STUDIES ON THE NATURALLY OCCURRING PENICILLINS. AN ASSAY METHOD FOR PENICILLIN G
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1 STUDIES ON THE NATURALLY OCCURRING PENICILLINS. AN ASSAY METHOD FOR PENICILLIN G BY THOMAS C. GRENFELL, JOHN A. MEANS, AND ELLIS V. BROWN (From the Research Laboratory of Chas. Pfizer and Company, Inc., Brooklyn, New York) (Received for publication, December 27, 1946) The present trend toward high purity in commercial penicillin, with emphasis on the penicillin G, has increased the need for more accurate methods of analysis. Since crude penicillin is known to be a mixture of two or more of the naturally occurring penicillins (1) and since biochemical methods of assay become complicated and uncertain when more than two penicillins are present in a mixture, chemical and physical methods seemed to offer the best hope for success. The first step in determining the percentage of penicillin G in a product is to ascertain the total amount of all the penicillins present. Several chemical methods for the assay of total penicillin have been proposed. Herriott (2) described a method based on heating penicillin in acetate buffer, ph 4.6, and determining the increase in ultraviolet absorption at 322 rnp with a spectrophotometer. Scudi (3) has reported a calorimetric method based on the reaction between a special dye and penicillin, which is followed by removal of the unused dye and measurement of the combined dye. The titrimetric procedures include the iodometric method of Alicino (4) and the penicillinase method of Murtaugh and Levy (5). Two titrimetric methods have been developed in this Laboratory.1 One is dependent on alkali inactivation in conjunction with hydrogen peroxide and is similar in principle to the penicillinase method but more rapidiy accomplished. The other method involves only inactivation with alkali, followed by back titration of the excess alkali. The method which we propose for the determination of total penicillin involves merely reading the optical rotation of a solution of known concentration. The specific rotation of pure sodium penicillin G is f 3, whence the molecular rotation is 106 f!i. X 103. This value appears, within the limits of experimental error, to be the same for the other varieties of penicillin, and since these differ relatively little in molecular weight the specific rotation of a sample may be taken as a measure of the total penicillin content. Some decomposition products of penicillin possess optical activity, but observations made on a large series of samples 1 Chas. Pfizer and Company report to the Committee on Medical Research, Office of Scientific Research and Development, August 13,
2 528 ASSAY FOR PENICILLIN G have shown this activity to be insignificant. In fact, almost all commercial penicillin, 800 units per mg. or over, has given extremely accurate values by this method, as shown in Table I. In cases of doubt, total penicillin may be checked by biological means (6) or by one of the chemical met,hods previously mentioned. Our procedure for the estimation of penicillin G is based on the fact that the absorption spectrum of sodium penicillin G in the region 250 to 280 rnp contains the characteristic breaks of the benzyl group at 258 and 263 rnp (Fig. 1, Curve 3). In contrast, other penicillins not containing an aromatic side chain give a non-characteristic absorption with very low values for the optical density in the range 260 to 280 mp. Penicillin X2 has a very high Comparison Sample of Polarisco No. TABLE I ic and Iodometric Assays for Total Penicillin Content Polariscopic Iodometric *e, cent per cent optical density at 280 rnp, whereas the other species have a minimal or low absorption at this wave-length. Macpherson s modification of the Pauly reaction (7) was adapted to the estimation of penicillin X in the presence of a large amount of penicillin G. Sodium penicillin G gives a definite yellow color with the Pauly test, but the presence of more than 0.5 per cent of penicillin X is indicated by the development of a pink color. As can be seen by a consideration of the absorption spectra, the presence of penicillin Xwill greatly affect the accuracy of the penicillin G determination, and the method reported here is not recommended for samples containing more than 1 per cent penicillin X. Fig. 2 shows the absorption spectrum for essentially pure sodium penicillin X (Curve I), while Fig. 1 shows the absorption for a mixture of 95 per cent penicillin G with 5 per cent penicillin X (Curve 1). The assay method for penicillin G consists of measuring the optical 2 We wish to thank Dr. Henry Welch of the Food and Drug Administration for a supply of sodium penicillin S.
3 GRENFELL, MEANS, AND BROWN 529 density at 263 mp and subtracting from this figure the optical density at 280 mp. This difference in optical density is then representative of the amount of penicillin G present. A graph showing the optical density difference plotted against per cent penicillin G in the sample is shown in WAVE LENGTH MU FIG. 1. Ultraviolet absorption spectra of crystalline sodium penicillins in 1 cm. cells (1.8 mg. per ml.). Curve 1,95 per cent penicillin G plus 5 per cent penicillin X; Curve 2, commercial penicillin G (1600 units per mg.) ; Curve 3, pure penicillin G (1667 units per mg.); Curve 4, penicillin K, also identical curve for a mixture of penicillin F species. Fig. 3. The instrument which we have used is a Beckman quartz spectrophotometer. The presence of small amounts of penicillin decomposition products causes a shift in the absorption spectrum which results in higher optical densities. When this decomposition is sufficient to give an optical density higher than 0.10 at 280 rnp, purification of the sample is necessary and can
4 530 ASSSY FOR PENICILLIN G be accomplished by the conversion of the penicillins to the ammonium salts. When the optical density at 280 rnp is 0.10 or less, spectrophotometric readings can be made directly in the manner used below for the purified ammonium salt. However, this condition is comparatively rare and for " WAVE LENGTH MU FIG. 2. Ultraviolet absorption spectra of crystalline sodium penicillins in a 1 cm. cell (1.8 mg. per ml.). Curve 1, penicillin X; Curve 2, penicillin G. the most part a purification will be necessary. Two procedures are used. Procedure A is useful with crystalline penicillin salts or yellow amorphous penicillins having a potency of 1400 units per mg. or better. Procedure B is necessary with some crude amorphous products. The need for a separate method for the more crude samples of penicillin will become obvious lrom a study of the yield of ammonium salts obtainable from different qualities of
5 GRENFELL, MEANS, AND BROWN 533: I w 30 Y g.to k! z.a d c PERCENT PENICILLIN - G FIG. 3. Change in optical density difference (263 to 280 mu), with increase in per cent penicillin G at a concentration of 1.8 mg. per ml. of penicillin. AMMONIUM SULFATE GMS/lOOCC FIG. 4. Recovery of ammonium penicillin with increasing ammonium sulfate concentration. Curve 1, crystalline sodium penicillin G (1667 units per mg.) ; Curve 2, amorphous sodium penicillin (1400 units per mg.); Curve 3, same (1250 units per mg.) ; Curve 4, same (1000 units per mg.); Curve 5, same (900 units per mg.). Penicillin concentration, l.s per cent; ph 7; -5 for 1 hour.
6 632 ASSAY FOR PENICILLIN G product (Fig. 4). In either case, the total penicillin must be determined as indicated previously. EXPERIMENTAL Procedure A-A 180 mg. sample of the product to be analyzed for total penicillin was dissolved in 10 ml. of freshly boiled and cooled distilled water at The angular rotation was determined within a period of 10 minutes and the specific rotation calculated in the usual manner. A solution of pure penicillin G, similarly prepared, is used as a standard in the polariscopic assay. Subsequent determinations of penicillin concentrates are based on this standard. Pure sodium penicillin G, as determined in our laboratory on numerous samples, has a specific rotation of +298 f 3 at 25. The per cent total penicillin was calculated as follows: Specific rotation of sample X 100 Specific rotation of pure sodium penicillin G = y0 total penicillin Results obtained with samples of penicillin from a number of manufacturers are shown in Table I. To each 5 ml. of a solution from the polariscopic assay were added 1.8 gm. of pure ammonium sulfate and 1 drop of 3 per cent ammonium hydroxide. The mixture was slowly cooled to -5 with stirring, and held at this temperature 1 hour. The crystalline slurry was filtered through a prechilled semimicro Biichner funnel and the filter cake was washed with 1 to 3 ml. of ice-cold 40 per cent ammonium sulfate solution. The precipitate was dissolved in 5 ml. of water and the penicillin determined in both this solution and the mother liquor polariscopically. On the basis of the polariscopic assay, the purified ammoniu,m salt was diluted to a concentration equivalent to 1.8 mg. of total sodium penicillin per ml. The solution was placed in a 1 cm. quartz cell and the optical density was determined for the wave-lengths 263 and 280 rnp. From these values, the optical density difference was calculated. (Optical density at 263 rns) - (optical density at 280 rnp) = optical density difference With this value, the per cent of sodium penicillin G in the sample may be determined from Fig. 3. Pure sodium penicillin G at a concentration of 1.8 mg. per ml. exhibits an optical density difference (263 to 280 mp) of Notes-By the ammonium salt precipitation method, materials of 1400 units per mg. quality are sufficiently purified to give absorption spectra approaching those of the pure penicillins (Fig. 1). In the case of the precipitated ammonium salt, more latitude is permissible in regard to the optical density at 280 rnp and this may be considerably higher than the
7 GRENFELL, MEANS, AND BROWN limit on the sodium salt without interfering with the accuracy of the determination. As will be seen from Fig. 4, the yield of ammonium salt from material of 1400 units per mg. is better than 90 per cent. It has been found that the proportion of the various penicillins in the mother liquor is essentially the same as in the precipitate and no correction is necessary for this difference from a quantitative yield. This is illustrated by two examples chosen from a large number of experiments. To 10 ml. of a solution containing 120 mg. of sodium penicillin G, 10 mg. of penicillin K, and 10 mg. of penicillin F was added sufficient ammonium sulfate to give a concentration of 36 gm. per 100 ml. On standing at lo- 15 for 2 hours, a 95 per cent recovery of the penicillins, as the ammonium salts, was obtained by filtering and washing with cold saturated ammonium sulfate. The polariscopic-ultraviolet absorption assay showed 83 per cent of penicillin G and 17 per cent of other penicillins. The amount of penicillin G as added to the original sample was 85.7 per cent. Another sample was prepared from 240 mg. of sodium penicillin G and 60 mg. of sodium penicillin F. It was treated with 20 ml. of saturated ammonium sulfate and 95 per cent of the penicillins were recovered. The polariscopic-ultraviolet absorption assay showed 80 per cent of penicillin G, which exactly corresponds with the amount introduced. Fig. 1 illustrates the absorption spectra of various samples. Curve 4 was obtained by plotting the values for sodium penicillin K and a mixture of F species penicillins in which the penicillin G content was essentially zero. These two penicillin species gave identical curves. Curve 3 represents crystalline sodium penicillin G of practically 100 per cent purity as regards both total penicillin and penicillin G. The minimal value at 280 rnp and the low peak at 320 rnp indicate the absence of penicillin degradation products. Curve 2 represents a crystalline sodium penicillin G in which decomposition has occurred, and the high optical density values at 280 and 320 rnp attest this. The per cent sodium penicillin G, as mentioned in this paper, refers to a ratio of the amount of this species divided by the amount of total penicillin present. The value thus obtained, when considered in comparison with the per cent of total penicillin present, as determined by the polariscopic assay, may be used to find the per cent of penicillin G in the sample. Procedure B-The sample was prepared for polariscopic assay in exactly the same manner as in Procedure A. A somewhat larger sample was used in this case because two different ammonium sulfate precipitations were performed. Total penicillin was determined as in Procedure A and onehalf of the solution was converted to ammonium salt exactly as previously outlined. The other half of the solution was treated with 2.1 gm. of pure ammonium sulfate and 1 drop of 3 per cent ammonium hydroxide for each
8 534 ASSAY FOR PENICILLIN G 5 ml. This mixture was cooled to -5 with stirring and held at this temperature for 1 hour. The ammonium penicillin was filtered and washed as before and then total penicillin on both precipitate and mother liquor was determined. The two purified ammonium salt solutions were diluted to 1.8 mg. per ml. on the basis of the polariscopic assay and the difference in optical density determined as described under Procedure A. Consideration of the different values for the percentage of penicillin G in the total penicillin ammonium salts compared with the yield from the crude penicillin will enable one to extrapolate wit,h reasonable accuracy to give the correct per cent of sodium penicillin G in the sample. Notes---The method of extrapolation gives results of sufficient accuracy for the more crude penicillins. For example, penicillin of 1000 units per mg., shown in Fig. 4 (Curve 4), gave a 62 per cent yield of total ammonium penicillins, of which 88 per cent was penicillin G, when the concentration of ammonium sulfate was 30 per cent (1.8 gm. added to 5 ml.). When this same sample was precipitated at 34 per cent ammonium sulfate concentration (2.1 gm. added to 5 ml.), the penicillin G content was determined as 90 per cent and the yield of penicillin was 74 per cent. Then the per cent of penicillin G was plotted against the per cent yield for ammonium penicillin. From experience with samples of somewhat higher potency, from which several points on the curve have been obtained, this was assumed to be a straight line function. Therefore, on drawing a straight line through the two points at 88 and 90 per cent, we find the value of 94 per cent for the theoretical 100 per cent recovery. DISCUSSION The method as outlined in this report has been for the determination of sodium penicillin G in sodium penicillin mixtures. It is obvious that by using other salts as standards, the method may be applied to any salts of penicillin wherein the base radical has no absorption which interferes in the 260 to 280 mp range. Moreover, other wave-lengths and other concentrations of ammonium sulfate may be used if desired. Variations will be apparent to any worker who applies the method to his particular problem, and further information about the sample may be gained by determining values at additional points. The method reported has been used routinely for several months in this laboratory and is felt to be extremely accurate on high quality material; even with amorphous penicillin having a potency of 900 units per mg. the accuracy is about ~t3 per cent. SUMMARY A polariscopic-ultraviolet absorption method, including an ammonium salt precipitation; is described by which penicillin G may be determined in
9 GRENFELL, MEANS, AND BROWN 535 salts of mixed penicillins. This method is recommended for white crystalline commercial products and for yellow amorphous penicillin of potency as low as 900 units per mg. BIBLIOGRAPHY 1. Committee on Medical Research, Office of Scientific Research and Development, Washington, and the Medical Research Council, London, Science, 102, 627 (1945). 2. Herriott, R. M., J. Biol. Chem., 164, 725 (1946). 3. Scudi, J. V., J. Biol. Chem., 164, 183 (1946). 4. Alicino, J. F., Znd. and Eng. Chem., Anal. Ed., 18, 619 (1946). 5. Murtaugh, J. J., and Levy, G. B., J. Am. Chem. Sot., 67, 1042 (1945). 6. McMahan, J. R., J. Biol. Chem., 188, 249 (1944). 7. Macpherson, H. T., Biochem. J., 36, 59 (1942).
10 STUDIES ON THE NATURALLY OCCURRING PENICILLINS. AN ASSAY METHOD FOR PENICILLIN G Thomas C. Grenfell, John A. Means and Ellis V. Brown J. Biol. Chem. 1947, 170: 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 0 references, 0 of which can be accessed free at ml#ref-list-1
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