Microbial & Biochemical Technology Isolation of Antioxidant Compounds from Aspergillus Terreus LS01

The antioxidative activity of an ethyl acetate extract from Aspergillus terreus LS01was evaluated using various in vitro assays: 1,1-diphenyl-2-picrylhydrazyl (DPPH), hydrogen-peroxide-scavenging free radical scavenging, and β-caroten-linoleat model assay and the antioxidants in the extract were isolated and identified. Silica gel column chromatography was used to separate the extract into five fractions (F1-F5). Fraction 3 possessed significant antioxidative activity with an IC 50 of 19.91µg/ml. Fraction 3 was further separated by column chromatography and yielded the crystalline compounds 1 and 2, which were identified as terreic acid and terremutin, respectively, on the basis of UV-vis spectra, MS and NMR analyses. Terreic acid and terremutin exhibited the highest level of DPPH free radical scavenging activity with IC 50 values of 0.115±4.02 and 0.114±2.19 mM, respectively. These compounds also showed scavenging activity of hydrogen peroxide radicals which terreic acid exhibited higher than terremutin with 74.07±1.48% and 33.74±2.81% respectively. In the β-caroten-linoleat model assay, the inhibition of terreic acid retained 26.01±1.14% and terremutin retained 32.29±2.23%. This is the first report on antioxidative activity of terreic acid and terremutin from A.terreus . The findings indicated that A. terreus could be considered as potential source of natural antioxidant products.


Introduction
Free radicals and oxidants play a dual role, since they can be either harmful or helpful to the body. They are produced continuously in all cells as part of normal cell metabolism in situ or from environmental sources (pollution, cigarette smoke, radiation, medications, etc). Overproduction of free radicals in the body can generate oxidative stress, a deleterious process that plays a major part in the development of chronic and degenerative diseases, such as arteriosclerosis, diabetes, and cancer initiation, and has also been implicated in the aging process [1][2][3]. Therefore, antioxidant therapy represents a promising treatment for the effects of oxidative stress. Antioxidants can prevent the oxidation of substrates when present at low concentrations [4], act as radical scavengers, and inhibit lipid per-oxidation and other free radical mediated processes; and so are able to protect the body from several diseases attributed to the reactions of radicals [2]. Therefore, attention has been directed toward the isolation of natural antioxidants to substitute for synthetic counterparts.
A number of plants and mushrooms (fruiting bodies) are known to produce antioxidants but there are few reports on lower fungi [5]. Filamentous fungi produce a diverse array of secondary metabolites, which have a tremendous impact on society and are exploited for their antibiotic and pharmaceutical activities such as anticancer, antitumor, and antioxidant effects [6]. Fungi belonging to the Aspergillus genera are among the major contributors to secondary metabolites of fungal origin [7]. Several antioxidants such as 3,3-di-OH terphenyllin, 3-OH terphenyllin, and candidusin B obtained from the extracts of Aspergillus candidus have been reported to have scavenging effects on DPPH radicals [8,9]. Exopolysaccharide has also been reported as an antioxidant, produced by the mangrove entophytic fungus Aspergillus sp [10].
Aspergillus terreus is a ubiquitous fungus isolated from both marine and terrestrial environments [7,11]. This microorganism produces a variety of secondary metabolites that are economically significant, such as the antihypercholesterolemic drug lovastatin and several other metabolites including sulochrin and terrein which have antibiotic activity [12]. There are relatively few studies of antioxidant from A. terreus compared to other Aspergillus species such as A. candidus [8,9], A. awamori [13], and A. fumingatus [14]. The objective of this study was to isolate and evaluate of the antioxidants activity of compounds in an ethyl acetate extract of Aspergillus terreus. Their antioxidant activity was determined by three different assays, namely, 1,1-diphenyl-2picrylhydrazyl (DPPH-radical) scavenging, hydrogen peroxide-scavenging, and β-caroten-linoleat model assay.

General experimental procedures
Melting points were recorded in a Micro melting point apparatus and are uncorrected. Optical rotation values were measured with a Jasco P-2100 polarimeter. The UV-Vis absorption spectra of the active compounds in methanol were recorded on a Hitachi U-1600 spectrophotometer. The mass spectra of the compounds were obtained using high resolution FAB-MS. The nuclear magnetic resonance (NMR) spectra were recorded at 500 MHz for 1 H and 125 MHz for 13 C on a JEOL 500 with TMS as an internal standard. Heteronuclear multiple quantum coherence spectroscopy (HMQC) and heteronuclear multiple bond correlation spectroscopy (HMBC) were used to assign correlations between 1 H and 13 C signals. The chemical shift values (δ) are given in parts per million (ppm), and coupling constant (J) in Hz. Silica gel (60-200 mesh Wako gel) was used for column chromatography. Precoated silicagel 60 F 254 TLC plates (Merck 5554) were used for analytical TLC. A mixture of chloroform and methanol (90:10, v/v) was used as mobile phase for TLC analysis. Compounds were visualized under UV light (at 254 nm) and on spraying with methanolic sulphuric acid (95:5, v/v) followed by heating at 120°C.

Medium and chemicals
Bacto peptone was obtained from Difco. Co (Detroit, MI). Malt extract was obtained from Becton Dickinson Company (Sparks, MD, USA). 1,1-diphenyl-2picrylhydrazyl (DPPH), β-carotene, and hydrogen peroxide were purchased from Wako Chemical Co.Ltd (Osaka, Japan). Tween 40 and quercetin were purchased from Sigma-Aldrich Co. Ltd. (Tokyo, Japan). Solvents used for column chromatography were obtained from Wako Pure Chemicals and distilled prior to use.

Fungal material
The fungus A. terreus LS01 was isolated from the Teluk Kodek area, Pemenang, West Lombok, Indonesia. It was identified as Aspergillus terreus Thom., according to its morphological characteristics and 28S rDNA sequence. The fungus has been deposited in the LIPI Microbial Collection (LIPIMC) Microbiology Division Research Center for Biology-LIPI. It was prepared on potato dextrose agar plates and stored at 4°C.

Fermentation and extraction
A. terreus LS01 was grown on PDA medium (dehydrated mashed potatoes 2% w/v, glucose 2% w/v, pH 5.5) for seven days at 25°C. Liquid culture experiments were conducted in a 250 ml Erlenmeyer -flask containing 50 ml of PMP medium (2.4% potato dextrose broth, 1% malt extract, and 0.1% peptone). The flasks were incubated at 25°C with shaking at 100 rpm for seven days. Ethyl acetate (EtOAc) (3x1 L) was added into the culture broth (3 L) and extracted for 20 min by vigorously shaking. The EtOAc extract was concentrated with a rotary evaporator under vacuum at 40°C to dryness and a brown paste (830 mg) was obtained.

Determination of DPPH radical-scavenging activity
The free radicals scavenging activities of extracts were measured by using 1,1-diphenyl-2-picryl-hydrazyl (DPPH). Each extract in methanol (4 ml) at a concentration of 10-200 µg/ml was mixed with 1 ml of a methanolic solution containing DPPH radicals at 1 mM. The mixture was shaken vigorously and left to stand for 30 min in the dark, and the absorbance was then measured at 517 nm against a blank [15]. The ability to scavenge the DPPH radical was calculated using the following equation: DPPH scavenging effect (%) = [(A0-A1/ A0) × 100], where A0 was the absorbance of the control reaction and A1 was the absorbance in the presence of the sample. The percentage of scavenging activity obtained was subsequently plotted against the sample concentration. The half maximal inhibitory concentration (IC 50 ) was calculated from the graph of antioxidant activity percentage against sample concentration. The assays were carried out in triplicate and the results expressed as mean values ± standard deviations. Quercetin was used as a reference compound.

Hydrogen peroxide radical scavenging (H 2 O 2 ) activity
A solution (4 mM) of hydrogen peroxide was prepared in phosphatebuffer saline (PBS, pH 7.4). Hydrogen peroxide concentration was determined spectrophotometrically from absorption at 230 nm with molar absorbtivity of 81 M -1 cm -1 [15]. Samples (200µg/ml) in 4 ml distilled water were added to hydrogen peroxide solution (0.6 ml). Absorbance of hydrogen peroxide at 230 nm was determined 10 min later against a blank solution containing samples in PBS without hydrogen peroxide. The percent inhibitory activity was calculated as follows: % Scavenged (H 2 O 2 ) = (A0 -A1/A0) X 100 . Where; A0 is the absorbance of control and A1 is the absorbance of sample. Ascorbic acid was used as positive control.

β-Carotene-linoleate model assay
Antioxidant activity of fractions and isolated compounds were determined using the β-carotene-linoleat model system [16]. A solution of β-carotene was prepared by dissolving 2 mg of β-carotene in 10 ml chloroform. Two milliliters of the solution were transferred into a boiling flask containing 20 mg linoleic acid and 200 mg Tween 40. Chloroform was removed using a rotary evaporator at 40°C for 5 min and, to the residue, 50 ml of distilled water was added, slowly with vigorous agitation, to form an emulsion. 4.8 ml aliquots of the emulsion were transferred into different test tubes containing 0.2 ml of samples in methanol at 0.5 mg/ml. The tubes were shaken and incubated at 50°C in a water bath. As soon as the emulsion was added to each tube, the zero time absorbance was measured at 470 nm using spectrophotometer. Absorbance readings were then recorded at 15 min interval until the control sample had change color. A blank, devoid of β-carotene, was prepared for background subtractions. Antioxidant activity was calculated using the following equation: % Antioxidant activity (AA)= 100 x [1-{(A s0 -A se )/(A c0 -A ce )}], where A s0 and A se were absorbance of the sample at 0 and 120 min, and A c0 and A ce were absorbance of the control at 0 and 120 min. The assays were carried out in triplicate and results expressed as mean values ± standard deviation. Ascorbic acid was used as standard.

Isolation and identification of antioxidative compounds
The major components in ethyl acetate extract of A. terreus LS01 were separated on a silica gel column chromatography into five fractions. Antioxidant activities of all the fractions obtained were measured using DPPH method at range concentrations 50-200 µg/ml. For each fraction, the IC 50 value was calculated and presented in Table  1. The IC 50 value is the concentration which causes a 50 % inhibition of the DPPH radicals. The lower IC 50 value, the greater antioxidant activity. The highest scavenging on DPPH free radical activity was shown by fraction F-3 (250 mg) with IC 50 =19.91µg/ml, and exhibit scavenging of hydrogen peroxide radicals (51.58±1.48%) and hider bleaching β-carotene (30.69±3.05%) at 200 µg/ml of sample solution (Table 1). Fraction 3 also gave a relatively good recovery (31.25%) from ethyl acetate extract than other active fractions; therefore it is more potential for further purification. F-3 was further fractionated by column chromatography (F3.1-F3.3). Fraction F3.1 (5mg) was not tested for antioxidant assays due to the sample limitation. Fraction F3.2 was recrystallized from ethyl acetate to yield pale yellow needles (compound 1,77 mg). Fraction F3.3 was recrystallized from acetone to yield a colorless solid (compound 2,30 mg The coupling constants (J = 3.7 Hz) between δ 5-H 3.89 and δ 6-H 3.86 and signal at C-5 (51.63) and C-6 (53.84) indicated these carbons to be attached to an oxygen atom forming the cis-epoxide, consistent with the findings of others [17,18]. Analysis of the 13 C NMR spectrum revealed the presence of two carbonyl ketone groups at δ C 182.55 (C-4) and 190.79 (C-1), suggesting that 1 is a quinone. In order to determine the connection between the partial structures and assign the NMR signals, 2D NMR spectra (HMQC and HMBC) were measured. In the HMBC spectrum (Figure 1), long range correlations between H-6 and C-1 and C-2 and correlations of H-5 and C-3 and C-4 established the position of the epoxide group [18]. From the data for 1 H and 13 C, the compound is an epoxy-quinone. Based on UV-vis, MS, 1 H NMR and 13 C NMR data, 1 was identified as (1S,6R)-4-hydroxy-3-methyl-7-oxabicyclo[4.1.0]hept-3-ene-2,5-dione (Terreic acid). The spectral characteristics of the compound were found to be identical with those in the literature [19][20][21][22]. The structure of 1 is shown in Figure 2.
The 1 H and 13 C NMR spectra of 1 and 2 were similar. However a significant difference was found around the quinone moiety of 2, with the appearance of a singlet proton signal at 4.68 ppm and a high field shift of the C4 carbon signal from 182.55 to 66.17 ppm. These results clearly indicated the carbonyl group of 1 to be replaced by a hydroxymethine group in 2. This structure was further supported by the HMBC data (Figure 1). In the 1 H NMR spectra of 2, coupling constants (J 4,5 = 1.4 Hz) indicated a trans-didydroquinone epoxide [19,24,25]. This value coincided with that for a trans-dihydroquinone epoxide [18]. The molecular formula of 2 was determined by high resolution FAB-MS [calcd and measured have same value for C 7

Antioxidative activities of isolated compounds
Terreic acid is a quinone epoxide whereas terremutin is a dihydroquinone epoxide. Both are produced by several members of the genus A. terreus and have been reported as antibiotic compounds Data are presented as the mean ± S.D. of triplicate measurements *Antioxidant activity determinant at concentration 200µg/ml of samples   [22, [25][26][27]. However, the antioxidative properties of these compounds are reported here for the first time. This study, we assessed the free radical scavenging activity of compounds by using DPPH radicals and hydrogen peroxide radicals and β-caroten linoleic system assay methods.
The DPPH method is considered to be a model compound of a lipophilic radical. DPPH is a free radical stable at room temperature,and produces a purple color solution in methanol. It is reduced in the presence of antioxidant molecule, giving rise to a yellowish methanol solution. One of the mechanisms involved in antioxidant activity assay is the ability of a molecule to donate a hydrogen atom to a radical, and the propensity of the hydrogen donation is the critical factor involved in free radical scavenging [28]. The DPPH radical scavenging activities of isolated compound are presented in (Figure 3). The compounds were able to scavenge DPPH free radicals in a concentration-dependent manner, with an IC 50 of 0.115±4.02 and 0.114±2.19 mM, respectively. However, the activity of quercetin with IC 50 of 0.033±1.02 mM was stronger than these compounds.
The ability of terreic acid and terremutin to effectively scavenge hydrogen peroxide were determined according to the method of Ruch [15], in which those were compared with ascorbic acid as standard ( Figure 4). Terreic acid had higher potency of hydrogen peroxide radical-scavenging activity (74.07±1.48%) than terremutin (33.74±2.81%) and similar with ascorbic acid (77.13±1.40%) at 200 µg/ ml concentration.
The study assessed the lipid peroxidation inhibitory activity of the isolated compounds by the β-caroten-linoleic model assay ( Figure 4). The system containing 200 µg/ml of samples, terreic acid retained 26.01±1.14% and terremutin retained 32.29±2.23% of the initial β-carotene after 120 min of the assay whereas ascorbic acid as standard retained only 11.8±0.5%. The mechanism of bleaching of β-caroten is a free-radicals-mediated phenomenon resulting from the hydroperoxides formed from linoleic acid. The linoleic acid free radical attacks the highly unsaturated β-caroten models. The presence of different antioxidants can hinder the extent of β-caroten-bleaching by neutralizing the linoleat-free radical and other free radicals formed in the system [16]. Accordingly, the absorbance decreased rapidly in sample without antioxidant whereas in the presence of an antioxidant, the colour could be retained for a longer time ( Figure 5).
In this study, terreic acid and terremutin showed potential scavenging activity toward free radicals. The antioxidative activity of these compounds is due to a quinine group which serves as an effective single electron acceptor, acts as a free radical chain-breaker by reacting with the alkyl radical [29]. This mechanism is similar to that of α-tocopherolhyroquinone [30].

Conclusion
The obtained result o in this study clearly demonstrated that terreic acid and terremutin from ethyl acetate extract of A. terreus exhibited significant antioxidant activity. To our knowledge, this is the first report on antioxidative activity of terreic acid and terremutin isolated from A.terreus. The findings indicated that A. terreus could be considered as potential source of natural antioxidant products. Further investigations to explore the antioxidant compound from A.terreus are underway in our laboratories.