Previously we have found that fucosterol-24,28-epoxide(V) was converted into desmosterol(IV) by a brief treatment with BF_3-etherate. The similar reaction was postulated to occur in the dealkylation step of β-sitosterol(II) in the phytophagous insects. To clarify this hypothesis, [2,4-^3H]_fucosterol-24,28-epoxide(V), fucosterol(III), isofucosterol(IX) and 24-methylene-cholesterol(VIII) were ingested in the larvae of the silkworm, Bombyx mori, and found to be transformed in cholesterol(I) in the yields of 15,10,1.9, and 3.2%, respectively. When a large amount of the epoxide(V) was feeded, followed by ingestion of ^3H-fucosterol(III), 2.4% of the ^3H-activity was trapped in V, while the incorporation in cholesterol(I) decreased to 1.6%. These data suggests that V is a possible metabolic intermediate between III and I. In conjunction with the previous findings, we propose II→III→V→IV→I (scheme 5), as one of the main dealkylation routes in insects. The other candidate, e.g. II→IX→VIII→IV→I, postulated as the reverse way of phytosterol biosynthesis (scheme 6) seems to be, if any, a by-path, from the small incorporation of IX and VIII. The nutrition studies showed I, II, III, IV, V, VIII, and IX satisfied the sterol requirement of B. mori, while the insects feeded with VI, VII, XI, XII or XIII were died during the first instar. The latter observations would exclude the third route(scheme 9), reminiscent of the side chain cleavage of cholesterol(I) in vertebrates.
