STEROL UTILIZATION AND METABOLISM BY HELIOTHIS-ZEA

Heliothis zea (corn earworm), an insect that fails to synthesize stero ls de novo, was reared on an artificial diet treated with 18 different sterol supplements. Larvae did not develop on a sterol-less medium. D elta(5)-Sterols with a hydrogen atom, a methylene group, an E-or Z-eth ylidene group, or an alpha- or beta-ethyl group (cholesterol, ostreast erol, isofucosterol, fucosterol, sitosterol, and clionasterol, respect ively) at position C-24, and Delta(5)-sterols doubly substituted in th e side chain at C-24 with an alpha-ethyl group and at C-22 with a doub le bond (stigmasterol) supported normal larval growth to late-sixth in star (prepupal: maturity). The major sterol isolated from each of thes e sterol treatments was cholesterol, suggesting that H. zea operates a typical 24-dealkylation pathway. The sterol requirement of H. tea cou ld not be met satisfactorily by derivatives of 3 beta-cholestanol with a 9 beta, 19-cyclopropyl group, gem dimethyl group at C-4, a Delta(5, 7)-bond or Delta(8)-bond, or by side-chain modi- fled sterols that pos sessed a Delta(25(27))-24 beta-ethyl group, Delta(23(24))-24-methyl gr oup or 24-ethyl group, or Delta(24(25))-24-methyl or 24-ethyl group. T he major sterol recovered from the larvae (albeit developmentally arre sted larvae)treated with a nonutilizable sterol was the test compound. Sterol absorption was related to the degree of sterol utilization. Th e most effective sterols absorbed by the insect ranged from 27 to 66 m u g per insect, whereas the least effective sterols absorbed by the in sect ranged from 0.6 to 6 mu g per insect. Competition experiments usi ng different proportions of cholesterol and 24-dihydrolanosterol (from 9:1 to 1:9 mixtures) indicated that abnormal development of H. tea ma y be induced on less than a 1 to 1 mixture of utilizable (cholesterol) to nonutilizable (24-dihydrolanosterol) sterols. The results demonstr ate new structural requirements for sterol utilization and metabolism by insects, particularly with respect to the posi- tion of double bond s in the side chain and functionalization in the nucleus. The novel st erol specificities observed in this study appear to be associated with the dual role of sterols as membrane inserts (nonmetabolic) and as pr ecursors to the ecdysteroids (metabolic).