Linkage of nutrition and silk production in Bombyx mori

M. Nakamura & C. Hirayama

Dept. of Sericulture, National Inst. of Sericultural and Entomological Sci., 1-2 Ohwashi Tsukuba Ibaraki 305-8634 Japan

Silkworm, Bombyx mori, is known to produce a large amount of silk-protein. There might exit unique mechanisms regarding silk production. Although Silkworm can be reared on artificial diets, silk production of the larvae reared on fresh leaves of mulberry, Morus alba, was larger than that of the larvae reared on artificial diets. This suggests mulberry leaves, the natural host of silkworm, have factors effective in silk production. Though usual artificial diets for silkworm contain mulberry leaf powder, it is possible to rear larvae on the diets contained no mulberry leaf powder. However, efficiency in silk production is lowered when larvae are reared on the diets without mulberry leaf powder. Larvae reared on fresh mulberry leaves shows the highest silk production, then larvae reared on usual artificial diet comes next and larvae reared on the diets without mulberry leaf powder is the lowest. These suggest that mulberry leaf have two kinds of factors effective in silk production. One exists only in fresh mulberry leaves and the effectiveness is lost during diet processing. The other shows its effectiveness even in mulberry leaf powder produced by drying in an oven. Though these factors are not clarified, mulberry leaf powder extracted with water and MeOH is effective in silk production. One of the factors kept during diet processing might be dietary fiber. One of the candidates that lose activity during diet processing is mulberry leaf urease that may make a significant contribution to silk production by converting useless urea into nitrogen source of silk-protein. It has been reported that concentration of urea in the hemolymph of larvae reared on artificial diets steeply increases from the beginning of spinning, while it decreases rapidly when reared on fresh mulberry leaves. Interestingly, urease activity was found in the hemolymph of the spinning larvae reared on fresh mulberry leaves, while no urease activity was detected in the larvae reared on artificial diets. Cleavage of urea by urease produces toxic ammonia. However, ammonia could be finally assimilated into the silk-protein by the pathway that involve glutamine systhetase (GS) and NADPH-dependent glutamate synthase (GOGAT). The experiment using methionine sulfoximine, a specific inhibitor of GS, ruled out the contribution of glutamate dehydrogenase, another enzyme which could possibly be responsible for ammonia assimilation. As GOGAT activity have not been demonstrated in other animals, the occurrence of GS/GOGAT pathway would be unique.

Index terms: Bombyx mori, Morus alba, urease, urea, ammonia

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