Gramicidin-S: Structure–activity relationship

J. Biosci., Vol. 7, Numbers 3 & 4, June 1985, pp. 323-329. © Printed in India.
Gramicidin-S: Structure–activity relationship*
G. NAGAMURTHI† and S. RAMBHAV‡
† Institute of Preventive Medicine, Hyderabad 500 029, India
‡ Department of Biochemistry, Osmania University, Hyderabad 500 007, India
Abstract. The two side chain amino groups of the two L-ornithine residues in gramicidin-S
seem to be important for the antibacterial activity of the molecule, since complete acetylation,
formylation, carbamylation, deamination, trinitrophenylation, succinylation, maleylation of
the antibiotic caused 90–95 % loss of the antibacterial activity of the antibiotic. However this
modification leads to only 12–30% loss of the hemolytic activity. Monoacetyl- and
monoformyl gramicidin-S with a free amino group retains nearly 50% of the antibacterial
activity of the molecule. It seems, therefore, that the two amino groups contribute equally to
the antibacterial activity of gramicidin-S.
Keywords.
Gramicidin-S; chemical modification; formylation; acetylation; carbamylation;
deaminatio; trinitrophenylation; succinylation; maleylation; biological activity.
Introduction
Gramicidin-S is produced by certain strains of Bacillus brevis and the pure peptide is
commercially available (Florey et al., 1949; Ayoagi et al., 1964; Izumiya et al., 1972). It is
a cyclic decapeptide consisting of two identical pentapeptides [D-Phe-L-Pro-L-Val-LOra-L-Leu]2. The exchange of ornithine for lysine or arginine in gramicidin-S, with
retention of the basic properties, did not change antibacterial potency (Schwyzer and
Sieber, 1959). However, exchange of proline for glycine increases the activity,
replacement of both proline and valine by glycine drastically reduces the activity, and
substitution of D -p-aminophenylalanine for D-phenylalanine leads to a nearly inactive
substance (Stepanov and Silaev, 1961). The linear decapeptides related to gramicidin-S
have some antimicrobial property (Katchalski et al., 1955).
Previous investigations from this laboratory have been successful in establishing
structure–activity correlations for linear gramicidins (Rambhav and Ramachandran,
1976), polymyxins (Srinivasa and Ramachandran, 1978) and bacitracins (Rambhav,
1981, 1982, 1984). However, chemical modification of gramicidin-S has not been
studied hitherto, and this paper deals with chemical modification of the side chain
amino groups of the two L-ornithine residues of gramicidin-S and their effects on the
antibacterial and hemolytic activity of the molecule.
*
Presented at 53rd Annual General Body Meeting of the Society of Biological Chemists (India), New Delhi,
October, 1984.
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Materials and methods
Gramicidin-S hydrochloride (potency 985µg/mg) was from Sigma Chemical
Company, St. Louis, Missouri, USA. All other chemicals and solvents were from
commercial sources and purified as and when necessary.
All colorimetric measurements were done with Spectronic 20 colorimeter and
spectrophotometric measurements with a Beckman model DB spectrophotometer.
Amino groups were estimated by the ninhydrin method (Rosen, 1957). Acetyl- and
formyl group contents in the acetyl- and formyl gramicidin-S were estimated by the
method described by Ushalakshmi and Ramachandran (1969a,b). Carbamyl group
content in carbamyl gramicidin-S was determined by the diacetyl monoxime method
(Wooten, 1964). The extent of deamination in nitrous acid treated gramicidin-S was
monitored by the ninhydrin method. Succinyl groups in succinyl gramicidin-S were
estimated by titrating the carboxyl groups of the derivative against standard alkali
using 1 % alcoholic Phenolphthalein as indicator and also by the determination of the
acid group (Fraenkel-Conrat and Cooper, 1944). The degree of substitution of
trinitrophenyl and maleyl groups in trinitrophenyl- and maleyl gramicidin-S were
estimated by spectral analysis according to the procedure of Okuyama and Satake
(1960) and Butler et al. (1969), respectively. Antibacterial activity of gramicidin-S and its
derivatives was determined using Streptococcus faecalis (ATCC 8043), Staphylococcus
aureus (NCIM 2079), two Gram-positive organisms and Escherichia coli (NCIB 8522),
Pseudomonas aeruginosa (NCIB 6750), two Gram-negative organisms, as described
earlier (Rambhav, 1981). Hemolytic activity of gramicidin-S and its derivatives was
carried out according to the method of Dimick (1943), where the rate of disappearance
of absorption at 660 nm in washed suspensions of red blood cells from the rat was
measured.
Preparation of gramicidin-S derivatives
Monoacetyl gramicidin-S: Gramicidin-S (30 mg) and sodium acetate trihydrate
(30 mg) were dissolved in ethanol (2 ml) in a 15 ml centrifuge tube and cooled in an ice
bath. The reaction mixture was then treated with acetic anhydride (10 µl) at 5 min
intervals. The pH of the reaction mixture was maintained at 6·5 by adding few drops of
dilute NaOH solution. A total of 50 µl of acetic anhydride was added and left in the cold
for about 1 h. Later the contents of the tube were diluted to 15 ml with cold water,
mixed well and the flocculum centrifuged. The supernatant was discarded and the
washing of the residue was repeated 2 or 3 times using 15 ml of cold water each time.
The final pellet was dried in vacuo over NaOH pellets. Yield: 22 mg, m.p. 270°C.
Diacetyl gramicidin-S: The procedure used for the preparation of diacetyl gramicidin-S
was similar to that of monoacetyl gramicidin-S, except that the reaction mixture was
treated with a total of 200 µl of acetic anhydride and left in the cold for overnight. Yield:
25 mg, m.p. 272°C.
Monoformyl gramicidin-S: Gramicidin-S (30 mg) was dissolved in 80 % formic acid
(3 ml) in a 15 ml centrifuge tube, cooled to about 5°C in an ice bath, and treated with
acetic anhydride (0·1 ml) at 5 min intervals. A total of 1·0 ml of acetic anhydride was
Gramicidin-S: Structure–activity relationship
325
added and then the reaction mixture was shaken for about 1 h at room temperature.
The derivative was precipitated and isolated according to the method given above.
Yield: 21 mg, m.p. 273°C
Diformyl gramicidin-S: The procedure adopted for the preparation of diformyl
gramicidin-S was similar to that of monoformyl gramicidin-S, except that 98 % formic
acid was used instead of 80 % formic acid and the reaction mixture was left at 4°C for
overnight. Yield: 24 mg, m.p. 270°C.
Succinyl gramicidin-S: Gramicidin-S (30 mg) was dissolved in ethanol (2 ml) in a 15 ml
centrifuge tube and cooled to about 5°C, pH was raised to about 8·0 by adding a few
drops of dilute NaOH solution. Solid succinic anhydride (8 mg) was added to the
reaction mixture and shaken well. The reaction mixture was maintained at pH 8·0 with
dilute NaOH solution for about 2 h. Later the derivatives was precipitated and isolated.
Yield: 27 mg, m.p. 266°C.
Maleyl gramicidin-S: Gramicidin-S (30 mg) was dissolved in ethanol (2 ml) in a 15 ml
centrifuge tube and the pH was raised to about 8·5 by adding a few drops of dilute
NaOH solution. The solution was treated with 1 Μ maleic anhydride in redistilled
dioxane (600 µl). The maleic anhydride solution was added in 6 additions and the pH of
the mixture was maintained at 8·5 by the additions of dilute NaOH solution. The
product was precipitated and isolated following the procedure given above. Yield:
36 mg, m.p. 226°C
Trinitrophenyl gramicidin-S: An ethanolic solution of gramicidin-S (30 mg in 2 ml), in a
15 ml centrifuge tube, cooled to about 5°C in an ice box, was treated with 4% sodium
bicarbonate (0·5 ml) and 0·1 % trinitrobenzene sulphonate in water (0·5 ml). The
solution was then incubated at 5°C in the dark for about 2 h. Later, the product
trinitrophenyl gramicidin-S was precipitated and isolated. Yield: 38 mg, m.p. 216°C.
Carbamyl gramicidin-S: An alcoholic solution of gramicidin-S (30 mg in 2 ml), in a
15 ml centrifuge tube, cooled to around 5°C in an ice bath was treated with 1 Μ
potassium cyanate (1·0 ml). The reaction mixture was kept at 5°C for about 6 h,
maintaining the pH at 7·0. The product, carbamyl gramicidin-S was precipitated and
isolated. Yield: 24 mg, m.p. 276°C.
Deamination of gramicidin-S: An alcoholic solution of gramicidin-S (40 mg in 3·0 ml),
in a 15 ml centrifuge tube cooled to around 4°C in an ice bath, was treated with gaseous
nitrous acid through a capillary tube for about 15 min, and left in the cold for 1 h. Later
the product was precipitated and isolated. Yield: 21 mg, m.p. 259°C.
Results
All gramicidin-S derivatives were dried in vacuo over NaOH pellets. The derivatives
were then dissolved in ethanol (nearly 2 ml) and ice-cold water was added to a slight
opalesence and then stored in the cold for several hours to achieve complete
precipitation. The precipitated derivatives were isolated by centrifugation. The
derivatives, maleyl gramicidin-S and trinitrophenyl gramicidin-S, were further purified
326
Nagamurthi and Rambhav
by gel-filtration on a column of Sephadex G-10 (1·1 × 14 cm), equilibrated with 5%
acetic acid. The weight yields of maleyl gramicidin-S (m.p. 242°C) and trinitrophenyl
gramicidin-S (m.p. 252°C) were 22 and 24 mg, respectively.
The maleyl gramicidin-S showed an increased absorption at 250 nm in 0·1 Ν
alcoholic sodium hydroxide solution compared to the native molecule. The difference
in molar extinction coefficient of 6653 corresponds to 1·98 maleyl groups per molecule.
The increased absorption at 250 nm (E250 = 3360 M–1 cm–1) at pH 8·0 has been used
to calculate the extent of maleylation (Freedman et al., 1968).
Spectral properties of trinitrophenyl amino groups makes it easy to detect these
groups. The molar extinction coefficient of trinitrophenyl-gramicidin- S in 66 % acetic
acid at 345 nm was found to be 27,832, which corresponds to 1·96 trinitrophenyl groups
per molecule. The molar extinction coefficient of ε-trinitrophenyl-α-acetyllysine was
14,200 at 345 nm at pH 4·5 (Goldfarb, 1966). The degree of substitution of different
reagent groups in gramicidin-S is shown in table 1.
Table 1. Degree of substitution of different reagent groups in gramicidin-S and
the Rf value of the derivatives.
Chromatographic homogeneity of gramicidin-S derivatives
Gramicidin-S derivatives were analyzed by thin layer chromatography on silica plates
(Merck, Germany) with pyridine: n-butanol: acetic acid: water (50:75:15:60, v/v) as
the solvent. Monoacetyl- and monoformyl gramicidin-S were detected with ninhydrin
reagent and as well as ultra-violet light. All other gramicidin-S derivatives were
visualized under ultra-violet light and were ninhydrin negative. The Rf values of
gramicidin- S and its derivatives are shown in table 1.
Antibacterial activity. The results on the biological activity of gramicidin-S and its
derivatives are shown in table 2. The 50% inhibitory concentrations (I50%) for
gramicidin-S in the assay using S. faecalis and S. aureus, were 2·0 and 4·2 µg/ml,
respectively. The minimal inhibitory concentrations (MIC) for gramicidin-S using E.
coli and P. aeruginosa, were 300 and 250 µg/ml, respectively. Complete acetylation,
formylation, carbamylation, deamination, trinitrophenylation, succinylation, maley-
Gramicidin-S: Structure–activity relationship
327
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Nagamurthi and Rambhav
lation of the antibiotic resulted in 90–95 % loss of antibacterial activity. On the other
hand monoacetyl- and monoformyl gramicidin-S showed nearly 50% of the anti
bacterial activity of the parent antibiotic. The antibacterial activity of the maleyl
gramicidin-S on demaleylation at pH 2·0 was the same as for gramicidin-S. GramicidinS and its derivatives were found to be less active towards the Gram-negative organisms,
E. coli and P. aeruginosa, and the MIC values were in the range of 250 to 420 µg/ml.
Gramicidin- S was found to cause hemolysis of red blood cell of rat at a cone, of about
50 µg/ml. The chemically modified forms of gramicidin-S were found to retain about
70–88 % of the hemolytic activity of the parent antibiotic.
Discussion
Gramicidin-S display high inhibitory activity towards Gram-positive organisms, with
2–50 µg/ml being effective. Relatively high concentrations (100–300 µg/ml) inhibit the
growth of Gram-negative organisms. The antibiotic binds to cell membranes and
disturbs the organization of lipoprotein systems and destroys the permeability barrier
of membranes. The orientation of the hydrophobic and hydrophilic groups in
gramicidin-S explains its mechanism of action as a surface-active agent. The antibiotic
causes rapid hemolysis of red blood cells at a concentration of about 50 µg/ml.
The results of the present chemical modification approach have established the
structure–activity correlations in gramicidin-S molecule. Complete acetylation, formylation, trinitrophenylation, maleylation, carbamylation, succinilation and deamination
of gramicidin-S resulted in great loss of the antibacterial activity of the molecule. On the
other hand monoacetyl and monoformyl gramicidin-S with a free amino group showed
nearly 50 % of the expected antibacterial activity of antibiotic (table 2). It seems
therefore, that the two amino groups equally contribute to the antibacterial activity of
the molecule. The chemical modification of both the amino groups that neutralized the
cationic charge on the molecule has inactivated the antibiotic and monosubstitution
has resulted in partial loss of activity. Masking of the amino groups has no appreciable
effect on the hemolytic activity of the molecule, as the modified forms of the antibiotic
have retained more than 70 % of hemolytic activity of the parent antibiotic.
Acknowledgements
The authors express their thanks to Dr. K. Rajyalakshmi and Prof. L. K.
Ramachandran for their keen interest and suggestions in this work.
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