Tryptophan catabolism in silkworm metamorphosis: Changes in total matter, DNA, free, and protein-bound tryptophan

In an attempt to correlate specific events in tryptophan metabolism to macroscopic changes during metamorphosis of the silkworm mutant rb, the following quantities have been recorded daily from the end of the feeding period through adult ecdysis: fresh and dry wieght, insoluble matter, DNA, free, and protein-bound tryptophan. Special attention has been directed towards methodology in the latter determinations.The data are summarized in Figs. 1 to 4. Based upon maximal development of fifth instar larvae the fresh weight decreases by 85 per cent, while the percentage of non-aqueous matter rises from below 20 to 26 and 44 in females and males, respectively. Total DNA content, which is considered to be the most reliable measure of animal size, decreases by 81 and 82 per cent in two series of experiments, the reduction continuing through the stage of apparently predominant histogenesis in the pharate adult. DNA concentration (9 to 20 μmoles DNA-P/mg insoluble matter) follows an inverted U-curve. The level of free tryptophan (in the range of 30 to 50 μg/g fresh wt. in feeding larvae) rises about fourfold in the beginning of metamorphosis, but subsequently decreases to less than 10 μg/animal. The overall reduction in protein-bound tryptophan is over 3 mg, two-thirds of which are lost during the first 2 days of metamorphosis.These results are discussed in relation to cocoon spinning and macroscopic changes. From a comparison with earlier data on 3-hydroxy-kynurenine accumulation in this mutant it is concluded that practically all tryptophan liberated is catabolized via the kynurenine pathway, thus extending the predominance of this metabolic sequence to insects.