Screening of Antioxidant Activity of Some Samia ricini (Eri) Silks: Comparison with Bombyx mori

MATERIALS AND METHODS

This study was performed for 3 months from March 10, 2011 to June 10, 2011. Most of the experiment was performed at Department of Chemistry, Faculty of Science, Mahasarakham University, Thailand.

Materials
Eri cocoons: The cocoons of 5 Eri silk varieties called China, Chiangmai, Lamphun, Thai and SIC and domesticated Thai silk names Nanglai and Dokbua were kindly supplied by Silk Innovation Center (SIC), Mahasarakham University, Maha Sarakham, Thailand. All chemical reagent of analytical grade were used.

Methods
Extraction of sericin solution: The cocoons of all silk varieties were boiled in distilled water with magnetic stirred at 70-75°C for 60 min to extract sericin. The weight ratio of 1 g silk per 10 mL of distilled water was used in this experiment. The sericin solution was then filtrated and collected at 4°C until investigation.

Determination of antioxidant activity and total phenolic content: This investigation of radical-scavenging activity was applied from Chew et al. (2008) by using DPPH (2,2’-diphenyl-1-picrylhydrazyl) method. Ferric Reducing/Antioxidant Power (FRAP) assay was also performed according to previously modified method of Kubola and Siriamornpun (2011) to confirm the activity. In addition, Total Phenolic Content (TPC) of the sericin extract was also investigated which applied from Bonoli et al. (2004).

Statistical analysis: All the results are shown as Mean±SD of triplicate measurement.

RESULTS

Antioxidant activity of the sericin extracts: In present study, the antioxidant activities of sericin extracts was determined by DPPH and FRAP assays as shown in Table 1 and 2, respectively.

DPPH radical scavenging activity: The amount of DPPH radical scavenging of sericin extracts was expressed by % inhibition (Table 1). From results, significant differences in scavenging activity were observed. The highest scavenging activity at concentration of 100 (%v/v) was found for Nanglai, then Dokbua which were 90.4866 and 83.7879% inhibition, respectively. The scavenging of Eri silk was lower than domesticated silk. When Eri silks are compared, SIC (62.6882%) and Chiangmai (62.1863%) demonstrated higher radical scavenging activities, followed by Lamphun (58.3242%), China (35.0862%) and Thai (34.4534%), respectively.

Ferric Reducing/Antioxidant Power (FRAP): The FRAP values of the sericin extracts are shown in Table 2. The results indicated that Dokbua had the greatest reducing power, followed by Nanglai silk. Eri silk had also lower reducing power than that of domesticated silk. Chiangmai, Lamphun and SIC varieties showed similar FRAP values followed by China and Thai varieties.

Total Phenolic Content (TPC): The Total Phenolic Content (TPC) of all silk extracts was investigated using a modified Folin-Ciocalteu method and results were expressed in terms of mg GAE/g as shown in Table 2. TPC of the extracts differed among the silk varieties and was in the range of about 0.2-1.2 mg GAE/g of sericin extracts. Both domesticated Thai silks; Dokbua and Nanglai had nearly equal with about 3 times of TPC higher than Eri silk. Comparison between Eri, the highest of TPC values was obtained from Lamphun, followed by SIC, Chiangmai, Thai while China had the lowest TPC.

DISCUSSION

Sericin, a glue-like protein, is a part of silk fiber that acts as coating substances of fibroin core proteins together. The sericin was excluded and became by-product in textile production process. Recently, it has been known for its potential activity such as antibacterial and antioxidant activities (Sarovart et al., 2003; Wu et al., 2007). Previously, almost antioxidant substances were phenolic compounds.

Table 1:	DPPH scavenging activities of different sericin extracts

Table 2:	Comparison of Total Phenolic Content (TPC) and Ferric Reducing/Antioxidant Power (FRAP) of different sericin extracts

They are widely distributed in plant and have been reported of their potential benefit for human health (Govindarajan et al., 2007). Total Phenolic Content (TPC) of the sericin extracts was investigated using the Folin-Ciocalteu assay. This method was suggested as a fast and reliability to quantify phenolic content (Kubola and Siriamornpun, 2011). The results found that significant differences were observed for TPC among the silk varieties. The TPC and antioxidant activity exhibited good correlation (Butsat et al., 2009). The DPPH radicals are widely used to investigate the radical scavenging activity of antioxidant compounds. Generally, antioxidant molecules acted as H-atom donor to stabilize the DPPH radicals. Scavenging activity of the sericin extract is indicated by % inhibition. The results found that % inhibition values varied from 34.45-90.50%. The highest of % inhibition was found by Nanglai. It may be suggested that Nanglai had higher sericin ratio per silk fiber than Dokbua and all of Eri. The varied radical scavenging activity of the sericin extracts depend on the TPC values in each silk varieties (Butsat and Siriamornpun, 2010). This result suggested that different types of silk composed of different antioxidant molecules and capacity. In addition, FRAP assay was used for investigation the ability of antioxidant to reduce Fe³⁺-Fe²⁺ (Benzie and Strain, 1996). The FRAP values of the sericin extracts indicated that domesticated Thai silk. Dokbua had the highest reducing power equally to Nanglai. This may be due to their higher TPC. The obtained result was positively associated with other reports (Butsat and Siriamornpun, 2010; Govindarajan et al., 2007). The finding results clearly evidenced that domesticated Thai yellow silk (Dokbua and Nanglai) had higher both TPC and antioxidant capacity than that of Eri silk. This may be due to domesticated silk composed of some pigments in their fibers. The main components of silk pigment are carotenoids and flavonoids which corresponding for antioxidant activity (Prommuak et al., 2008). Differences of TPC and antioxidant activity in same types may depend on many factors such as host plant, environmental weather and habitat.

CONCLUSION

The different Total Phenolic Content (TPC) of the sericin extracts was found to be important factors on antioxidant activity. The different values of both TPC and antioxidant activity depend on silk varieties. However, the TPC and antioxidant activity of the sericin extracts showed good correlation. When types of silk were compared, domesticated Thai silk have higher TPC and antioxidant capacity than those of Eri. This may be due to the sericin content of domesticated silk was higher than Eri as well as the pigments composition on the silk fibers.

ACKNOWLEDGMENTS

Author would like to thank Miss Jenjira Jiram and Miss Yardpirun Boonsod my M.Sc students as well as Miss Jiraporn Krasaetep, for their help to do my study. Author also great fully thank Department of Chemistry, Faculty of Science, Division of Research Facilitation and Dissemination, Mahasarakham University and Center of Excellence for Innovation in Chemistry, Commission on Higher Education, Ministry of Education, Thailand for financial support of this study.