JANUARY 13. 1940
ANALYTICAL EDITION
27
permission to use the division’s potentiometric equipment and also to show a portion of the data on a soil profile prior to publication.
a well-stoppered bottle and did not change appreciably, its pH value being checked before each titration was started. K h e n taking readings, the potentiometer was brought into balance in the regular way; however, the zero point was always confirmed by throwing the potentiometer out of balance equal amounts on each side (usually 0.0010 volt) to note if equal deflections on the light scale were obtained. When once the system was set up, readings were obtained with no more effort than is required to weigh a sample of material on a Chain-omatic balance. Acknowledgment is given to F. J. Alway and C . 0. Rost of the Division of Soils, University Farm, Saint Paul, hlinn., for
Literature Cited (1) Hissink, D. G., Trans. Faraday SOC.,20, 551 (1924). (2) Kelley, TT. P., and Brown, S. SI.,“Proceedings and Papers of First International Congress of Soil Science”, p. 491, 1928. (3) Parker, F. W., Ibid., p. 164. (4) Robinson, G. W.,“Soils, Their Origin, Constitution and Classification”, 2nd ed , p. 111, New York, D. Van Nostrand Co., 1936. (5) Schollenberger, C. J., Soil Sci., 30, 161-73 (1930).
A Method for Determining Glutamine in Plant Tissues GEORGE W.PUCHER AND HUBERT BRADFORD VICKERY Connecticut Agricultural Experiment Station, New Haven, Conn.
Glutamine is converted to ammonia and pyrrolidone carboxylic acid by hydrolysis in neutral solution. Conditions have been ascertained under which this acid can be quantitatively extracted from the mixture of plant tissue components. On being hydrolyzed, pyrrolidone carboxylic acid is converted into glutamic acid; accordingly the increase in amino nitrogen during this operation furnishes a measure of the glutamine amide nitrogen originally present in the tissue. The details of a method to determine glutamine, founded on these reactions, have been developed and it is shown that satisfactory results are secured. The new procedure is not proposed as a substitute for the convenient and accurate amide hydrolysis method, but as a more specific means of estimating glutamine when present in small amounts or in especially unfavorable conditions, such as in the presence of a large excess of asparagine or other substances that might interfere with the simpler hydrolytic method.
T
HE method of determining glutamine, originally proposed
by Chibnall and Westall (1) and subsequently modified in certain details ( S ) , depends upon the complete hydrolysis of this somewhat unstable amide when it is heated to 100” C. for 2 hours at a reaction in the range p H 6 to 7 . The ammonia liberated under these conditions has been shown to be a reasonably trustworthy measure of the glutamine amide nitrogen present, and the method leaves little to be desired in most practical cases on the scores of convenience or precision. Severtheless there are other plant components that liberate ammonia, though usually in small amounts, under the same conditions. Urea and allantoin are two that are found in certain plant species, and asparagine, the other widely
,
distributed plant amide, is not entirely unaffected. Accordingly, if reliance is placed on the determination of the unstable amide nitrogen, the results may be misleadingly high in cases TI-here the proport’ion of glutamine present is unusually smal1,l and especially in cases where the t’issues are unusually rich in asparagine. Furthermore, no account is taken of the possibility that unknown plant components that also interfere with the method may be occasionally encountered. The development of a procedure which depends on a more specific property of glutamine than the hydrolysis of the unstable amide group therefore seemed desirable. The present method is not suggested as a substitute for the convenient amide hydrolysis method, but to be applied as a confirmation both qualitative and quantitative in cases of doubt, and especially when the proportion of glutamine present is unusually small. It is necessary to emphasize that no method of plant analysis short of the isolation of a characteristic crystalline derivative is thoroughly trustworthy in the present inadequate state of our knowledge of plant tissue composition. When glutamine is heated with water at pH 6 to 7, hydrolysis of the amide group and ring closure to pyrrolidone carboxylic acid take place. The liberation of ammonia is, so far as is known, precisely quantitative, but the ring closure may not be strictly so. The loss of amino nitrogen has been shown, horneyer, to be a t least 98 per cent, and the silver salt of pyrrolidone carboxylic acid has been isolated in an amount equivalent to nearly 90 per cent of that calculated from the amount of amide hydrolysis. The hydrolysate obtained was slightly colored, however, and a trace of brown flocculent precipitate separated on the addition of the first drop of silver nitrate. This suggests the presence of a small amount of some by-product of the reaction (3). Consideration of the solubility of the silver salt and the conditions under which i t was isolated indicates, however, that the amount of glutamine destroyed by side reactions is, for most purposes, negligible. Pyrrolidone carboxylic acid is a relatively strong imino acid (pK = 3 . 3 2 , 6 ) that can be quantitatively extracted with ethyl acetate from aqueous solution buffered in the range p H 2 to 3. Furthermore i t is easily hydrolyzed, when heated for a 1 The petioles of rhubarb leaves furnish a case in point. It is shown in another communication ( 4 ) that the small amount of glutamine present was overestimated by the hydrolytic method since negative values for the calculated asparagine amide nitrogen were obtained. With the present method, the calculated asparagine amide nitrogen values were very emall but positive and there is reason t o suppose that they were in fact zero.
28
INDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 12, NO. 1
addition of 1.2 N sulfuric acid: 1.5 ml. are usually required but the exact quantity must be found by trial. The Anal volume is TABLE I. RECOVERY OF PURE GLUTAMINE BY THE PYRROLIDONE then brought to 40 ml. by the addition of water. The other flask CARBOXYLIC ACIDMETHOD of the extraction apparatus is charged with 40 ml. of 0.5 N Reaction sodium bicarbonate. Sufficient ethyl acetate is then added, the of flasks are stoppered, and the device is oscillated at the rate of Solution rlmino Nitrogen Glutamine After Before Extrac- Exabout 144 cycles per hour for 18 hours. At the expiration of this Amide Nitrogen tion tracted, hybperiod, the alkaline solution contains the whole of the pyrrolidone drolysis drolysis Taken Found Recovery Time PH carboxylic acid derived from the glutamine of the tissue extract, MQ. MQ. MQ. Hours MQ. % together with most of the malic and citric and other organic 0.015 0.0158 1.00 0.0008 0.08 20 6.3 acids; traces of amino acids and other substances are also present. 0.015 0,0796 1.00 0.0646 6.46 20 5.88 0.814 0.023 1.00 0.791 79.1 20 3.51 The alkaline solution is quantitatively drawn from beneath the 2.91 0.005 0.995 1.00 0,990 99.0 20 solvent by a fine-bore pipet attached to a distilling flask and a 105.0 0,010 1.06 1.00 1.0.3 2.48 20 source of vacuum. Two successive 5-ml. portions of water are 0,000 0.986 1.00 0.986 98 6 2.43 17 0.007 0.980 97.3 1.00 0.973 2.05 18 gently agitated with the solvent and likewise withdrawn. The 1.00 1.03 103.0 1.75 0 , 0 0 5 1.035 20 solution on the acid side of the extractor is usually sampled at 0,010 0.116 0.100 0.106 106.0 20 1.82 the same time for a confirmatory test of the reaction by means of 0.205 0,200 0.199 99.5 20 1.34 0.006 0.016 1.00 0.981 0.997 98.1 0.92 20 the glass electrode. The alkaline solution is then neutralized with 4 N sulfuric acid, a feiv drops of bromophenol blue indicator solution being added, is rapidly concentrated in U ~ C U Oto a volume of about 10 ml., and is transferred to a 50-ml. flask and made to volume. short time with dilute mineral acid, to glutamic acid. Accordingly the amount present in a solution may readily be ascertained from determinations of the increase in amino Hydrolysis of Pyrrolidone Carboxylic hcid nitrogen after acid hydrolysis. The increase so found is A 15-ml. aliquot of the neutralized solution is transferred to a equivalent to the amide nitrogen of the glutamine from which 50-ml. Erlenmeyer flask, 3 ml. of concentrated hydrochloric acid the pyrrolidone carboxylic acid was derived. This combinaare added, together with a few angular fragments of quartz, and tion of properties-i. e., the conversion of glutamine at p H the flask, covered with a small glass bulb, is heated in a boiling water bath for 2 hours. Alternatively it may be boiled under a 6 to 7 into an acid that can be extracted with ethyl acetate reflux condenser on a hot plate. The flask is then transferred to and subsequently hydrolyzed with the production of amino a hot plate and the solution is concentrated to about 5 ml. (about nitrogen-is highly specific and, if the steps are conducted 15 minutes). -4 drop of bromophenol blue indicator solution is quantitatively, forms the basis of an accurate and satisfactory added and the solution is made distinctly alkaline with 10 N sodium hydroxide and is then acidified with 1 ml. of glacial acetic method to determine glutamine. acid. The solution is transferred to a test tube graduated at 15 ml. and diluted to the mark-that is, to the original volume of the Reagents aliquot before hydrolysis. Amino nitrogen is determined in 5-ml. portions of this solution, Buffer of pH 8.5: 4.957 grams of potassium dihydrogen phosand also in suitable aliquots of the solution before hydrolysis, in phate and 12.167 grams of sodium tetraborate (borax) made to 1 liter. Sodium sulfate, 20 per cent: 200 grams of anhydrous salt made to 1 liter. OF VALUES FOR GLGTAMINE AMIDE TABLE11. COMPARISON Sulfuric acid, 1.2 N: 33.4 ml. of concentrated sulfuric acid NITROGEN made to 1 liter and adjusted betLveen the limits 1.20 and I .22 N. [Determined by pyrrolidone carboxylic acid and amide (PH 6.5) hydrolysis Sulfuric acid, 4 N : 111 ml. of concentrated sulfuric acid made methods] to 1 liter. Reaction Glutamine Amide Nitrogen of Pyrrolidone Amide Sodium bicarbonate, 0.5 N : 42.0 grams made to 1 liter. Solution carboxylic acid hydrolysis Sodium hydroxide, 10 N: 400 grams made to 1 liter. method, Extracted, method, Ethyl acetate, 95 t o 99 per cent by volume: 1 ml. shaken with Sample PH % % 10 ml. of water must not require more than 0.1 ml. of 0.1 A; 0.270 1.9 Tomato leaf sodium hydroxide to give a faint pink color with phenolphthalein 0.278 0.274 when titrated at once. The solvent may be recovered after use by being washed twice, in 1-liter lots, with 200 ml. of 10 per cent 0.294 1.5 sodium carbonate and with water until neutral. It is then dried 0.290 0.298 over calcium chloride and distilled. 1.4 0.274
Preparation of Dry Tissue Extract A 5-gram sam le of the dry powdered tissue is mixed with 80 ml. of water a n i heated at 80" C. for 10 minutes with careful stirring. The suspension is cooled at once, transferred to a centrifuge tube graduated at 100 ml., diluted to the mark, mixed, and centrifuged. The clear fluid is decanted through a plug of glass wool in a dry funnel. Such extracts can be preserved by toluene for several months unchanged for the purposes of the present method if stored in a refrigerator, although considerable amide hydrolysis may have occurred in this time.
0.276
0.538
Tomato stem
1.9 1.4
0,542 0.554 Av. 0 , 5 4 8
Rhubarb petiole
1.9
0.112 0.113 Av. 0.113
0.118
Rhubarb leaf
1.9
0.490
0.450
Tobacco leaf FLNA
1.5
0.012 0.016 0.017 0.016 Av. 0.015
0.014
Tobacco leaf LW2
2.1
0.062 0.067 Av. 0.065
0.064
Tobacco leaf LTV5
2.0
0.117
Hydrolysis of Glutamine A suitable aliquot of the extract-usually 10 m1.-is transferred to a 200 X 25 mm. test tube and 10 ml. of pH 8.5 buffer are added. The reaction of the mixture should be in the vicinity of pH 6.5. The tube is lightly stoppered and is heated in a boiling water bath for 2 hours. Loss of ammonia is immaterial.
0.290 Av. 0.284
0.109
0.113
0.114
0.107 0.109 0.112 0.107 Av. 0.109
0.117
Av.
Extraction of Pyrrolidone Carboxylic Acid The solution is transferred to one of the flasks of a modified Widmark extraction apparatus (500-ml. size, Z), the tube is rinsed into the flask with 10 ml. of 20 per cent sodium sulfate solution, and the reaction is adjusted to pH 2.4 * 0.2 by the
Tobacco leaf LW6
2.0
JANUARY 15, 1940
ANALYTICAL EDITION
the manometric Van Slyke apparatus. The increase in amino nitrogen brought about by the hydrolysis is equivalent to the glutamine amide nitrogen of the original sample and, if the dilutions and quantities mentioned have been adhered to, when multiplied by 2000 gives the percentage of glutamine amide nitrogen in the dried tissue taken.
Recovery of Glutamine Table I s h o m the results of a study of the reaction a t which pyrrolidone carboxylic acid may be extracted. A standard solution of pure glutamine TYas used after having been hydrolyzed as described. It is evident that recoveries of from 95 to 105 per cent are obtained, provided the extraction is made within the limits p H 3 t o 1. The mean of the six determinations of l mg. of glutamine amide nitrogen in this range is 100.2 f 3.1 per cent. The small quantities of amino nitrogen present before hydrolysis are to be attributed to reagent blanks. An examination of the period of hydrolysis required to convert pyrrolidone carboxylic acid to glutamic acid under the conditions described showed that 90 minutes a t 100" C. followed by an evaporation time of about 15 minutes on the hot plate was sufficient to give 97 per cent hydrolysis, while a 2hour period gave complete hydrolysis.
Analysis of Tissues The choice of the quantity of dry tissue for analysis is dependent upon the proportion of glutamine present. A consideration of the range of values usually encountered in leaves suggested that the aliquot part of the solution actually taken for the analysis might most conveniently represent 0.5 gram; with this amount, if only 0.01 per cent of glutamine amide nitrogen were present, the measurement would involve the determination of 0.05 mg. of amino nitrogen which should be possible within 5 per cent. The precision is greater for larger quantities. The adjustment of the reaction previous to the hydrolysis of glutamine need not be exact, since small losses of ammonia are immaterial. Glutamine is completely hydrolyzed in 2 hours in the range pH 6 to 7 , and the velocity of hydrolysis increases rapidly outside of this range. Experience with several tissues showed that, if 10 ml. of buffer a t pH 8.5 are mixed with 10 ml. of extract prepared as described above, the reaction of the mixture fell within this range. The adjustment of the reaction previous to the extraction of the pyrrolidone carboxylic acid in the case of glutamine determinations in tissue extracts does not appear to be particularly critical; successful results have been obtained within the range pH 1.4 to 2.9. But, as will be noted below, the recovery of added glutamine requires extraction a t a reaction adjusted fairly closely in the range p H 2.4 * 0.2. It was found desirable to increase not only the buffering effect but also the salt concentration of the solution previous to extraction in order to avoid loss of water by migration in solution in the organic solvent to the aqueous phase of higher salt content. Accordingly 10 ml. of 20 per cent sodium sulfate were added and the quantity of 2 N sulfuric acid necessary to bring the reaction to p H 2.4 * 0.2 mas ascertained. I n many cases this was found to be close to 1.5 ml. The reaction of the acid solution a t the end of the extraction should also be noted and conditions adjusted if necessary. The choice of ethyl acetate as solvent was dictated by experience with the rate of extraction of citric acid with several solvents. It is, however, by no means a specific solvent under the conditions employed; more or less a-amino nitrogen always finds its way into the alkaline solution although no evidence mas found that hydrolyzable amino nitrogen-e. g., peptide nitrogen-accompanies it. This is not a serious dis-
29
TABLE111. RECOVERY OF GLUTAMINE ADDEDTO PLANT TISSUE EXTRACTS Reaction of Solution Extracted,
Glutamine Added,
%
Recovery,
PH 1.00 t o 1.7 1 . 7 to 1.93 2 09 2.27 2.49 1.52 2.91 2.91
0.04 to 0.20 0.2 0.2 0.2 0.2 0.2 0.2 0.2
0 to 82.0 8 2 . 0 t o 93.0 96.5 96.5 96.5 94.0 96.5 101.0
%
advantage since the datum required is the increase in amino nitrogen brought about by acid hydrolysis under fairly mild conditions.
Comparison with -4mide Hydrolysis Method Table I1 shows a series of determinations on several dried plant tissues compared with determinations made by the amide hydrolysis method ( 3 ) . The reactions of the fluids on the acid side of the extraction apparatus a t the end of the extraction period are also recorded to show the range over which satisfactory results have been secured. The data also show t o what extent replicate determinations may be expected to agree. The eight determinations on tomato leaf gave a mean value of 0.284 * 0.011, a standard error of *3.7 per cent, and the value by the hydrolysis method lies well within these limits. The analyses given cover a range of glutamine concentrations from tissues that contained little more than a trace to one that contained over 5 per cent of the dry weight as glutamine.
Recovery of Glutamine Added to Tissue Extracts Attempts to recover glutamine added to tissue extracts gave low and erratic results until i t was found that the reaction of the solution from which the pyrrolidone carboxylic acid is extracted must be carefully controlled. Data that show the phenomena encountered are given in Table 111. They refer to several different tissues and may be summarized by the statement that the recovery may be less than 80 per cent a t reactions more acid than pH 1.7 and that the recovery is less than 93 per cent a t reactions in the range 1.7 to 1.9. I n the most favorable reaction range, pH 2.1 to 2.9, the recovery still fell a little short of being quantitative but was reasonably satisfactory. I n view of this it seems best to suggest that the reaction should be adjusted to p H 2.4 * 0.2 for general application. The difficulty of obtaining complete recovery of added glutamine appears to be attributable to a failure of complete extraction owing perhaps to an obscure reaction between the pyrrolidone carboxylic acid and some tissue extract component. Prolonged extraction (46 hours) did not improve the results. N o explanation of this behavior can be advanced a t the present time. Conditions have been found empirically by which the difficulties can be approximately overcome in practical cases, and considerable experience with a wide range of tissues has indicated that the effect is confined to the case of glutamine added to the extract.
Literature Cited (1) Chibnall, A. C., and Westall, R. G., Biochem. J.,26, 122 (1932). (2) Pucher, G . W., and Vickery, H. B., ISD. EXQ.CHmr., Anal. Ed., 11, 656 ( 1 9 3 9 ) .
(3) Vickery, H. B., Pucher, G. W., Clark, H. E., Chibnall, A. C., and Westall, R. G., Biochem. J., 29, 2710 (1935).
(4) Vickery, H. B., Pucher, G. W., Wakeman, A. J., and Leavenworth, C. s., Conn. Agr. Expt. Sta., Bull. 424 (1939). (5) Wilson, H., and Cannan, R. K., J . Bid. Chem., 119, 309 (1937).
