Kynureninase and its activity during metamorphosis of the silkworm, Bombyx mori

Abstract Kynureninase which converts kynurenine and 3-hydroxykynurenine to anthranilic and 3-hydroxyanthranilic acid, respectively, was studied in the silkworm, Bombyx mori . The enzyme activity was found only in Malpighian tubules. The enzyme has a pH optimum of 8.0, the K m values are 7.4 × 10 −5 M for kynurenine and 4.2 × 10 −5 M for 3-hydroxykynurenine. The enzyme probably occurs as a holoenzyme which partially dissociates into apo- and co-enzyme (pyridoxal phosphate). A standard assay for silkworm kynureninase was established, and the enzyme activity during metamorphosis was investigated. Kynureninase activity in Malpighian tubules is high during the larval stages. It disappears at the onset of spinning, and thereafter no activity is observed up to the first half of adult development. The enzyme activity reappears when the meconium begins to accumulate in the newly formed rectal sac, and thereafter increases rapidly. The possibility that kynureninase activity in Malpighian tubules can account for the changes in 3-hydroxykynurenine concentrations observed in the whole insect during metamorphosis is discussed.