Utilization of Δ^5,7 ‐ and Δ⁸‐sterols by larvae of Heliothis zea

Abstract

Larvae from two populations of Heliothis zea were reared on artificial diets containing various sterols, which supported suboptimal growth, and their tissue sterols were characterized in order to determine how these dietary sterols are utilized by this insect. The sterols studied included A5r7-sterols (7-dehydrocholesterol or ergosterol), A8-sterols (lanosterol andlor 2Cdihydro-lanosterol), and a A5-sterol (4,4-dimethylcholesterol). Although larvae did not develop on 4,4dimethylcholesterol, those fed primarily A8-4,4,14trimethyl-sterols developed to the third instar. When the latter sterols were spared with cholesterol, the larvae reached the sixth instar and contained 4,4,14-trimethylsterols as well as cholesterol in their tissues. When larvae were fed 7-dehydrocholesterol, < 1 % of the larvae from one population developed to the sixth instar and these larvae contained 7-dehydrocholesterol as their principal sterol. The other larvae successfully completed their larval stage when they were transferred from the diet containing 7-dehydrocholesterol (or no sterol) to a diet containing cholesterol within at least 9 days. The sterol composition of larvae transferred from a diet containing cholesterol to a diet containing 7-dehydrocholesterol, after they had reached 60% of their final weight, was 54% cholesterol and 46% 7-dehydrocholesterol. The major sterol isolated from the tissues of the larvae fed ergosterol was also 7-dehydrocholesterol. Therefore, although the larva of H. zea can dealkylate and saturate the side chain of the A5,7,22-24fhnethylsteroI, it carries out little metabolism of the B ring of the nucleus. These studies demonstrate that, when A5r7-or A8-sterols are the principal sterols in the diet of H. zea, they are absorbed and incorporated into i t s tissues, although they slow the rate of growth and may prevent complete development of the larva.

Sterol utilization and metabolism by Heliothis zea

“…The corn earworm can tolerate certain modifications in the nucleus, as shown earlier by Ritter (8)(9)(10), studying a series of ∆ 0 -, ∆ 5 -, and ∆ 7 -, and ∆ 8 -sterols. However, we found that many phytosterols that are transformed naturally by plants, such as those with a 4,4-dimethyl group (24-dihydrolanosterol, lanosterol, and cycloartenol), a 9β,19-cyclopropane Structures of sterols are reported in Figure 2.…”

“…When cholesterol is not available and the dietary phytosterol cannot undergo metabolism to cholesterol by the insect, then the nutritional sterol is nonutilizable and larval death should ensue. Ritter (7)(8)(9)(10), studying sterol structure-activity relationships in H. zea, has shown that the type and amount of sterol available to the insect diet influence growth and development. In our comparative studies on metabolism of 4-desmethyl, 4-monomethyl and 4,4-dimethyl sterols fed to Manduca sexta, we found that sterol utilization was associated with the level of phytosterol in the medium preparation and the degree of sterol metabolism by the insect (11).…”

“…We chose to study H. zea for three reasons: (i) a relatively sterolfree diet has been designed to grow the insect (7); (ii) there has been a series of sterol structure-function studies performed on the insect (7-10); and (iii) H. zea is a major pest of corn, causing substantial crop damage annually. Heliothis zea has been shown to convert sitosterol to cholesterol (10) and to synthesize ecdysone and 20-hydroxyecdysone (12), suggesting that H. zea operates the sitosterol-to-ecdysteroid pathway commonly found in insects (3).…”

“…group (24-dehydropollinastanol and cycloartenol), a ∆ 8(9)bond (cholest-8(9)-en-3β-ol) were ineffective nutritional supplements and were recovered from larvae in unchanged form (Table 1), suggesting a much stricter sterol specificity than was recognized by Ritter (8)(9)(10). For mechanistic reasons, 24dihydrolanosterol should pass through cholest-8(9)-en-3β-ol onto cholesterol (19).…”

Metabolism of dietary Δ8‐sterols and 9β, 19‐cyclopropyl sterols by Locusta migratoria

“…However, it has been shown in H . zea that the percentage of esterification of sitosterol and stigmasterol was much smaller than that of cholesterol and other C-27 sterols [7]. 2.…”