Ecdysone Metabolism in Insects

The current status of the pathway of ecdysone biosynthesis and inactivation in insects is discussed. Evidence is presented demonstrating that rβ-ecdysone is generated from α-ecdysone by fat body, Malpighian tubules, gut, and body wall, but not by blood, oenocytes, muscle, salivary gland, or the prothoracic gland itself (in vitro experiments with prepupal Manduca sexta tissues). Since α-ecdysone does not appear to have demonstrable hormonal activity at physiological concentrations in in vitro tissue systems which cannot metabolize it to rβ-ecdysone, it is suggested that α-ecdysone serves as a prohormone rather than as a hormone. The role of the prothoracic gland in the production of α-ecdysone remains to be defined.