LI, S.; WAGNER, C. A.; FEI, H.; FRIESEN, J. A.; BORST, D. W.; Illinois State University, Normal: HMG-CoA reductase activity in the mandibular organ of the lobster, Homarus americanus
3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) is the rate-limiting step in vertebrate isoprenoid synthesis. Since the lobster mandibular organ (MO) produces high levels of the sesquiterpene methyl farnesoate (MF), we investigated the molecular characteristic and regulation of HMGR in lobsters. Unlike the vertebrate enzyme, most of the lobster HMGR was detected in the microsomal supernatant suggesting that it is not membrane bound. The enzyme has a Km of 11 µM, and is inhibited by lovastatin, a recognized competitive inhibitor of HMGR. Other potential regulators (MF, farnesoic acid, cholesterol, ecdysteroids, progesterone) had no effect on its activity. HMGR purified by anion exchange had a MW of about 40,000 by SDS-PAGE. Western blots using an antiserum for HMGR from the bacterium Pseudomonas mevalonii showed a single band of similar size. A fragment (314 bp) of lobster HMGR was amplified using degenerate primers based on the conserved regions of other HMGR enzymes. The deduced amino acid sequence of this fragment had 50-60% similarity to other eukaryotic HMGRs. The lobster MO was the only tissue with detectable HMGR activity. There were no differences in HMGR activity in different MO regions, in spite of regional differences in MF synthesis. Eyestalk ablation (ESA), a procedure that increases MF synthesis, caused a 4-fold increase in HMGR activity in the MO, and treatment of ESA animals with an extract of the sinus gland (SG; a neurohemal organ in the eyestalk) lowered HMGR activity. Incubation of lobster MO fragments with SG extracts also lowered HMGR activity. These results suggest that HMGR has a role in the regulation of MF synthesis by the MO. (Supported by NIH HD37953 to DWB).
