Process and Reagent for the Determination of N-Carbamoylsarcosine With the Use of a New Enzyme

The present invention is concerned with a process and a reagent for the determination of N-carbamoylsarcosine with the use of a enzyme, N-carbamoylsarcosine-amidohydrolase, and is also concerned with obtaining this enzyme.

We have found that micro-organisms possess the ability to convert creatinine into N-carbamoylsarcosine. For the investigation of this breakdown path, a specific process would be desired for the determination of N-carbamoylsarcosine. Furthermore, such a process could also be employed for the determination of creatinine itself by a previous conversion thereof into N-carbamoylsarcosine.

Therefore, it is an object of the present invention to provide a process for the specific determination of N-carbamoylsarcosine.

Thus, according to the present invention, there is provided a process for the determination of N-carbamoylsarcosine, wherein a solution containing N-carbamoylsarcosine is reacted with N-carbamoylsarcosine-amidohydrolase to give sarcosine which is then determined.

The present invention is based upon the discovery of the new enzyme N-carbamoylsarcosine-amidohydrolase, abbreviated CSH, which forms sarcosine specifically, with the splitting off of carbon dioxide and ammonia according to the following equation: ##STR1##

Sarcosine formed by the process according to the present invention can be determined by known methods, for example with the use of sarcosine dehydrogenase and measurement of NADH formed or by means of sarcosine oxidase and measurement of hydrogen peroxide formed or of oxygen consumed. The determination with the use of sarcosine oxidase and measurement of the hydrogen peroxide formed is preferred. The latter can be determined not only titrimetrically but also potentiometrically, polarographically and colorimetrically, as well as enzymatically. Amongst these, there are preferred the enzymatic methods using catalase or peroxidase (POD) since these are extremely specific and dependable. The determination by means of catalase takes place in the presence of a .beta.-diketone, for example acetylacetone, and methanol, ethanol or methylene glycol. The determination with peroxidase takes place in the presence of one or more chromogens. Examples of appropriate chromogens include 2,2'-aminobenzthiazoline-sulphonic acid (ABTS), the indicator system according to Trinder (Ann. Clin. Biochem., 6, 24-27/1969) in which phenol or another aromatic alcohol or aromatic amine is oxidatively coupled with 4-aminophenazone or with a 4-aminophenazone derivative to give a coloured material. Appropriate aromatic alcohols or amine include, for example, p-chlorophenol, aminophenols, naphthol and its derivatives, naphthylamine and its derivatives, aminoquinolines, hydroxyquinolines, dihydroxyphenyl-acetic acid and the like. Instead of 4-aminophenazone, there can be used appropriate 4-aminophenazone derivatives, phenylenediamine-sulphonic acid, methylbenzothiazolohydrazone (MBTH), sulphonated methylbenzothiazolohydrazone (S-MBTH) and derivatives thereof.

The determination process according to the present invention for N-carbamoylsarcosine is preferably carried out in a buffered solution, the preferred pH value being from about 6 to 9.

The present invention also provides a reagent for the determination of N-carbamoylsarcosine, which contains N-carbamoylsarcosine-amidohydrolase and a system for the detection of sarcosine.

A preferred reagent of the above-indicated type contains, as the system for the detection of sarcosine, sarcosine oxidase, peroxidase and a chromophore.

A further preferred system for the detection of sarcosine consists of sarcosine dehydrogenase and NAD.

The reagents according to the present invention can also contain a buffer (pH 6 to 9) and possibly a surface-active agent.

The present invention also provides the enzyme N-carbamoylsarcosine-amidohydrolase (CSH).

The following properties have been determined for this enzyme:

Stability optimum: pH 6 to 6.5; good stability properties are present in phosphate buffer, the best stability being achieved by the addition of glycerol.

Heating of CSH 20 mg./ml. in 50% glycerol, pH 6.5, for 20 minutes:

residual activity %:

40.degree. C./100

50.degree. C./48

60.degree. C./24

80.degree. C./0

At +4.degree. C. in 50% glycerol, 100% activity is maintained after 4 weeks.

Creatinine, creatine, sarcosine, N-methylhydantoin and hydantoin are not reacted. Michaelis constants for N-carbamoylsarcosine at 25.degree. C., pH 8.0, 0.1M/liter tris buffer: 3.3 to 3.8.times.10.sup.-3 M; in 0.1M/liter potassium pyrophosphate buffer, pH 8.5: 5.0 to 7.1.times.10.sup.-3 M.

The pH optimum depends upon the buffer substance. In the case of potassium pyrophosphate buffer, it was found to be 8.5 and in the case of tris buffer and hopes buffer was found to be 8.0.

MW about 40,000. Determination by disc electrophoresis.

5 .mu.g. merthiolate/ml. inhibits up to 95%.

The equilibrium was determined with 0.5 U/ml. CSH in 90 mM/liter tris buffer, pH 8.0: ##EQU1##

CSH is obtained by culturing appropriate microorganisms and obtaining the enzyme from the biomass or from the culture broth. Especially preferred micro-organisms include those of the genus Arthrobacter, Micrococcus and Moraxella, the best results having been achieved with Arthrobacter DSM 2563, DSM 2564, Micrococcus spec. DSM 2565 and Moraxella DSM 2562.

The micro-organisms can be used in the form of a cell suspension for the determination process according to the present invention. However, it is preferable to use a purified enzyme preparation. This can be prepared by digesting a micro-organism which has been cultured on N-methylhydantoin and which has a sufficiently high content of N-carbamoylsarcosine-amidohydrolase to make working up worthwhile, followed by separating off insoluble material. The digestion can be carried out by using conventional methods, such as high pressure dispersion, ultrasonics, disintegration mills or lysozyme addition. The clear extract obtained is mixed with 0.3 to 0.5% by weight of polyethyleneimine, the precipitate formed is separated off and the supernatant is diluted with water, the enzyme thereby precipitating out, whereafter it can be separated off.

For high purification, the so obtained enzyme can be subjected to the usual methods of enzyme high purification. The enzyme preparation produced as described above is preferably dissolved in an appropriate buffer and fractionated at an ammonium sulphate concentration of from 1.3M to 2.2M. Chromatography over phenyl-sepharose and DE-52-cellulose, as well as gel filtration over Sephacryl S 200, gives an enzyme preparation which is free from foreign activities and has a specific activity of 2 U/mg. protein, in a yield of about 70%, referred to the amount of enzyme found to be present in the cell extract. The so obtained enzyme can be stored, without loss of activity, in 50% glycerol/water solution, pH 6.5, at +4.degree. C. for at least six months.

The present invention makes it possible specifically to determine N-carbamoylsarcosine. Such a method is a considerable contribution especially for the elucidation of creatinine metabolism. In combination with the conversion of creatinine into N-carbamoylsarcosine, the process according to the present invention can also be used directly for the determination of creatinine.

The following Examples are given for the purpose of illustrating the present invention:

Aqueous solution with 0 to 35.6 .mu.mol/liter N-carbamoylsarcosine.

2 ml. reagent

100 .mu.l. sample

Mix sample and reagent, leave standing for 30 minutes at ambient temperature, transfer to cuvette and read extinction of the sample against reagent blank at .lambda.=546 nm.

There is found to be a linear dependency between the extinction and the N-carbamoylsarcosine concentration in the sample.

10 ml. of an N-methylhydantoin-containing medium (composition see below) are inoculated with Arthrobacter DSM 2563, BMTU 2194-1, from a tilt agar culture of the same composition and shaken in 100 ml. Erlenmeyer flasks for about 24 hours at 28.degree. C. and 160 r.p.m. Subsequently, it is further inoculated in an amount of 1% into medium of the same composition so that 500 ml. of this culturing stage are available for a 50 liter fermenter (5.times.1 ml. of the 1st culture in each of 100 ml. per 500 ml. Erlenmeyer flask, 24 hours/28.degree. C. and 160 r.p.m.).

This 500 ml. amount is transferred into 50 liters of medium of the same composition and further fermented in a fermenter equipped with 2 plate stirrers per baffle plate, with 1000 liters air/hour and 500 r.p.m. at 28.degree. C.

After 10 to 12 hours, the formation of the N-methylhydantoin-decomposing enzyme commences and continues up to the end of the logarithmic growth phase. After 18 hours of culturing, harvesting takes place. The micro-organisms are separated off in a cooled centrifuge and the biomass is subsequently washed with 0.1M phosphate buffer, again centrifuged and frozen. In this way, from 50 liters of culture, there are obtained about 800 g. biomass.

per liter: 7 g. Na.sub.2 HPO.sub.4 .times.2H.sub.2 O, 3 g. KH.sub.2 PO.sub.4, 0.5 g. MgSO.sub.4 .times.7H.sub.2 O, 10 g. N-methylhydantoin (sterile filtered), 5 g. yeast extract/Difco, 1 ml. trace solution 1.sup.+, 0.1 ml. trace solution 2.sup.++, 1 ml. vitamin solution.sup.+++ (sterile filtered).

100 mg. MnCl.sub.2 .times.4H.sub.2 O, 100 mg. FeCl.sub.3 .times.6H.sub.2 O, 100 mg. CaCl.sub.2 .times.2H.sub.2 O are dissolved in 100 ml. double distilled water and sterilised. 1 ml. of this solution is used per liter of medium.

1 mg. CuCl.sub.2 .times.2H.sub.2 O, 1 mg. of ZnCl.sub.2, 1 mg. (NH.sub.4).sub.2 MoO.sub.4, 1 mg. CoCl.sub.2 .times.6H.sub.2 O are dissolved in 1000 ml. double distilled water and sterilised. 0.1 ml. of this solution is used per liter of medium.

0.1 mg. biotin, 0.1 mg. pyridoxol, 0.1 mg. pyridoxamine hydrochloride, 0.1 mg. PABS, 1.0 mg. riboflavin, 1.0 mg. nicotinamide, 1.0 mg. folic acid, 10.0 mg. thiamine hydrochloride are dissolved in 100 ml. double distilled water and sterile filtered. 1 ml. of this solution is used per liter of medium.

The washed cells (1 kg. dry weight) are made up in 50 mM/liter potassium phosphate buffer (pH 6.5) to 15 liters and digested at about 600 bar (ats.) by high pressure dispersion. The cell residue is separated off and the clear extract obtained contains the CSH. It is mixed with 4% by weight of 10% by weight polyethyleneimine (G-35) solution (pH 6.5). After separating off the nucleic acids, the enzyme is precipitated by dilution with water. The small amount of protein precipitate is taken up with 50 mM/liter potassium phosphate buffer (pH 6.5) and fractionated at an ammonium sulphate concentration of from 1.3 to 2.2M/liter. Sarcosine oxidase is completely removed by subsequent hydrophobic chromatography on phenylsepharose.

In decreasing gradients of 20 mM/liter potassium phosphate buffer (pH 6.5) containing 1.0M/liter ammonium sulphate, to 20 mM/liter potassium phosphate buffer (pH 6.5) the CSH is eluted at about 0.4M/liter ammonium sulphate.

The eluate is dialysed cold against 20 mM/liter potassium phosphate buffer (pH 6.5) and applied to a column of DE-52-cellulose which has been equilibrated with the same buffer. The CSH is eluted by an increasing gradient with the above buffer of from 50 to 500 mM/liter sodium chloride. The eluate is concentrated by ultrafiltration.

After subsequent gel filtration in 0.1M potassium phosphate buffer (pH 6.5), using Sephacryl-S-200, the CSH is obtained in 53% yield, with a specific activity of 2.1 U/mg. The enzyme can be stored without loss of activity for more than 6 months in 50% glycerol solution, pH 6.5, at +4.degree. C.

The following Table gives the details of the above-described purification process:

From a culture batch of Moraxella DSM 2562 (BMTU 2913-1), cultured on N-methylhydantoin as described in Example 2a), there are obtained 180 g. of dry mass. The CSH is purified similarly to Example 1B). However, instead of by high pressure dispersion, the cells are lysed with 0.05% lysozyme (g./g. dry mass) at pH 7.0.

The following Table gives the details of the purification process:

Dilution with cold 50 mmol/liter KPO.sub.4 buffer (pH 6.5)

436 nm; 25.degree. C.; V=2.00 ml.; d=1 cm.; .epsilon.=8.3 cm.sup.2 .times..mu.mol.sup.-1