a study of lysine assay using 1-fluoro-2,4-dinitrobenzene

A Study of Lysine Assay Using 1-Fluoro-2,4-DinitrobenzeneAuthor(s): Brenda WheelerSource: Irish Journal of Agricultural Research, Vol. 10, No. 1 (Apr., 1971), pp. 49-58Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25555592 .

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Ir. J. agric Res. 10: 49-58, 1971

A STUDY OF LYSINE ASSAY USING l-FLUORO-2,4-DINITROBENZENE

Brenda Wheeler

An Foras Taluntais, Animal Production Research Centre, Dunsinea, Castleknock, Co. Dublin

ABSTRACT

The method of Selim for the determination of lysine in protein hydrolysates has been examined to ascertain its suitability for the analysis of cereal-based animal feedstuffs. In this method the a-amino

group of lysine is protected by the formation of its copper complex and then the s-amino group is reacted with l-fluoro-2,4-dinitrobenzene. After removal of the copper the s-dinitrophenyl-lysine is

measured colorimetrically. When the method was applied to pure lysine and to amino acid solutions, the concentration of

amino acids affected lysine recovery, which decreased with decreasing amino acid concentration. For commercial animal feedstuffs non-lysine constituents of the hydrolysate influenced the results and indicated higher levels than actually were present.

These two factors, the amino acid concentration and the non-lysine components of the hydroly sate, make the method unsuitable for compound feedingstuffs.

INTRODUCTION

The method of Carpenter (1) for the determination of available lysine in animal

protein concentrates is less suitable for vegetable materials as there appears to be some destruction of s-dinitrophenyl (DNP)-lysine during hydrolysis if carbohydrates are present. Selim (2) devised a micro-method based on that of Porter and Sanger (3) for the estimation of total lysine in hydrolysed pure protein in which lysine is con

verted to s-DNP-lysine. Amino acids, in which the a-amino group is protected by the formation of a stable copper chelate with the carboxyl group, are made to react with

l-fluoro-2,4-dinitrobenzene (FDNB) under mildly alkaline conditions. The s-amino

group of lysine, the imidazole group of histidine, the sulphydryl group of cysteine and the phenolic group of tyrosine combine with the reagent. The solution is acidified and the copper precipitated with hydrogen sulphide, leaving s-DNP-lysine as the only coloured substituted amino acid derivative in solution. Excess FDNB and its break down products dinitrophenol and dinitroaniline are removed by extraction with ether.

The absorbance of the e-DNP lysine is measured at 390 nm. At this wavelength,

Sanger (4) demonstrated that a 20^m solution of e-DNP-lysine has an absorbance ol

0.204 and that Beer's law is obeyed in solutions of concentrations of less than 50 jim

49



50 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 10, NO. 1, 1971

Selim (2) determined lysine in quantities ranging from 0.1 to 0.5 umole in pure

lysine solutions, in amino acid solutions and in pure protein hydrolysates. Recoveries from the first two were 100 % and results for the hydrolysates compared well with

published values. The recovery of lysine added to protein hydrolysates was quan titative.

The assay o( total lysine in cereal-based feeds requires a simple, rapid method that would be suitable for smaller quality control laboratories. The aim ofthe present study was to assess the suitability of Selim's (2) method for the determination of total lysine

in such materials.

EXPERIMENTAL Materials

Anhydrous I-lysine monohydrochloride and crystalline bovine serum albumin

(n = 15.05%) were obtained from British Drug Houses Ltd. The feedstuff examined

was a commercial cereal-based pig feed. Copper phosphate suspension and borate

bulVcr pi I l>.2 were prepared according to Selim (2).

Four types of solution containing lysine were analysed:

1. Pure lysine solutions.

2. Amino acid solutions prepared according to Selim (2). 3. Hydrolyscd bovine serum albumin.

4. Hydrolysed pig feeds.

Method Pure lysine and amino acid solutions were taken to dryness and dissolved in borate

buffer at pH 9.2, and from this point the formation of the e-DNP derivative was carried out according to Selim's (2) procedure.

The samples of albumin and pig feed (0.3 g) were refluxed under nitrogen for 18

hours, with 6 n HCl (200 ml). The hydrolysates were filtered and diluted to 250 ml with aqueous washings. Aliquots of sample solutions, based on expected levels of

lysine, were evaporated to dryness on a rotary evaporator under nitrogen. Larger

samples of feed (1.0 and 1.8 g) were hydrolysed using the same volume of HCl in order to check whether smaller aliquots of more concentrated hydrolysates would be suitable for analysis. Hydrolysate residues were titrated to pH 9.6 with 0.2 n NaOH

containing thymol blue as indicator (1 drop 0.1 % thymol blue per ml NaOH). (The indicator was removed during the final washing of the solution with ether). Borate buffer (pH 9.2) was then added to make the solution up to the required volume

(usually 2 ml) for reaction with copper phosphate suspension. The formation of the 8-DNP derivative was carried out following Selim's (2) procedure.



WHEELER: LYSINE ASSAY 51

To check whether, in the hydrolysed feeds, the absorbance at 390 nm was caused

only by the s-DNP-lysine, the absorbance of solutions was measured before and after

removal of s-DNP-lysine with methoxycarbonyl chloride, using Carpenter's (1) method starting from stage 2 of his procedure.

Recoveries of s-DNP-lysine from pure lysine solutions, containing 0.1 to 0.5

umole of lysine per ml reaction volume, were lower than expected, so the various steps of the method were checked. The formation of the lysine-copper complex was studied

using the method of Spies and Chambers (5), and shown to be complete. Solutions of

s-DNP-lysine were prepared and shaken with copper phosphate suspension to give an s-DNP-lysine-copper complex. Volumes of 1 ml containing from 0.1 to 0.5 umole

lysine were treated according to Selim's (2) method to remove copper and the s-DNP

lysine concentrations checked. No losses of s-DNP-lysine occurred during these

stages of the procedure. Therefore the discrepancy must have occurred at the stage

where FDNB reacted with the lysine-copper complex. Consequently lysine concentration, reaction volume, time, temperature and pH

of the reaction medium were all examined, as were the concentration of FDNB and

the method of its addition, whether concentrated or in solution. In order to check the effect of lysine concentration and reaction volume on the formation of s-DNP

lysine, a solution of lysine hydrochloride (20 ml), containing 5 jimoles of lysine per ml, was evaporated to dryness under nitrogen in a rotary evaporator, and the residue

dissolved in borate buffer (5 ml). This gave a solution containing 20 jimoles per ml which was within the range of 5 to 40 pmoles per ml stated by Selim (2) for reaction with 1 to 2 ml of copper phosphate suspension. The suspension was added (5 ml), and the mixture was shaken before centrifugation. The supernatant liquid and three

successive dilutions prepared from it were diluted with aliquots of buffer, to prepare 40 lysine-copper solutions ranging in concentration from 0.1 to 2 p moles lysine in four volumes of 0.25 to 1 ml for reaction with FDNB.

RESULTS AND DISCUSSION Lysine solutions

The recovery of lysine from pure solutions varied. Altering Selim's (2) recom mendations for the concentration of FDNB, its method of addition, time of reaction,

temperature and pH did not improve recovery. Reaction volume and concentration

of lysine influenced its recovery. The results of an experiment repeated on 4 days, involving the recovery of lysine at 10 concentrations in each of four reaction volumes,

were analysed by the method of least squares, so that the effects of replicates, reaction volumes and concentration means could be studied independently of one another.

Tests of significance were carried out using the Student-Newman-Keuls test as set out

by Sokal and Rohlf (6).




TABLE 1: Percentage recovery of lysine (mean of four replicates) _fc standard deviation ofthe mean, from lysine solutions

of varying concentration and volume ,

Reaction volume (ml) Lysine cone.

(Mtnolc) 0.25 0.50 0.75 1.00

0.1 67.2?8.8 48.1?22.5 24.0?11.9 23.8?12.8 0.2 72.6?4.4 59.2?12.6 43.6? 4.8 37.6? 2.0 0.3 80.7?3.1 65.6? 6.1 58.1 ? 6.0 49.5? 3.1 0.4 8L0?1.8 70.4? 4.0 61.5? 2.6 58.4? 2.5

0.5 83.1 ?4.5 72.4? 4.2 64.5? 3.9 63.1 ? 3.2 0.6 82.0?8.7 78.4? 4.7 68.0? 7.4 68.6? 1.6 0.8 85.9?6.2 79.8? 5.4 72.7? 3.6 67.0? 6.4 1.0 89.0?5.5 83.4? 3.4 77.7? 1.9 76.8? 3.3

1.5 90.5?2.4 85.8? 3.0 82.5? 6.4 80.5? 4.1 2.0 93.2?5.1 83.5? 5.0 82.1 ? 1.3 82.0? 4.3

TABLE 2: Analysis of variance

Sources of variation df ms F

Replicates (R) 3 666.08 14.57* Reaction volume (V) 3 3860.82 84.46**

Lysine concentration within subclasses (L) 9 3249.76 71.09 (R X V X L) 11 24.86

Pooled error 143 45.71

*p< 0.025 **p< 0.005 j

TABLE 3: Least squares estimates of replicate means

Replicate Mean3 sd

1 70.13 ab ?1.51 2 64.86 b ?1.51 3 74.90 a ?1.53

4 70.20 ab ?1.51

a Values followed by the same letter are not significantly different from each other (p< 0.05)




The means and standard deviations are given in Table 1, which shows a decrease

in recovery as reaction volume increased and an increase in recovery as lysine concen

tration increased. The analysis of variance is shown in Table 2. Table 3 indicates that there was a systematic difference between replicates for which there is no obvious

explanation as the experimental conditions were identical on each day. Tables 4 and 5 and Figures 1 and 2 show that there was a greater percentage

recovery with the lower reaction volumes and a steady increase in recovery with in

creasing concentration of lysine. Reproducibility diminished when the reaction volume was increased beyond 0.25 ml and there was a marked difference in the percentage recovery when the lysine increased to 1.0 pmole. At concentrations above this value there was no significant difference in recovery.

These results do not agree with those of Selim (2) who obtained quantitative re

covery with concentrations of lysine over the range of 0,1 to 0.5 pinole in a reaction volume of 1 ml. Hocquellet (7) quoted a recovery of 67.5% when he followed Selim's

TABLE 4: Least squares estimates of lysine recovery by reaction volume

Mean lysine Volume (ml) recovery (%)a sd

0.25 82.5 ?1.5 0.50 72.7 ?1.5 0.75 63.6 a ?1.6 1.00 60.7 a ?1.5

a Values followed by the same letter are not significantly different from each other; all others differ significantly (p < 0.05)

TABLE 5: Least squares estimates of lysine recovery by concentration

Lysine reacted Mean lysine Oimole) recovery (%)a sd

0.1 40.8 ?2.4 0.2 53.3 ?2.4 0.3 63.5 d ?2.4 0.4 67.8 cd ?2.4 0.5 71.1 be ?2.5 0.6 74.3 b ?2.4 0.8 76.3 b ?2.4 1.0 81.7 a ?2.4 1.5 84.8 a ?2.4

2.0 85.1 a ?2.4

a Values followed by the same letter are not significantly different from each other; all others differ significantly (p < 0.05)




(2) method but did not state the concentration of lysine reacting. The results in the

present paper are similar to those obtained by Schroeder and Le Gette (8) who, in the

quantitative dinitrophcnylation of amino acids, found that recovery was raised by increasing the concentration of amino acid in the reaction mixture. Subsequently

Heikens et al (9) obtained more complete conversion of amino-caproic acid into

dinitrophenyl-aminocaproic acid after evaporation of the reaction mixture until dry.

Heikens et al (9) and Tonge (10) showed that certain losses in quantitative dinitro

phenylation of a, fl and co-amino acids were due to polymerisation. An analogous

ciTcct may account for the low recoveries of s-DNP-lysine in the present study.

Amino acid solutions

Recovery of s-DNP-lysine from solutions of mixed amino acids is shown in

Table 6. Obviously the method gave satisfactory recovery only when there was a

concentrated solution, of lysine alone, or in a mixture with other amino acids; This

trend is well illustrated in Tables 1 and 5 but is at variance with the results published by Selim (2). Hocquellet (7) obtained quantitative yields of s-DNP-lysine when glycine was added to the lysine solution before copper complexing. IJe explained this phen omenon in terms of the formation of a more stable and reactive mixed chelate.

However, Tables 1, 5 and 6 show that it was the total amount of amino acid present in the reaction volume that affected recovery of lysine rather than any change in the

chelate.

90_ T 9 Standard deviation

2 80- N.

9 X & 70- \.

60j \ 0 0-25 0-50 0-75 V00

Reaction volume (mi)

Fig.1: Effect of reaction volume on recovery of lysine (least squares estimates)




90-i

T--?i-1 80- -^"1

2-70- X^l

% Y ?60- /L

- ?*

V 2

Standard deviation

50- j

T ." ._. -j--,-,-,-j-1-,-,-,?-,--,-,-, U 06 VO V5 20

Lysine (p. mole)

Fig. 2: Effect of concentration on recovery of lysine (least squares estimates)

TABLE 6: Effect of concentration on recovery of lysine from solutions of

pure lysine and amino acid mixturesa

Lysine recovery (%)

Reaction volume (ml) Total amino acids Lysine

-

reacted (|i mole) (|i mole) 0.5 1,0

0.3 0.3 65.6 49,4 0.5 0.5 72.4 63.0 1.0 1.0 81.8 78.3 1.0 0.3 83.3 72.5 1.2 0.5 83.1 75.3

a Amino acid solutions prepared according to Selim (2)

Protein hydrolysate The result of repeated determinations for lysine in a sample of bovine serum

albumin compared well (Table 7) with those quoted by other workers in this field, except Selim (2) and his only cited reference, Keller and Block (13), The total amino acid concentration in the reaction volume was 1700 ug per ml copper-amino acid

solution, which is 45 % in excess of the amount of lysine shown in Table 1 to give an

average recovery of 93 %.



56 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 10, NO. I, 1971

Solomons and Irving (17) used Porter and Sanger's (3) method to determine lysine in protein hydrolysates and obtained quantitative results. However, they did not

quote the quantity reacted.

Lysine added to a protein hydrolysate Lysine added to hydrolysed bovine serum albumin was completely recovered. This

result agrees with Sclim's (2) quantitative recovery of lysine from the same material and also with recoveries obtained for the more concentrated lysine solutions as shown

in Table 1.

It was noted that while the completion of the bonding of FDNB with the copper lysine complex in the hydrolysate took 55 minutes, the lysine added to the hydrolysate

before copper complexing had an average recovery of 98.5% (95.8 to 101.6%) after 25 minutes.

Cereal-based pig feed Selim's (2) method was followed to determine the total lysine in a pig feed. There

was a progressive decline in the level of recovery of lysine with increasing size of the

sample hydrolysed (Table 8) and a corresponding decline in the precision of deter mination as shown by the coefficients of variation. This decrease in recovery was

expected, as Roach (18) stated that for hydrolysis the volume of acid should be 500 times that of the sample, in order to reduce interference from carbohydrate material

and to limit the formation of humin. This condition was not fulfilled for the 1.0 g and 1.8 g samples.

Aliquots of the final s-DNP-lysine solutions obtained from the feed hydrolysates were treated according to Carpenter (1), following his method from stage X with the omission of the ether washing at that stage. The apparent lysine levels were lowered, indicating that non-lysine components contributed to the colour of the s-DNP-lysine solution obtained by Selim's (2) method.

TABLE 7: Lysine content of bovine serum albumin as obtained in this analysis compared with values quoted by other authors

Kakade Keller Spahr Stein & & & &

This Brand Liener Block Selim Shemin Edsall Moore analysis8 (11) (12) (13) f2) (14) (15) (16)

Lysine 11.83?0.40 10.9 12.86 8.6 8.1 10.9 11.98 11.25 (gperl6gN)

a Mean of six determinations




TABLE 8: Lysine content of a pig feed determined by two methods

Quantity hydrolysed 0.3 1.0 1.8

(g) ^_^__ __

Method8 C S C S C S

Lysine^' (g per 100 g feed) 0.61(0.77) 0.74 0.52(0.69) 0.65 0.52(0.78) 0.56

sd ?0.03(0.01) ?0.03 ?0.03(0.04) ?0.05 ?0.07(0.08) ?0.07

CV(%) 5.4(1.30) 4.2 5.4(5.7) 7.7 13.5(10.8) 12.1

a C = Carpenter tl) method carried out from stage 2 on final Selim (2) solution; S = Selim (2) b Mean of four analyses

Values in parentheses are those before subtraction of the 'blank' obtained after treatment with methoxy-carbonyl chloride

The feed was analysed by two independent laboratories, one using a microbio

logical technique and the other an autoanalyser. The value obtained by the former method was 0.75% lysine which agreed with that of 0.74% using Selim's (2) method on a 0.3 g sample. The second laboratory was sent hydrolysates of samples of 0.3 g

and 1.8 g. The results obtained were 0.60% and 0.54% lysine which compared well with the values of 0.61 % and 0.52% obtained when Carpenter's (1) method (from stage 2) was followed on final e-DNP-lysine solutions obtained by Selim's (2) method.

CONCLUSION

The total concentration of amino acid in solutions of lysine or mixed amino acids affects the determination of lysine by Selim's (2) method. Incomplete recovery is obtained from dilute solutions. For a hydrolysed pure protein such as bovine serum

albumin the amount of amino acids present is sufficient for correct results to be obtained.

When Selim's (2) method was applied to a cereal-based animal feedstuff, non

lysine components contributed to the colour of the final e-DNP-lysine solution, in

dicating a higher level of lysine than was actually present. Consequently the method is unsuitable for the determination of total lysine in these feeds.

ACKNOWLEDGMENTS

The author is grateful to Dr. M. F. Maguire, Head of the Animal Nutrition and

Biochemistry Department, for helpful discussion, Miss Louie Lavery for technical




assistance, Mr. V. E. Vial, Head of the Animal Breeding and Genetics Department and Miss P. McGloughlin of the same Department, for statistical analyses. Thanks are due to Mrs. E. Carey of the Chemistry Department, University College Dublin, for nitrogen analyses, and to Mr. F. Smith, Faculty of Veterinary Medicine, University College Dublin, for lysine determination in feed hydrolysates by autoanalyser. Com ments on the manuscript by Dr. V. Booth, late of the Department of Agricultural Science and Applied Biology, University of Cambridge, and Dr. J. G. Heathcote, Department of Chemistry, University of Salford, are much appreciated.

REFERENCES

1. Carpenter, K. J., Biochem. J. 77: 604, 1960. 2. Selim, A. S. M., J. agric. Fd Chem. 13: 435, 1965. 3. Porter, R. R. and Sanger, F., Biochem. J. 42: 287, 1948.

4. Sanger, F., Biochem. J. 45: 563, 1949. 5. Spies, J. R. and Chambers, D. C, /. biol. Chem. 191: 787, 1951. 6. Sokal, R. R. and Rohlf, F. J.,'Biometry,' W. H. Freeman and Co., San Francisco, p. 240,1969. 7. Hocquellet, P., Annls Falsif. Expert, chim. 61: 155, 1968. 8. Schroeder, W. A. and Le Gette, Joann, /. Am. chem. Soc 75: 4612, 1953. 9. Heikens, D., Hermans, P. H. and van Velden, P. F., Nature, Lond. 174: 1187, 1954. 10. Tonge, B. L., Nature, Lond. 195: 491, 1962. 11. Brand, E., Ann. N.Y. Acad Sci. 47: 187, 1946. 12. Kakade, M. L. and Liener, I. E., Analyt. Biochem. 27: 273, 1969. 13. Keller, S. and Block, R. J., Archs Biochem. Biophys. 85: 366, 1959. 14. Shemin, D., /. biol. Chem. 159: 439, 1945. 15. Spahr, P. F. and Edsall, J. T? /, biol. Chem. 239: 850, 1964. 16. Stein, W. H. and Moore, S., /. biol. Chem. 178: 79, 1949. 17. Solomons, C. C. and Irving, J. T., Biochem. J. 68: 499, 1958. 18. Roach, A. G.( 4th Amino Acid Colloquium, Technicon Instruments Co. Ltd., Chertsey, Surrey,

p. 61, 1966.

Received November 2, 1970



a study of lysine assay using 1-fluoro-2,4-dinitrobenzene

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