Host plant urease in the hemolymph of the silkworm, Bombyx mori

Urease activity was detected in the hemolymph of the silkworm, Bombyx mori from the beginning of spinning to the pharate adult stage if the larvae were reared on mulberry leaves throughout the 5th-instar (the last larval instar). In contrast, no urease activity was detected in the hemolymph of insects fed artificial diets, resulting in accumulation of urea during the spinning stage. To identify the hemolymph urease, the enzyme was highly purified from the hemolymph of the spinning larvae that had been reared on mulberry leaves and the properties of the purified enzyme were compared with those of the mulberry leaf urease. Four out of six monoclonal antibodies raised against jack bean seed urease cross-reacted equally with the silkworm hemolymph urease and the mulberry leaf urease. Under reducing conditions, the hemolymph urease and the mulberry leaf urease migrated at 90.5 kDa on SDS-PAGE gels. The first 20 N-terminal sequence of the hemolymph urease revealed complete identity with that of the leaf urease. The optimum pH for activity and Km value for urea were almost the same for the two enzymes. In conclusion, these two ureases are very likely identical, strongly suggesting that the mulberry leaf urease passes through the larval gut wall into the hemolymph without being digested. In addition, oral administration of mulberry leaf urease just before spinning induced considerable urease activity in the hemolymph of the larvae, but the same treatment did not induce enzyme activity in the hemolymph of the larvae three days before the onset of spinning. These results suggest that the silkworm larvae acquire the host plant urease specifically at the end of the feeding stage in order to degrade urea accumulated in the hemolymph.