BIOSYNTHESIS OF ECDYSTEROIDS IN ZEA-MAYS

The incubation of several radiolabelled substrates with Zea mays plants demonstrated the incorporation of radiolabels into ecdysteroid conjugates. Radiolabelled [C-14]ecdysone and [C-14]20-hydroxyecdysone, were biosynthesized from [2-C-14]mevalonic acid (MVA) in Z. mays. These ecdysteroids can be released from their conjugates by treatment with wheat germ acid phosphatase or mild acid hydrolysis. The C-24 reduced side chain sterols, lathosterol and cholesterol were endogenously biosynthesized from [2-C-14]MVA and were identified by H-1 NMR. Lathosterol accounts for ca 0.5% of the total sterol composition, whereas cholesterol; is present at 2.5%. However, lathosterol was consistently found to have a specific activity three to six times that of cholesterol. Incubation of [4-C-14]cholesterol with Z. mays leaves also demonstrated incorporation into a 20-hydroxyecdysone conjugate. Incubation of [22,23-H-3]ecdysone with Z. mays demonstrated incorporation into radiolabelled ecdysone diphosphate and ecdysone polyphosphate, which were cleaved upon acid phosphatase treatment. Z. mays biosynthesizes primarily ecdysteroid conjugates and does not appear to produce detectable levels of non-conjugated ecdysteroids.