturer's recommendations. All of the purification procedures were carried out at 4 °C. The purity and sizes of the recombinant proteins were assessed by SDS-PAGE. N-terminal Amino Acid Sequence AnalysisTo confirm the N-terminal sequences of the recombinant LOXL4 proteins, the purified proteins were subjected to N-terminal analysis by Edman degradation. After purification by the nickel-chelating affinity chromatography, 5 µg each of LOXL4-p2, LOXL4-p3, LOXL4-p4, and LOXL4-p5 were subjected to SDS-PAGE, transferred to polyvinylidene difluoride membranes (Hybond-P, Amersham Biosciences), and stained with Ponceau S. The visualized protein bands were excised and loaded into a CLC Capillary 492 sequencer (Applied Biosystems). The 10 N-terminal residues of each purified recombinant protein were analyzed by the automated N-terminal sequencing, and all of the recombinant LOXL4 proteins contained the expected sequences in the N terminus.
Refolding of the LOXL4 ProteinsFor stepwise dialysis, the purified LOXL4 proteins were diluted to 100 µg/ml in elution buffer (6 mM urea, 250 mM imidazol, and 10 mM K2HPO4, pH 8.2). The protein samples were dialyzed overnight first against a buffer of 10 mM K2HPO4, pH 9.6, 200 µM CuCl2, and 2% sodium N-lauroylsarcosinate and then against a buffer of 10 mM K2HPO4, pH 9.6, and 5 µM CuCl2. The proteins were further dialyzed twice against the phosphate buffer. The concentration of dialyzed protein samples was determined by the BCA method (18). After dialysis, the protein solutions were aliquoted and lyophilized in the presence of 10 mM trehalose using a Freeze dryer (Labconco).
Amine Oxidase AssaysAmine oxidase assays were performed with a UV spectrophotometer (Kontron Instruments). The assay reactions included 10 mM benzylamine and 100 µg of a recombinant LOXL4 protein in 1 ml of the phosphate buffer. The reaction mixtures were incubated at 37 °C with simultaneous detection at 250 nm. For the assays with BAPN, the LOXL4 protein was preincubated with 100 molar excess of BAPN in the phosphate buffer at 37 °C for 1 h, and then 10 mM benzylamine was added to the reaction mixture for amine oxidase assays.
| RESULTS AND DISCUSSION |
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Upon induction by 1 mM IPTG at 37 °C, the recombinant LOXL4 proteins, except for LOXL4-p1, were expressed at high levels from the expression constructs (Fig. 1B). Fractionation of the cell lysates into different cellular compartments, such as cytoplasmic extracts, periplasmic extracts, and inclusion body fractions, revealed that most of the recombinant proteins were expressed in insoluble forms within inclusion bodies (data not shown). It was not clear yet why the LOXL4-p1 construct did not show any detectable expression, but it might be because of the presence of rare codons, such as CTA (Leu16), CCC (Pro19 and Pro21), and AGG (Arg23), in the N terminus of LOXL4, which are infrequently used in E. coli. Clusters of rare codons were shown to cause translation errors and reduction of the expression level in E. coli (21).
The apparent sizes of the expressed recombinant LOXL4 proteins were in good agreement with the deduced molecular masses; 62 kDa for LOXL4-p2, 50 kDa for LOXL4-p3, 38 kDa for LOXL4-p4, and 22 kDa for LOXL-p5 (Fig. 1, B and C). The hexa-histidine-tagged recombinant LOXL4-proteins were over 95% pure on SDS-PAGE gels (Fig. 1C). Automated N-terminal sequencing confirmed the presence of the designed N-terminal sequence in each LOXL4 recombinant protein, indicating that each protein band in the SDS-PAGE gels represents the desired length of the corresponding recombinant LOXL4 protein. The LOXL4-p2, LOXL4-p3, LOXL4-p4, and LOXL4-p5 proteins contained the open-reading frame of LOXL4 starting from Val159, Val311, Val421, and Asp570, respectively, with the additional Met-Ala-Ser tag at the N terminus, as expected.
Amine Oxidase Activity of the Purified LOXL4 ProteinsIn an attempt to refold the proteins denatured by urea during purification, we tried stepwise dialysis in the presence of N-lauroylsarcosinate and Cu2+, an oxidative catalyzer. In the first dialysis buffer, 2% (w/v) sodium N-lauroylsarcosinate and 200 µM CuCl2 were added. The second dialysis was carried out without any detergent but in the presence of 5 µM CuCl2, and the protein samples were further dialyzed twice in the potassium phosphate buffer.
The refolded recombinant proteins were assessed for amine oxidase activity toward a nonpeptidyl substrate, benzylamine. One unit of amine oxidase activity was defined as the activity resulting in oxidation of 1 µmol of benzylamine per min at 37 °C. The conversion of benzylamine substrate to benzaldehyde was monitored at 250 nm using spectrophotometry. The specific activity of LOXL4-p2, LOXL4-p3, LOXL4-p4, and LOXL4-p5 was 0.022 units/mg, 0.025 units/mg, 0.027 units/mg, and 0.032 units/mg, respectively (Table II). All of these activities were inhibited to a background level by 100 molar excess of BAPN, an irreversible inhibitor of LOX (Table II).
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0.04 units/mg, performed with the LOX protein purified from the bovine aorta (22, 23). The differences in specific activity within the tested recombinant LOXL4 proteins were primarily due to the differences in molar concentration of the recombinant proteins in the assays. These assay results indicate that all of the rested recombinant proteins, including the potentially bone morphogenic protein-1 processed form of LOXL4, function as an active amine oxidase toward benzylamine and, moreover, that the conserved C-terminal domains of LOXL4 are sufficient to confer the amine oxidase function to this protein. The SRCR Domains in LOXL4 In our assays, the recombinant LOXL4 proteins containing one or more SRCR domains (LOXL4-p2, LOXL4-p3, and LOXL4-p4) showed BAPN-specific amine oxidase activity toward benzylamine (Table II). Although we have not yet tested the catalytic activity of these SRCR domain-containing recombinant proteins toward more physiological substrates, such as collagen and elastin, our results indicate that the presence of the SRCR domains does not interfere with the amine oxidase activity of LOXL4 at least in in vitro assays. The SRCR superfamily proteins are found either on the cell surface membranes or as secreted proteins. The SRCR domains within these proteins are known to be involved in protein-protein interactions for cell adhesion or cell signaling (15, 16, 24). The presence of SRCR domains within LOX4, therefore, may indicate possible existence of modulators that may affect the amine oxidase activity of LOXL4 through interaction with the SRCR domains. Identification of the interactive proteins of LOXL4 will be critical in further characterization of the functional significance of the SRCR domains in LOXL4.
In summary, the purpose of this study was to assess the amine oxidase activity of LOXL4. With purified recombinant forms of LOXL4, we have shown that LOXL4 functions as an amine oxidase. Furthermore, by using a series of deletion constructs, we confirmed that neither the presence nor the deletion of SRCR domains affects the amine oxidase activity of LOXL4. The availability of these enzymatically active forms of LOXL4 will pave the way for further functional characterization of this latest member of amine oxidases, which should be critical in understanding the mechanistic bases for the diverse functions played by the LOX family members.
| FOOTNOTES |
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** To whom correspondence should be addressed. Tel.: 82-2-3010-5914; Fax: 82-2-486-3312; E-mail: ykim{at}amc.seoul.kr.
1 The abbreviations used are: LOX, lysyl oxidase; SRCR, scavenger receptor cysteine rich; BAPN,
-aminopropionitrile; IPTG, isopropyl-1-thio-
-D-galactopyranoside; PMSF, phenylmethyl sulfonyl fluoride. ![]()
| REFERENCES |
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