Silkworm and silkworm droppings have long been used in China and Korea as a folk remedy for the treatment of diabetes.1-Deoxynojirimycin (1-DNJ) is the hydrogenation product of nojirimycine, which was firstly discovered in Streptomyces. Natural DNJ was first isolated from the mulberry tree (Yoshikaki and Hivonu, 1976), and to date, more than 20 polyhydroxy alkaloids have been identified in mulberry and silkworm (Asano et al., 1994a; 1994b; 2001). As a piperidine alkaloid, DNJ is known to possess highly effective α-glycosidase inhibition activity (Yoshikuni, 1988; Yoshikuni et al., 1988; Hughes and Rudge, 1994) and is an effective anti-hyperglycemia agent. Currently, DNJ and its analogs have been extracted from a wide range of plants and microbes (Asano et al., 1998; 2000; Kim et al., 1999), but its content in the mulberry tree is the highest compared with other plants (Kimura et al., 2007; Yatsunami et al., 2008). Mulberry has been used in Chinese medicine against diabetes mellitus for a long time. Ryu et al. (1997) first reported that the silkworm larval powder of the 5th instar (prepared by lyophilization) had a positive effect on diabetic patients (Ryu et al., 1997; 1999), and the action of lowering the blood sugar level was further proved by subsequent research (Han et al., 2007). Silkworm powder possesses blood glucose-lowering effects (Ryu et al., 2002), and mulberry leaves, which form the diet of silkworms, effectively inhibit α-glucosidase in the human small intestine (Oku et al., 2006). DNJ from silkworm powder is stable upon heating to 121℃ for 15 min (Yatsunami et al., 2011). In recent times, in Korea, Japan, and China, mulberry and silkworm larva products are becoming a popular auxiliary therapy for diabetes mellitus. In this report, we measured the content of 1-DNJ in the powder according to different heating temperatures and investigated the glucoselowering effect of heated silkworm powder on db/db mice.

Silkworm larvae (Bombyx mori) were reared by feeding mulberry leaves during the spring season in 2012 at the National Academy of Agricultural Science. The silkworm varieties used for the experiment was Yeonnokjam. The larvae of 3^rd d of the 5th instar were quickly frozen with in liquid nitrogen and lyophilized.

Heat treated at 60℃, 100℃ and 150℃ for 30 min in the oven.

DNJ content was measured according to the method reported by Kim et al. (2003). DNJ in the silkworm extract powder was extracted with 0.05 mol/L HCl, treated with 9-fluorenylmethyl (FMOC) to produce the DNJ-FMOC complex, and finally analyzed by high-pressure liquid chromatography (HPLC).

Male C57BL/KSJ-(db/db) mice (6 wk old) were purchased from Japan SLC Inc. (Japan). Mice were housed in a conventional cage at the appropriate temperature (23℃ ± 3℃) and humidity (55% ± 15%) under a 12-h light/dark cycle, and had free access to food and water. All the groups were fed a standard diet (certified irradiated global 18% protein rodent diet). After a 1-wk adaptation period, the 7-wk-old mice were divided into five groups (n = 10 in each group): G1( control group : no silkworm powder), G2(T0 ; silkworm extract powder no- heating), G3(T60 ; 60℃ / 30 min), G4(T100 ; 100℃/30 min), and G5(T150 ; 150℃/30 min), the silkworm powder was remade with diet.

The mice were fasted for 3 h, and then blood samples were taken after autopsy. The biochemical analysis of blood included the measurement of the levels of TG(Triglyceride), TCHO(Total cholesterol), LDL(Low density lipoprotein), GLU(Glutamic acid), AST(Aspartate aminotransferase), and ALT(Alanine aminotransferase), which was performed with a blood biochemical analyzer (AU680, Beckman Coulter, Japan). The fat weight was estimated by measuring the circumference of the perirenal area and of the epididymis at autopsy.

The content of 1-DNJ in silkworm powder of Yeonnokjam pupae at d 3 of the 5^th instar heated at different temperatures was determined. The content of 1-DNJ in the powders heated at 60℃ and 100℃ for 30 min was similar to that of the powder without heat treatment. However, in the powder treated at 150℃ for 30 min, the 1-DNJ content was approximately half of that in the other samples (6.83 mg/dL). The amount of 1-DNJ remained the same when the powder was heated to 121℃ for 15 min (Yatsunami et al., 2011). Therefore, 1-DNJ in the silkworm powder was relatively stable at up to 100℃ for 30 min and 121℃ for 15 min (Table 1). The mice were fed these diets for 3–4 wk.

[Table 1.] 1-Deoxynojirimycin content in silkworm powder as a function of the heating temperature

Our analysis focused on the comparison between the changes in body weight of the treatment groups and the control group. In the T60 group, a statistically significant weight gain inhibition was observed compared to the T0 group (Fig. 1).

[Fig. 1.] Change in body weight of db/db mice.

The weekly diet used in the experiments consisted of a fixed amount. The feed intake was observed once per week for 4 wk after silkworm powder administration. Mice in the T0, T60, T100, and T150 groups showed statistically significant reduction in feed intake compared to the control group (no silkworm powder administration) at 3-4 wk after administration (Fig. 2).

[Fig. 2.] Change in body weight of db/db mice administered silkworm powder.

We prepared a fixed amount of water the day before water intake, for determining water intake quantity. The water that remained unused in the period of 24 h was subtracted from the total amount of water that was offered daily, and the difference was considered as the water intake quantity (g/mouse/d). The water intake quantity of each cage was the sum of each individual water intake value in each cage. The result showed no significant difference among the different groups (Fig. 3).

[Fig. 3.] Change in water consumption in db/db mice administered silkworm powder.

In the T0 and T60 groups, a statistically significant reduction in the blood glucose level was observed compared to the control group (P < 0.05 and P < 0.01, respectively). However, in the T100 and T150 group, no significant reduction in the blood glucose level occurred. This result indicates that the silkworm powder as a supplement raw material has to be sterilized at temperatures below 100℃ (Fig. 4). Heat-treated silkworm powder also had a decreased effect in blood glucose levels, but the non-heat-treated silkworm powder was 6-52% better than the heat-treated silkworm powder in reducing blood glucose level.

[Fig. 4.] Blood glucose-lowering effect of silkworm powder in db/db mice.

Blood biochemical analysis indicated that in the T150 group, the level of AST was lower than that in the other groups. The levels of TCHO in the T100 group significantly decreased compared to the control groups. The levels of TG in the T0 and T60 mice were significantly decreased compared to the control groups. For GLU level, no treatment showed significant changes compared to the control groups, but GLU decreased by approximately 20% compared to control group without silkworm powder treatment (Fig. 5).

[Fig. 5.] Clinical biochemical analysis of the blood of db/db mice administered silkworm powder.

The epididymal fat weight of mice in the T0, T60, and T150 groups significantly decreased compared to the control groups, while the perirenal fat weight of mice in the T60 and T150 groups significantly decreased compared to the control groups (Fig. 6).

[Fig. 6.] Change in body fat in db/db mice administered silkworm powder.