Ecdysone 20-monooxygenase activity during larval-pupal development of Manduca sexta

Profiles of ecdysone 20-monooxygenase (E-20-M) activity in the fat body and midgut of Manduca sexta were determined during larval-pupal development. The E-20-M activities for both tissues were found to exhibit a single major fluctuation during this 10-day period of development: fat body, a 10-fold fluctuation with peak activity on day 4; midgut, a 60-fold fluctuation with peak activity on day 5. Substrate kinetics revealed that the apparent Km values of fat body and midgut monooxygenases for ecdysone were fairly constant during the instar, 2.42 X 10(-7) M and 4.67 X 10(-7) M, respectively. By contrast, the monooxygenase Vmax values in each tissue fluctuated in a manner both quantitatively and temporally coincident with the fluctuations in enzyme activity. These findings suggest that changes in E-20-M activity are a function of changes in the titer of the enzyme. The possible developmental significance of the fluctuations in E-20-M activity are discussed.