| Abstract: | The prothoracic glands of Manduca sexta synthesize dehydroecdysone, which is rapidly converted to ecdysone through the mediation of a hemolymph enzyme, a 3 β-forming-3-ketosteroid reductase. The hemolymph protein fraction (HPF) containing this enzyme was obtained from diapausing and non-diapausing pupae, isolated abdomens, surgically manipulated pupae, etc., and in all cases had the capacity to affect the conversion of dehydroecdysone to ecdysone. The enzyme is heat labile, is inactivated by trypsin, and has a molecular weight of between 20,000 and 30,000. The data indicate that the conversion of dehydroecdysone to ecdysone exhibits linear kinetics and may be dependent on both the enzyme concentration and the concentration of NADPH at the beginning of the reaction but may be limited by the absolute amount of reducing equivalents after 10 min, under the experimental conditions utilized. The capacity of the enzyme to reduce dehydroecdysone was titered in the hemolymph during the last larval instar and during prepupal and pupal life with maximum capacity exhibited at the beginning of the instar, on day 8 of larval life and at day 1 of pupal life. Even at its lowest point at day 5, 1 ml of hemolymph was able to convert 77 pmol (～35 ng) dehydroecdysone to ecdysone in 1 min. These results require a new interpretation of the control of molting in the Lepidoptera. |
