Studies on the metabolism of amino acids in the silkworm: - arginase, ornithine- transaminase and side chain transaminases
Hsu, T-s.; Chu, J-h.; Lin How.; Tsou, B-s.
Acta Entomologica Sinica 231: 1-8
ISSN/ISBN: 0454-6296 Accession: 021820128
Nutritional studies indicate that insects, in general, are unable to synthesize arginine. In some insects this nutritional requirement can be replaced by citrulline, but not ornithine, and the evidence for the existence of the urea cycle in insects is scanty. In the fat body tissues of Bombyx mori and Philosama cynthia ricini the existence of ornithine transcarbamylase or argininosuccinase was not observed. This indicates that insects cannot synthesize arginine from ornithine in the urea cycle. Arginase, ornithine-.delta. transaminase and side chain transaminases (valine, leucine, isoleucine-glutamate transaminases) were detected in the fat body of B. mori and P. cynthia ricini at different stages during development. Arginase and side chain transaminases were the highest in the fat body of 5th instar larvae and adults and lowest in the embryo and pupae. The ornithine-.delta. transaminase was highest in the fat body of the 5th instar larvae, but in contrast to the arginase and side chain transaminases, higher in the pupae. Tissue preparations of the fat body form ornithine, urea, glutamic acid and proline at the expense of arginine and .alpha.-ketoglutaric acid. In the presence of boric acid less ornithine, urea and proline were found, but ornithine-.delta. transaminase was not inhibited by boric acid. The pathway from arginine to proline via ornithine with the formation of urea occurs in the fat body of the silkworm, and the formation of proline at the expense of arginine and .alpha.-ketoglutarate are the results of 3 sequential enzymatic reactions, i.e., arginase, ornithine-.delta. transaminase and pyrroline-5-carboxylate reductase. The larger amount of urea in the feces of P. cynthia ricini as compared with B. mori agrees with the higher activity of arginase in P. cynthia ricini. The biochemical regulation of these enzymes was also discussed.