(Received for publication, February 26, 1945)

Transcription

1 TRYPTOPHANE METABOLISM XI. CHANGES IN THE CONTENT OF TRYPTQPHANE, UREA, AND NON PROTEIN NITROGEN IN THE BLOOD AND OF TRYPTOPHANE IN THE TISSUES AFTER THE ORAL ADMINISTRATION OF TRYPTOPHANE OR TRYPTOPHANE DERIVATIVES* BY DOROTHY M. BUCK AND CLARENCE P. BERG (From the Biochemical Laboratory, State University of Iowa, Iowa City) (Received for publication, February 26, 1945) Most of the information available on tryptophane met,abolism is concerned with growth or the excretion of metabolites. It seemed to us that an examination of the blood at intervals after the administration of tryptophane might afford further evidence particularly as to rate and course of metabolism in animals known to excrete kynurenic acid. Accordingly the blood of rabbits fed tryptophane was assayed at intervals for tryptophane, urea, and nonprotein nitrogen. Ultimately similar tests were undertaken after feeding derivatives of tryptophane known to be unavailable for growth in the rat. The distribution of tryptophane in the tissues of the rat fed tryptophane was also investigated. The studies are similar to those on other amino acids recorded some years ago by Luck (I), Johnston and Lewis (2), and Shambaugh, Lewis, and Tourtellotte (3). They differ chiefly in the use of a relatively specific test for the amino acid fed instead of a test for amino nitrogen, and in the administration of the tryptophane and tryptophane derivatives as suspensions, without neutralization or conversion to the sodium salt. The data show differences which seem to be significant and probably associable with known peculiarities in the metabolism of tryptophane. Downloaded from by guest on April 9, 218 EXPERIMENTAL Comparisons of several methods for determining the relatively small concentrations of tryptophane in the blood after feeding tryptophane led to the adoption of the Shaw and McFarlane procedure (4) with minor modification; this is essentially a quantitative adaptation of the Hopkins Cole glyoxylic acid test. The extinction coefficient of the colored solution was measured in a Zeiss Pulfrich photometer; Filter S53 (53 mp) was used and the test solution balanced against a blank prepared in the same * The experimental data in this paper are from a dissertation submitted by Dorothy M. Buck in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biochemistry in the Graduate College of the State University of Iowa. 11

2 TRYPTOPHANE METABOLISM. XI way, but without the glyoxylic acid. The tryptophane content was established with the aid of a calibration curve plotted from readings made on aqueous solutions of known concentration. With.2 mg. or more of tryptophane the error of analysis did not exceed 1 per cent; for smaller amounts the estimation was less accurate. Spectral transmittance curves plotted from measurements in the Coleman1 spectrophotometer showed maximum absorption at 55 rnp. Kynurenine, kynurenic acid, and indolepyruvic acid, all metabolites of tryptophane, developed no color, but indole, skatole, and indolepropionic acid, which may be produced by bacterial action in the intestine, did. In the region of maximum absorption the colors with skatole and indolepropionic acid were qualitatively similar to the color formed with tryptophane; the color with indole was quite different. Colors produced by acetylltryptophane, acetyldltryptophane, phenacetyltryptophane, and methylenetryptophane also showed maximum absorption at about the same wavelength as tryptophane, as had already been observed for acetyltryptophane by Shaw and McFarlane (4). Although these derivatives are not normally found in the body, they resemble peptides closely enough to warrant the assumption that peptides produced metabolically from tryptophane cannot be distinguished from the free amino acid by this method. No satisfactory procedure was found for estimating indolepyruvic acid in small concentrations, such as might appear transiently in blood and tissues as the result of oxidative deamination of the tryptophane fed. Obviously the urea and nonprotein nitrogen analyses conducted instead cannot serve specifically as an index of oxidative ordeamination; ammonia is produced also when kynurenic acid is formed and its production from tryptophane by nonoxidative deamination, e.g. hydrolytic, cannot be precluded. In the tests on blood, the use of potassium oxalate as an anticoagulant and of tungstic acid as a deproteinizing agent did not interfere with the quantitative estimation of tryptophane added before the deproteinization. Blood urea was determined by the method of Koch (5), nonprotein nitrogen by a minor modification of the procedure of Keys (6). Small oral doses of tryptophane in the rabbit and the rat (.35 gm. per kilo) produced concentrations in the blood which rose to a maximum abruptly and declined to zero again in 5 hours. Unlike the inconsistent and erratic changes after subcutaneous injection, the general trend following oral administration was reproducible. Therefore, in the studies involving the simultaneous estimation of tryptophane, urea, and nonprotein nitrogen, male rabbits which had been fasted for 18 hours were given relatively large doses of tryptophane (1 gm. per kilo, suspended in Downloaded from by guest on April 9, The double monochromator model was used.

3 D. M. BUCK AND C. P. BERG 13 water) by stomach tube. With one exception the dosage caused no ill effects, but larger amounts of tryptophane were sometimes toxic. All blood samples were drawn from the marginal ear vein. The analyses TABLE I Changes in Concentration of Tryptophane, Urea, and NonProtein Nitrogen in Blood of Rabbit after Administration of Tryptophane* by Stomach Tube Re.it Weight kg ike after feeding hrs. Tryptophanet Urea N mg. per 1 cc (3.3) 4.8 (1.7) 6 2. (.3) 1.6 (.2) (2.8) (1.2) (2.9) (2.4) (2.6) (.4) (.4) (2.2) 4 8. (1.) %i Ionprotein nitrogen T Undetermined 7% 2: % 2: N Downloaded from by guest on April 9, 218 * The dosage was 1 gm. per kilo of body weight. t Tryptophane nitrogen is indicated in the parentheses. (Table I) showed a maximum concentration of tryptophane in 2 hours, less in 4 hours, and little or none in 6; a primary rise in urea to a maximum in the 2 or 4 hour sample, a decline to a minimum in the 6 or 8 hour speci

4 14 TRYPTOPHANE METABOLISM. XI men, then a secondary rise; and a concentration of nonprotein nitrogen which in most instances reflected the changes in urea and tryptophane. The depression of urea which accompanied or soon followed the disappearance of tryptophane, to be succeeded later by a secondary rise, made it desirable to determine whether tryptophane is readily withdrawn for a time into the tissues. Rats, which had been fasted for 18 hours, then fed.35 to 1.3 gm. of tryptophane per kilo of body weight, were killed with ether, usually 1 to 3 hours later. The gastrointestinal tract was dissected out and washed thoroughly with boiling water to recover the unabsorbed tryptophane. The gastrocnemius muscles, liver, and kidneys were removed and ground separately with sand. The skin was peeled off, together with the fatty layer and the tail, and rinsed free of adhering blood. The rinsings were combined with the blood for analysis with the remainder of the carcass. The (skin and carcass fractions were ground separately in a meat chopper. Each of the several fractions was extracted eight times with a total of approximately 1 times its weight of boiling water acidified with 1 gm. of trichloroacetic acid per liter to facilitate coagulation. Complete precipitation of protein at this stage required concentrations of trichloroacetic acid high enough to interfere with the tryptophane determination. The small amount of residual protein was therefore removed in a subsequent step, by transferring the extract to a 2 cc. volumetric flask and adding 2 cc. of 25 per cent cupric sulfate and enough 1 per cent calcium hydroxide suspension (usually about 2 cc.) to make the mixture alkaline to litmus. When the volume of the extract was too large, it was first concentrated in vacua to about 1 cc. Several other methods of removing proteins proved to be less satisfactory than this combination. Large amounts of fatty substances in the skin extracts made precipitation and filtration difficult. Calcium sulfate, which sometimes separated during the assay of the filtrates for tryptophane, was removed by centrifugation just before measuring the color in the photometer. Some of the carcass and skin blanks showed noticeable coloration. As outlined, analysis accounted fairly completely, but variably, for tryptophane which had been added to carcass tissue (85 to 1 per cent) and to liver (93 to 1 per cent). Recoveries were rather poor from kidney tissue (43 to 54 per cent) and from the skin (44 per cent), and intermediate from leg muscle (52 to 8 per cent). Freezing the tissue afforded no advantage and seemed even to interfere with tryptophane recovery. Table II shows that in rats fed tryptophane less than a third of the amount absorbed could be detected in the body, even in the 1st hour after the feeding. That none was found in the muscle may be ascribed in part, but not entirely, to the small sample analyzed; this observation Downloaded from by guest on April 9, 218

5 D. M. BUCK AND C. P. BERG 15 agrees with that of Luck (1) that feeding amino acids induced no significant change in the amino nitrogen content of muscle tissue. Except in a single instance no tryptophane could be found in the kidney. Its concentration in the liver varied widely, being reasonably consistent only in the 3 hour tests, in which it accounted for about a fifth of the total tryptophane found in the body. Either of the two processes ascribed to the liver, deamination to indolepyruvic acid and conversion to kynurenine and kynurenic acid, produces metabolites unresponsive to the tryptophane Rat TABLE Absorption and Distribution of Tryptophane in Rats Fed Tryptophane after 18 Hour Fast No. Rat weight Tryptophane fed Time for absorption Tryptophane absorbed II Skin Tryptophane found* Liver I carcast Total sm. w. hrs. m&t. %. mg. +w. m&t * No tryptophane was found in any of the gastrocnemius muscle samples analyzed and, except in one test, none was found in the kidneys. Hence these analyses are not tabulated. t Includes blood and all of the body except the gastrointestinal tract and the parts analyzed separately. $ The kidneys of this animal contained.4 mg. Downloaded from by guest on April 9, 218 test. Appreciable urinary excretion of kynurenic acid would likely stimulate the kidney to produce ammonia, possibly even from tryptophane, if this were readily available. The skin retained tryptophane temporarily in amounts representing 13 to 37 per cent of the total in the 1st and 2nd hours, and 2 to 8 per cent in the 3rd. Usually over half of the total tryptophane detected was in the carcass fraction, which included the blood. The capacity of these tissues to destroy tryptophane is unknown. Retention of tryptophane as such by the tissues appears to be only temporary and of secondary significance in lowering the tryptophane content of the blood.

6 16 TRYPTOPHANE &tetabolism, XI The introduction of certain substituents into the aamino group prevents tryptophane from promoting growth in the rat or yielding kynurenic acid in the rabbit. Obviously, therefore, the substituent must either block metabolism entirely or alter its course profoundly. In the latter event, measurable differences in rate of metabolism could conceivably facilitate interpretation of the observations with tryptophane. With this in mind, methylenetryptophane and phenacetyltryptaphane were tested for their influence upon the urea and the nonprotein nitrogen content of the blood, as tryptophane had been. Neither of these derivatives promotes growth (7,8) ; hence neither would be expected to produce kynurenic acid (methylenetryptophane is known not to do so (9), but phenacetyltryptophane has not been similarly tested). Both compounds are relatively insoluble in acid solution and consequently highly susceptible to precipitation with or adsorption on the tungstic acid precipitate. This probably explains why blood filtrates prepared after the feeding of these derivatives failed to produce a color in the test for tryptophane (Table III), except in the 2nd hour after phenacetyltryptophane administration, when a weak response was noted. Both compounds caused a rise in urea which was not followed by the fluctuations observed consistently in the tests with tryptophane. This seems to argue against failure to undergo absorption and catabolism, though partial excretion unchanged (9) and incomplete catabolism are probable. The rise in urea noted after feeding phenacetyltryptophane was particularly striking. To determine whether phenylacetic acid could have been liberated and thus involved, phenylacetic acid was fed as the sodium salt. The blood urea did not increase steadily as after the ingestion of phenacetyltryptophane, but showed a rather long initial depression which probably reflected diversion of amino nitrogen for detoxication of the phenylacetic acid. Since the acid was fed as the salt, appreciable diversion for the synthesis of urinary ammonia would seem unlikely. The marked rise in blood urea which followed this depression may reflect readjustments incident to restoration of the normal metabolic state upon release of the stress of mobilizing amino nitrogen for the conjugation. Downloaded from by guest on April 9, 218 DISCUSSION Analysis of the data in the light of previous observations seems to admit of the following interpretation pertaining to animals which excrete kynurenic acid. The early rise in urea and the rapid removal of tryptophane from the blood after tryptophane is fed indicate that the initial steps of its metabolism are begun promptly. This is substantiated in the rat by failure to account for more than a third of the tryptophane absorbed, even

7 D. I& BUCK AND C. P. BERQ 17 TABLE III Changes in Concentration of Urea and NonProtein Nitrogen in Blood of Rabbit Given Methylenetryptophane, Phenacetyltryptophane, or Phenylacetic Acid by Stomach Tube Weight kg I Substance fed Hethylenetryptophane (2.34 gm.) Methylenetryptophane (2.22 gm.) Phenacetyltryptophan (2.4 gm.) Phenacetyltryptophan (2.2 gm.) Phenylacetic acid (1.O gm. as Na salt). e e T ime after feeding hrs Urea N %I Nonprotein Gtrogen Jndeternined 1. _ zr % ::r % % * * t * The color produced in the tryptophane test was equivalent to.2 mg. of tryptophane N per 1 cc. of blood in the initial period,.8 mg. in the 2 hour period. t The color produced in the tryptophane test was equivalent to.3 mg. of trvntnphane N per 1 cc. in this period. N Downloaded from by guest on April 9, 218

8 18 TRYPTOPHANE METABOLISM. XI in the 1st hour. Earlier studies on rats fed even larger doses of Ztryptophane suggest that excretion in the urine was not involved (1). Theoretically, at least, ammonia can be produced metabolically from tryptophane either before or after rupture of the indole ring. The noticeable depression in blood urea which followed the initial peak in every test on tryptophane suggests diversion of nitrogen from urea synthesis, possibly to provide ammonia for the excretion of acidic metabolites, such as kynurenic acid. This supposition is strengthened by the observation that interference with deamination and kynurenic acid synthesis by blocking the aamino group of tryptophane prevented the fluctuations in blood urea noted with t.he free amino acid. If the gradual increases observed in the urea nitrogen were produced by the metabolism of the methylene or the phenacetyl derivative fed, initial rupture of the indole ring must be assumed. Operation of the process involved in the synthesis of kynurenine would yield a derivative of kynurenine containing a free amino group. Since rats fed free kynurenine excrete only a portion as kynurenine or kynurenic acid (lo), at least one other metabolic path, quantitatively even more important than conversion to kynurenic acid, must be available for kynurenine to follow (1). The only paths obviously closed to kynurenine derivat,ives such as the ones suggested are those allowing ultimate production of kynurenic acid. SUMMARY Analysis of the blood of rabbits at intervals after the ingestion of sizable doses of tryptophane reveals changes in the tryptophane, urea, and nonprotein nitrogen contents which begin promptly and follow a characteristic and reproducible sequence. The changes which occur after the ingestion of the methylene or the phenacetyl derivative of tryptobhane follow a different sequence. Suggestions as to the probable course of the metabolism of free tryptophane and the derivatives are made on the basis of these data and on the assumption that the substituent groups alter the course of metabolism chiefly by preventing ready ardeamination and the synthesis and excretion of kynurenic acid. Analysis of a series of rats killed at intervals after they had been fed try@ophane in amounts too low to cause any considerable excretion showed no tryptophane in the muscle or kidney, but appreciable amounts in the liver, skin, and the remainder of the animal during the first few hours. At no time did the total amount found in the rat represent more than a third of the tryptophane absorbed. This supports the conclusion based on observations with rabbits that at least the initial stages of tryptophane metabolism must proceed rapidly. Downloaded from by guest on April 9, 218

9 D. M. BUCK AND C. P. BERG 19 BIBLIOGRAPHY 1. Luck, J. M., J. Biol. Chem., 77, 13 (1928). 2. Johnston, M. W., and Lewis, H. B., J. Biol. Chem., 78, 67 (1928). 3. Shambaugh, N. F., Lewis, H. B., and Tourtellotte, D., J. Biol. Chem., 92, 499 (1931). 4. Shaw, J. L. D., and McFarlane, W. D., Canad. J. Res., Xect. B, 16, 361 (1938); J. Biol. Chem., 132, 387 (194). 5. Koch, F. C., Practical methods in biochemistry, Baltimore, 2d edition, 1 (1937). 6. Keys, A., J. BioZ. Chem., 132, 181 (194). 7. Berg, C. P., Rose, W. C., and Marvel, C. S., J. BioZ. Chem., 86,219 (19293). 8. Berg, C. P., and Hanson, H. E., Proc. Iowa Ad. SC., 41, 165 (1934). 9. Berg, C. P., J. BioZ. Chem., 91, 513 (1931). 1. Borchers, R., Berg, C. P., and Whitman, N. E., J. BioZ. Chem., 145, 657 (1942). Downloaded from by guest on April 9, 218

10 TRYPTOPHANE METABOLISM: XI. CHANGES IN THE CONTENT OF TRYPTOPHANE, UREA, AND NONPROTEIN NITROGEN IN THE BLOOD AND OF TRYPTOPHANE IN THE TISSUES AFTER THE ORAL ADMINISTRATION OF TRYPTOPHANE OR TRYPTOPHANE DERIVATIVES Dorothy M. Buck and Clarence P. Berg J. Biol. Chem. 1945, 159:1119. Access the most updated version of this article at Alerts: When this article is cited When a correction for this article is posted Click here to choose from all of JBC's alerts This article cites references, of which can be accessed free at tml#reflist1 Downloaded from by guest on April 9, 218