Early stages of ecdysteroid biosynthesis. The role of 7-dehydrocholesterol

The synthesis of [4-14C]cholesta-4,6-dien-3-one and [4-14C]3β-hydroxy-5α-cholestan-6-one is described. Both [4-14C]cholest-4-en-3-one and [4-14C]cholesta-4,6-dien-3-one were not incorporated significantly into ecdysteroids compared to [1α,2α-3H]cholesterol in fifth instar and maturing adult female Schistocerca gregaria. Similarly, [4-14C]3β-hydroxy-5α-cholestan-6-one was not incorporated significantly in the latter system. The results suggest that none of the three 14C-substrates are intermediates in ecdysteroid biosynthesis from cholesterol, although possible complications from permeability barriers cannot be discounted. [4-14C, 7-3H]7-dehydrocholesterol has been synthesized and incorporated into ecdysteroids in adult female Schistocerca gregaria and in Spodoptera littoralis pupae. Although approximately half the tritium was eliminated during ecdysteroid synthesis in S. gregaria, there was essentially complete retention of the tritium in Spodoptera. The results support the direct incorporation of 7-dehydrocholesterol into ecdysteroids and not via cholesterol. A possible explanation for the loss of appreciable tritium in S. gregaria is discussed. © 1986.