WAGNER, C.A.; LI, S.; FRIESEN, J.A.; BORST, D.W.; Illinois State University, Normal: HMG-CoA Reductase acitivity in the fat body of the lubber grasshopper, Romalea microptera.

Isoprenoid dolichols are associated with the glycosylation of insect vitellogenin (Vg), a major yolk protein produced by the fat body (FB). Therefore, we studied the activity of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR, the rate-limiting step in isoprenoid synthesis) during the oviposition cycle of the lubber grasshopper. HMGR from the FB had a Km for HMG-CoA of approximately 25 &microM. HMGR activity varied with temperature (max. at 30 &degC) and pH (max. pH 7.2-8.0) and was linear with protein concentration and incubation time. The enzyme required NADPH as a cofactor. HMGR activity in the FB was between 10 to 100 fmol/mg protein/min. Higher HMGR activity (>150 fmol/mg protein/min) was detected in the corpora allata (CA), reflecting its synthesis of juvenile hormone (JH). HMGR activity was low in the brain (20 fmol/mg protein/min) and ovary (5 fmol/mg protein/min), and no HMGR activity was detected in the gut and muscle. During the oviposition cycle, HMGR activity in the FB increased 7-fold during the first 18 days, decreased 4-fold by day 24, and then increased 3-fold on day 32 (possibly due to the onset of the next oviposition cycle). The initial rise in HMGR activity was correlated with the rise in hemolymph Vg and JH levels during this period, supporting the hypothesis that HMGR is important for Vg synthesis and may be stimulated by JH. However, FB from allatectomized animals on day 18 of the oviposition cycle had HMGR activity (60 fmol/mg protein/min) similar to that of intact animals, indicating that JH is not necessary for HMGR activity. An understanding of HMGR regulation in the FB may provide insight into the control of vitellogenesis. (Supported by NSF DBI 9978810 and NIH HD37953 to DWB).