Chemical characterization of cytoplasmic lipid droplets in the pheromone-producing cells of the silkmoth, Bombyx mori

Abstract

Accumulation of lipid droplets within the cytoplasm is a common feature of the pheromone gland cells of many lepidopteran species. The cytoplasmic lipid droplets in the pheromone-producing cells of the silkmoth, Bombyx mori, were effectively extracted by dipping the trimmed glands in acetone for 10 min. In order to analyze the components originating from the lipid droplets, we separated the acetone extracts prepared before and after adult eclosion using HPLC, and specified the peaks showing a similar pattern of stage-dependence to that in the morphological change of the lipid droplets previously reported by Fónagy et al. (Arthropod Struct. Dev. 30 (2001) 113). Finally, we specified the peaks #1–5 and #1a–4a separated by reversed-phase HPLC as lipid droplet contents. Structure elucidation using FAB-MS and MS-MS analyses confirmed that they were triacylglycerols (TGs), and 12 species of TGs were identified as lipid droplet contents. Fatty acyl groups contained in these TGs were limited to five unsaturated C 16 and C 18 fatty acyl groups (Δ11-hexadecenoate, Δ10,12-hexadecadienoate, Δ9-octadecenoate, Δ9,12-ocatadecadienoate, and Δ9,12,15-ocatadecatrienoate), including the pheromone precursor Δ10,12-hexadecadienoate as a major component. Digestion with porcine pancreatic lipase confirmed that three major TGs eluted in the peaks #3–5 all contained C 18 fatty acyl groups at the sn-2 position, indicating that the pheromone precursor is sequestered preferentially at the sn-1 and/or sn-3 position. Present results combined with the fact that the morphological change of the lipid droplets is under the control of PBAN indicate that the role of the cytoplasmic lipid droplets in the pheromone-producing cells is to store the pheromone precursor in the form of TGs and to provide it for pheromone production in response to the external signal of PBAN.