The influence of substrate concentrations on the rate of insect juvenile hormone biosynthesis by corpora allata of the desert locust in vitro

Abstract

The rate at which isolated corpora allata of adult female Schistocerca gregaria incorporate [³H]farnesenic acid and [¹⁴C]methionine into C₁₆juvenile hormone in vitro was examined at different concentrations of farnesenic acid, methionine, O₂ and H⁺ ions. Maximum juvenile hormone biosynthesis is obtained at a farnesenic acid concentration of 20μm. The range of optimum l-methionine concentrations (0.1–0.4mm) encompasses the physiological concentration of this substrate in the haemolymph. Hormone biosynthesis is dependent on O₂, but is not stimulated by hyperbaric oxygen tension. The glands had a maximum synthetic activity at pH8.0, but their activity was more reproducible in the the physiological range pH7.0–7.5. At pH6.5 and less, the synthetic ability was considerably decreased. The relative incorporations of the labelled substrates into methyl farnesoate and C₁₆ juvenile hormone indicate that [³H]farnesenic acid comes into isotopic equilibrium within the gland more rapidly than [¹⁴C]methionine. The incorporations into methyl farnesoate become stoicheiometric after 20min incubation and into C₁₆ juvenile hormone after a further 10min. Labelled juvenile hormone is detectable after 10min incubation and the rate of incorporation is constant for up to 4h. It is proposed that the described method may be usefully employed to assess the physiological changes in the enzymic competence of the glands to effect the last two stages in C₁₆ juvenile hormone biosynthesis.