Biosynthesis of 3-hydroxy-3-methylglutaryl-CoA, 3-hydroxy-3-ethylglutaryl-CoA, mevalonate and homomevalonate by insect corpus allatum and mammalian hepatic tissues.

Both radioactively labeled 3-hydroxy-3-methyglutarylcoenzyme A and its homolog 3-hydroxy-3-ethylglutarylcoenzyme A are produced by a cytosolic fraction obtained from corpora allata-corpora cardiaca complexes of the moth Manduca sexta, incubated with [1-14C]acetylcoenzyme A plus unlabeled propionylcoenzyme A. A particulate fraction isolated from the same tissue was able to reduce [Me-3H]hydroxymethylglutaryl-CoA and [3-14C]hydroxyethylglutaryl-CoA to mevalonate and homomevalonate, respectively, when NADPH was used as the electron donor. These reactions are in keeping with the proposed homoisoprenoid biosynthetic pathway for certain of the insect juvenile hormones. Measurement of products produced in picomole levels was accomplished by their conversion to ultraviolet absorbing derivatives which were subsequently analyzed by thin-layer chromatography and liquid chromatography followed by collection and liquid scintillation counting of appropriate fractions. Similar assays conducted with cell-free preparations from corpora allata of the grasshopper Schistocerca nitens, the mealworm beetle Tenebrio molitor, and from rat liver, species in which homoisoprenoid compounds have not been detected, also resulted in formation of the normal and homologous products (although no hydroxymethylglutaryl-CoA/hydroxyethylglutaryl-CoA reductase activity could be demonstrated in T. molitor). The latter results indicate a rather loose substrate specificity of the enzymes leading to mevalonate formation.