METHOD FOR THE DETERMINATION OF LEUCINE AMINOPEPTIDASE (LAP)

Abstract: The present invention provides a method for the deter­ mination of leucine-aminopeptidase (hereinafter referred to as LAP) activity or Y-glutamyltranspeptidase (hereinafter referred to as γ-GTP) activity in a sample which comprises converting the substrate for the enzyme to be determined represented by the general formula (I): wherein Z represents (CH₃)₂CHCH₂CH(NH₂)- or (NH₂) (COOH) CHCH₂CH₂-, R₁ represents hydrogen, alkyl or substituted alkyl, R₂ represents alkylene or hydroxyalkylene, R₃ and R₄ are the same or different and represents hydrogen, haloge­ no, nitro, hydroxyl, sulfo, carboxyl, alkyl, and alkoxy, and salts thereof into the compound represented by the general formula (II): wherein R₁, R₂, R₃ and R₄ have the same meanings as defined above [hereinafter referred to as compound (II)] by the action of LAP or γ-GTP, (a) reacting the compound (II) with a chromogen to form a pigment or (b) converting the compound (II) into diazonium salt and reacting the diazo­ nium salt with coupling agent to form azo dye, and measuring the absorption of the colored reaction solution.

Description: The present invention relates to a method for the determination of the activity of the enzyme leucine aminopeptidase (hereinafter referred to as LAP) and a composition for use therein. In the determination of LAP activity it is known to react the enzyme with a suitable substrate obtained by binding leucine to a chromogen, to form a colored compound which is then determined colorimetrically. The chromogens proposed for this purpose include p-nitroanilide, β-naphtylamide, 4-hydroxy-3-carboxyanilide, p-hydroxyanilide, 4-N,N-dialkylamino-3-carboxyanilide and 4-N,N-dialkylaminoanilide. However, the solubility of these compounds is, at most, 6.5 mg/ml, and it is this solubility that controls and limits the rate of reaction. This method of determination of LAP activity is therefore slow since a long period of time is required to complete the enzymatic reaction. Other disadvantages are that the results are influenced by other components in the sample, the reaction has to be carried out under strongly alkaline conditions and the sensitivity is low because of the low molecular extinction coefficient of the pigments formed. Thus altogether the accuracy of this method of determining LAP activity is low. In GB-A-2 022 079 there are described novel L-leucyl-4-hydroxyanilide derivatives for use in the determination of LAP activity and which are alleged to be of high solubility and high reactivity and which are alleged to overcome the problems the then known chromogens for the determination of LAP activity, namely, safety, reproducibility, stability, reactivity and selectivity. The novel L-leucyl-4-hydroxyanilide derivatives described in GB-A-2 022 079 are of the formula where R is carboxy or a sulfo-group. Of the two compounds described (R equals -COOH or -SO₃H) the highest substrate reactivity (263 units) is ascribed to the compound L-leucyl-3-sulfo-4-hydroxyanilide. In accordance with the present invention, it has been found that good results are obtained using L-leucyl anilides of aniline derivatives having a sulfoalkyl-substituted amino group in the 4-position. Such aniline derivatives are known per se and have been proposed as diffusion-fast, colour-forming developing agents in photographic processing, see GB-A-1309 133. However, the L-leucyl anilides of those derivatives are not known, and it is those L-leucyl anilide derivatives that form the subject matter of the present invention both as novel compounds per se and as chromogenic substrates in the determination of LAP activity, and for which purpose they show excellent solubility and provide a pigment of high stability and with a high molecular extinction coefficient so that it is possible to carry out the colorimetric determination of the pigment in the visible region of the spectrum. In accordance with one aspect therefore of the invention LAP activity is determined by reacting the enzyme with a substrate of the formula (I) wherein Z represents (CH₃)₂CHCH₂CH(NH₂)-, R₁ represents hydrogen, alkyl or substituted alkyl, R₂ represents alkylene or hydroxyalkylene, R₃ and R₄ are the same or different and represent hydrogen, halogen, nitro, hydroxyl, sulfo, carboxyl, alkyl or alkoxy, or a salt thereof, thereby to produce a compound of the formula (II) wherein R₁, R₂, R₃, and R₄ are as defined for formula I, and either reacting compound (II) with a chromogen to form a pigment, or converting compound (II) into diazonium salt and reacting the diazonium salt with coupling agent to form azo dye, and measuring the absorbency of the colored solution comprising said pigment or said azo dye. In accordance with other aspect, the invention provides novel chromogenic substrates for the determination of LAP activity being the compounds of formula I above, and reagent solutions for the determination of LAP activity comprising such a substrate in combination with either a chromogen reactive with the product of the reaction of that substrate with LAP to produce a pigment, or with a diazotising reagent which is reactive therewith and then with a coupling agent to produce an azo dye. In the definition of R₁ - R₄, alkyl includes alkyl having 1 - 5 carbon atoms such as methyl, ethyl, propyl, butyl and pentyl, halogen includes chlorine, bromine or iodine, the alkyl moiety in alkoxy has the same meaning as defined above for alkyl, alkylene includes alkylene having 1 - 5 carbon atoms such as methylene, ethylene, tetramethylene and pentamethylene, the alkylene moiety in hydroxyalkylene having the same meaning, and the possible substituents in substituted alkyl include sulfo, hydroxyaryl, nitro, carboxyl, and halogen such as chlorine or bromine. The compounds (I) can be synthesized by the following method. Where Z' is a group as defined for Z but in which the amino group is protected by an amino protecting group. Examples of suitable protecting groups include benzyloxycarbonyl and tertbutoxycarbonyl (BOC). An example of compound (III) is N-BOC-L-leucine. As compound (IV), phenylenediamine or phenylenediamines having substituents corresponding to R₃ or R₄ of the desired compound may be used. A suitable condensing agent is, for example, DCC. The first reaction is carried out at room temperature to 60⁰C for 1 to 10 hours in an organic solvent. To the reaction mixture as such, or compound (V) isolated therefrom, is added the Compound (VI). The second reaction is then carried out at room temperature to 60⁰C for 15-30 hours. Suitable examples of Compound (VI) are shown in reference Example 2 described later. The desired compound, Compound I , is finally obtained by isolating compound (VII) after completion of the second reaction and removing the amino protecting group in a manner known per se. Examples of suitable chromogens used to form the pigment by reaction with the compound (II) formed by the enzymatic reaction include amines such as N,N-diethyl-m-toluidine, N-ethyl-N-hydroxyethyl-m-toluidine, N-ethyl-N-3-methylphenyl-N'-acetylethylenediamine (EMAE), N-ethyl-N-(3-methylphenyl)-N'-succinylethylenediamine (EMSE), N,N-di-3-sulfopropyl-m-toluidine, N-ethyl-N-2-hydroxy-3-sulfopropyl-m-toluidine (TOOS), N,N-dimethyl-m-toluidine, N-ethyl-N-2-cyanoethyl-m-toluidine, N-(2-hydroxy-3-sulphopropyl)-m-toluidine, aniline derivatives, such as N-ethyl-N-(3-sulfopropyl)-3,5-dimethoxyaniline, N-ethyl-N-(2-hydroxy-3-sulfopropyl)-3,5-dimethoxyaniline, N-ethyl-N-(3-sulfopropyl)-3-methoxyaniline, N-ethyl-N-2-cyanoethyl-aniline, N,N-dimethylaniline and N,N-diethyl-aniline, phenols such as α-naphthol, β-naphthol, o- or p-bromophenol, o- or m-anisole, 2,6-xylenol, 3,5-xylenol, 2,5-xylenol, 2,3-xylenol and o- or m-cresol. These chromogens are used in a concentration of 0.01 - 5 mg/ml with a surfactant such as Triton X-100, Brij-35 and Brij-80 in a concentration of 0.01 - 2% especially in the case of chromogens having low solubility. Cinnamaldehydes such as p-N,N-dimethylamino cinnamaldehyde, p-N,N-diethylamino cinnamaldehyde may also be used as chromogens at a concentration of 0.01 - 100 mg/ml. In this case an azo dye is formed as the pigment. In the alternative method of converting a compound (II) into diazonium salt, sodium, potassium or ammonium nitrite may be used at a concentration of 0.1 - 100 mg/ml, and as the final coupling agent, phenols such as phenol and 3,5-xylenol may be used in a concentration of 0.01 - 100 mg/ml. When the formed pigment is not an azo dye, the reaction of compound (II) with a chromogen is usually carried out in the presence of an oxidizing agent or oxidase. Examples of suitable oxidizing agents are inorganic compounds such as hydrogen peroxide, sodium or potassium metaperiodate, sodium or potassium permanganate, sodium or potassium dichromate, sodium or potassium matavanadate, pentacyanoiron complexes, and peroxy acids, such as peracetic acid. Suitable oxidases are hydrogen peroxide-peroxidase (HRP), and oxidases such as cerulopiasmin and laccase (E C. 1. 10. 3. 2.). The inorganic oxidizing agent may be used in a concentration of 0.01 - 10 mg/ml and the oxidases at a concentration of 0.1 - 10 U/ml. Tables 1 and 2 show the solubility, maximum absorption wavelength, sensitivity and stability of the formed pigments: Numbers in parenthesis of R₃ or R₄ show the position of the group R₃ or R₄ on the benzene nucleus. λ max : Maximum absorption wavelength SE : Sensitivity of pigment ST : Stability of pigment S : Solubility Sensitivity: 50 µl of 1 mM each free compound was added to 3 ml of 0.1 M phosphate buffer containing 0.5 mg/ml EMSE and 1 U/ml laccase and the mixture was incubated at 37⁰C for 5 minutes. The absorption of the reaction solution at λ max was measured. As a control, 5-aminosalicylic acid was used and the reaction of the compound with 2,6-xylenol was carried out in the presence of metaperiodate under strong alkaline conditions. The results are shown, defining the sensitivity of 5-aminosalicylic acid as 100. The enzymatic reaction is usually carried out in a buffer solution having a pH of 6 to 10 preferably around 8. As the buffer, good buffer, phosphate buffer, acetate buffer, etc. may be used. The present method is very simple and excellent. For example, the solubility of the present substrate is very high and therefore it is not necessary to add surfactant or organic solvent to increase the solubility. The measurement of absorption is carried out at a wavelength of 560 nm or more and therefore the results are not influenced by other components present in the test serum. Since the sensitivity and stability of the present substrates are excellent, more accurate results are obtainable. Another aspect of the present invention resides in a test composition for the determination of LAP activity which comprises a compound of formula (I) and a chromogen. The composition may also contain an oxidizing agent, an oxidase, a diazotizing agent, a coupling agent or a buffer. Certain specific embodiments of the invention are illustrated by the following representative examples. Example 1 Reagent solution (pH 7.3) Compound No. 2 0.03 g/ml EMSE 0.5 mg/ml Magnesium nitrate 1 mg/ml Good buffer 8 mg/ml Laccase 1 U/ml 3 ml of the reagent solution was incubated at 37°C for ten minutes. To the solution was added 0.05 ml of serum sample and the mixture was immediately stirred and the change in absorption of the solution at 745 nm was measured for one minute. LAP activity in the sample can be calculated using the following equation. ΔE means the amount of absorption change per minute. The same procedures as described above were repeated except that the following materials were used. (a) Substrate : L-leucyl-3-carboxy-4-hydroxyanilide Oxidizing agent : sodium metaperiodate Coupling agent : p-xylenol Wavelength for measurement : 635 nm (b) Substrate : L-leucyl-p-nitroanilide Coupling agent : p-dimethylaminocinnamaldehyde Wavelength for measurement : 565 nm (c) Substrate : L-leucyl-p-N,N-diethylaminoanilide Oxidizing agent : m-periodate Coupling agent : 1-naphtol-2-sulfonic acid Wavelength for measurement : 675 nm The amounts of absorption change per one minute were measured for ten samples and the results are shown in Table 3. (a) (b) (c) The method according to the present invention Serum(1) 0.0188 0.0071 0.0158 0.0454 (2) 0.0127 0.0054 0.0095 0.0395 (3) 0.0198 0.0090 0.0150 0.0543 (4) 0.0203 0.0089 0.0145 0.0541 (5) 0.0148 0.0071 0.0116 0.0382 (6) 0.0158 0.0071 0.0126 0.0437 (7) 0.0153 0.0074 0.0113 0.0400 (8) 0.0191 0.0086 0.0150 0.0532 (9) 0.0190 0.0082 0.0138 0.0537 (10) 0.01840 0.0079 0.0133 0.0476 Example 2 The reagent solution wherein laccase was excluded from the reagent solution of Example 1 was used. 3 ml of the reagent solution was incubated at 37°C for 10 minutes and to the mixture was added 0.05 ml of serum sample. The mixture was immediately stirred and incubated at 37°C for 10 minutes. After ten minutes passed, (a) 0.03 ml of 10 mg/ml α-naphtol methanol solution and 0.1 ml of 1N-NaOH solution containing 50 mg/ml of sodium meta-periodate were added to the solution. The mixture was incubated at 37°C for 10 minutes and the absorption of the reaction solution at 600 nm was measured. (b) 1 ml of 10 mg/ml p-N,N-dimethylamino cinnamaldehyde ethanol solution and 1 ml of 0.4N-HCl solution were added to the solution and the mixture was incubated at 37°C for 10 minutes. The absorption of the reaction solution at 570 nm was measured. (c) 0.5 ml of 1N-HCl and 0.5 ml of 1 mg/ml sodium nitrite were added to the solution and the mixture was incubated at 37°C for 10 minutes. Then 0.1 ml of 1 mg/ml 3,5-xylenol ethanol solution was added thereto and the absorption of the reaction solution at 650 nm was measured. The results are shown in Table 4 (a) (b) (c) Example 1 Serum 1 0.331 0.386 0.277 0.0454 2 0.291 0.336 0.241 0.0395 3 0.389 0.466 0.330 0.0543 4 0.395 0.460 0.325 0.0541 5 0.277 0.320 0.230 0.0382 6 0.322 0.372 0.270 0.0437 7 0.291 0.333 0.244 0.0400 8 0.386 0.453 0.325 0.0532 9 0.392 0.460 0.331 0.0537 10 0.346 0.405 0.292 0.0476 Ratio of sensitivity 73 85 61 100 Reference Example 1 Production of hydrobromide of L-leucyl-p-N,N-disulfopropyl aminoanilide. 2.49 g of N-BOC-L-leucine (hereinafter referred to as BLL) was dissolved in 100 ml of dioxane. To the solution were added 4.12 g of DCC and 2.16 g of p-phenylene-diamine (hereinafter referred to as PDD), and the mixture was incubated at room temperature for 4 hours. The mixture was filtered and the filtrate was concentrated to dryness under reduced pressure. The product was redissolved in methanol/water = 80/20 (vol/vol) and the solution was subjected to chromatography using Diaion HP-20 (Produced by Mitsubishi Chemical Industries Ltd.) and methanol/water = 80/20 was used as eluting agent. Methanol was removed from the eluate under reduced pressure to obtain N-BOC-L-leucyl-p-aminoanilide in the yield of 1.25 g. 1 g of the product was dissolved in 50 ml of chloroform. To the mixture were added 2 ml of triethylamine and 5 g of 1,3-propanesultone and the mixture was incubated at 50° for one day. 50 ml of ethylether was added to the mixture which was separated into two layers. To the underlayer were added 5 ml of acetic acid and 20 ml of 25% hydrogen bromide-acetic acid to remove BOC group and the mixture was allowed to stand at room temperature. To the mixture was added 50 ml of ethylether to form L-leucyl-p-N,N-disulfopropyl aminoanilide · HBr as a precipitate. After filtration with glass filter, the precipitate was dissolved in 50 ml of methanol and the solution was neutralized with 6N-NaOH. After removal of formed NaCl by filtration, ethylether was added to form a precipitate. The precipitate was separated by filtration to obtain 0.63 g of desired compound having a melting point of 221 -223°C Elemental analysis C H N Found (%) 46.29 6.58 9.07 Calculated (%) 46.45 6.67 9.03

Claims: A method for the determination of leucine-aminopeptidase (LAP) activity in a sample which comprises reacting the enzyme with an enzyme substrate to produce a product compound determinable by means of a colorimetric reaction, characterised by the steps of: (a) reacting the enzyme with an enzyme substrate of the formula (I): wherein Z represents (CH₃)₂CHCH₂CH(NH₂)-, R₁ represents hydrogen, alkyl or substituted alkyl R₂ represents alkylene or hydroxyalkylene, R₃ and R₄ are the same or different and represent hydrogen, halogen, nitro, hydroxyl, sulfo, carboxyl, alkyl and alkoxy, or a salt thereof, thereby to produce a product compound of the formula (II): wherein R₁, R₂, R₃, and R₄ are as defined above; (b) reacting the compound of formula (II) with a chromogen to form a pigment or azo dye or converting the compound of formula (II) into diazonium salt and reacting the diazonium salt with coupling agent to form an azo dye; and (c) measuring the absorbtion of the colored reaction solution. A method according to claim 1, wherein in step (b), the reaction of compound (II) with the chromogen is carried out in the presence of an oxidizing agent, that reaction producing a coloured product which is other than an azo dye. A method according to claim 2, wherein said oxidizing agent is an oxidase. A method according to claim 1, 2 or 3, wherein, in the formulae (I) and (II), R₁ is sulfoalkyl. A method according to claim 1, 2 or 3, wherein the enzyme substrate used in step (a) is L-leucyl-p-N,N-disulfopropylamino anilide. A method according to any one of claims 1 to 5, wherein, in step (b), the compound of formula (II) is reacted with an amine, aniline derivative or a phenol as said chromogen. A method according to claim 6, wherein the chromogen is EMSE. A method according to claim 1, 4 or 5, wherein, in step (b), the compound of formula (II) is reacted with cinnamaldehyde to produce an azo dye for measurement in step (c). A reagent for the determination of LAP activity comprising a compound of formula (I) as defined in claim 1 and a chromogen reactive with the reaction product of that compound with LAP to produce a pigment or azo dye. A reagent according to claim 9, which further contains an oxidizing agent. A reagent according to claim 10, wherein the oxidizing agent is an oxidase. A reagent for the determination of LAP activity comprising a compound of formula (I) as defined in claim 1, and a diazotising agent reactive with the reaction product of that compound with LAP. A compound of formula (I) as defined in claim 1 or a salt thereof.

Inventors: KATSUMATA, YUKIO, MIIKE, AKIRA, TATANO, TOSHIO
Applicants: KYOWA MEDEX CO. LTD.
Application year: 1985
Main classification: C12Q1
IPC codes: C12Q1