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On the mechanism of iridoid and secoiridoid monoterpene biosynthesis

1970; Elsevier BV; Volume: 136; Issue: 2 Linguagem: Inglês

10.1016/0003-9861(70)90221-3

1096-0384

Carmine Coscia, Luigi Botta, Rocco Guarnaccia,

Plant biochemistry and biosynthesis

Mevalonate (11) labeled with 14C at C-2 and with tritium stereospecifically at C-2 or C-4 was incorporated into the iridoid monoterpene glucoside, loganic acid (1), and the secoiridoid, gentiopicroside (8), in the higher plant Swertia caroliniensis.5 From the 3H14C ratios of these monoterpenes as well as their degradation products, 7-oxologanin tetraacetate (5), bromolactol (7), and gentianine (10), it was possible to determine the fate of the isotopic hydrogens. The pro-R hydrogen of the C-4 methylene group of mevalonate (11) was retained in loganic acid (1) whereas the pro-S hydrogen was lost. Likewise gentiopicroside contained no pro-S hydrogen from C-4 of mevalonate but retained one pro-R-derived hydrogen at C-9 as expected from its revised structure. A pro-R, hydrogen from the C-2 methylene group of mevalonate is found at C-7 of loganic acid whereas the pro-S hydrogen had been essentially eliminated from this hydroxylated position. The C-3 carbon of loganic acid contained the remainder of the tritium from 2-3H-mevalonate. Gentiopicroside obtained in the 2R and 2S-tritio mevalonate incorporations had the same 3H14C ratio as loganic acid. A doubly labeled loganic acid (1) isolated in the 4R-tritiated mevalonate experiment was directly incorporated into gentiopicroside (8) in a Swertia plant. These results suggest a mechanism of isoprenoid biosynthesis from mevalonate (11) to geranyl pyrophosphate (14) similar to that of cholesterol and other terpenoids, and establish the precursor relationship of iridoid monoterpenes to the secoiridoids.