Tissue bioassays ecdysone activity

In insects, ecdysone is secreted from the prothoracic glands (PG) at every moulting and pupal stage. Conversion of ecdysone to 20-hydroxyecdysone occurred in fat body, Malpighian tubules, gut and body wall tissues [189]. Although ecdysone may have direct hormonal effects, it is generally held that ecdysone serves as a prehormone that is converted to 20-hydroxyecdysone which functions as the active hormone. This is because in many bioassay systems, 20-hydroxyecdysone appears to be much more active than ecdysone. 

---