Recycling of urea associated with the host plant urease in the silkworm larvae, Bombyx mori

Urea concentration and urease activity in the midgut content were compared between larvae of the silkworm, Bombyx mori fed an artificial diet and thos e fed fresh mulberry leaves. A considerable amount of urea was found in the midgut content of the both larvae, however it was significantly lower in t he larvae fed fresh mulberry leaves than in the larvae fed the artificial d iet; average urea concentrations in the midgut content of the larvae fed fr esh mulberry leaves and the artificial diet were 2.9 and 4.6 mu mol/g, resp ectively. Urea in the midgut content seems to be secreted from the insect i tself since the amount of urea in both diets were negligibly small. Urease activity was detected only in the midgut content of the larvae fed fresh mu lberry leaves but not in other tissues of the larvae. On the other hand, no urease activity was detected in the midgut content of the larvae fed the a rtificial diet. Subsequently, to elucidate the role of mulberry leaf urease in the midgut lumen, larvae that had been reared on the artificial diet we re switched to fresh mulberry leaves. The diet switch caused a rapid decrea se in urea concentration in the midgut content and an increase in ammonia c oncentration in the midgut content, suggesting that secreted urea could be hydrolyzed to ammonia by mulberry leaf urease in the midgut lumen. Furtherm ore, to investigate the physiological significance of mulberry leaf urease on urea metabolism of the silkworm, N-15-urea was injected into the hemocoe l, and after 12 h the larvae were dissected for N-15 analysis. A. considera ble amount of N-15 was found to be incorporated into the silk-protein of th e larvae fed fresh mulberry leaves, but there was little incorporation of N -15 into the silk-protein of the larvae fed the artificial diet. These data indicate that urea is converted into ammonia by the action of mulberry lea f urease in the midgut lumen and used as a nitrogen source in larvae fed mu lberry leaves. (C) 1998 Elsevier Science Ltd. All rights reserved.