In vivo transformations of artemisinic acid in Artemisia annua plants

Dihydroartemisinic acid, not artemisinic acid, is the precursor of artemisinin.

“recent feeding experiments with artemisinic acid [13] and dihydroartemisinic acid [14] have shown that the latter substance is the precursor of artemisinin.”

There is no conversion in planta of artemisinic acid to dihydroartemisinic acid or vice versa.

“According to previous investigations, there is no conversion in planta of artemisinic acid to dihydroartemisinic acid or vice versa [13].”

Feeding experiments with both labeled precursors appear to show that artemisinic acid (473) and dihydroartemisinic acid (480) are not mutually interconvertible.

“In addition, feeding experiments with both labeled precursors appear to show that artemisinic acid (473) and dihydroartemisinic acid (480) are NOT mutually interconvertible”

The biological transformations of (473)/(480) have been proposed to involve spontaneous autoxidation of the Δ4,5 double bond and subsequent rearrangements of the resultant allylic hydroperoxides.

“These biological transformations have been proposed to involve spontaneous autoxidation of the Δ4,5 double bond in (473)/(480) and subsequent rearrangements of the resultant allylic hydroperoxides [155,185].”

The most abundant metabolite from feeding artemisinic acid (473) was arteannuin B (462), followed by epi-deoxyarteannuin B (478) and the seco-cadinane (494).

“The most abundant metabolite from feeding artemisinic acid (473) was arteannuin B (462), followed by epi-deoxyarteannuin B (478) and the seco-cadinane (494) as shown in Scheme 7”

The remaining four metabolites from feeding artemisinic acid (473) - annulide (457), isoannulide (490), deoxyarteannuin B (477) and artemisinic acid methyl ester (474) - were isolated in trace amounts.

“(the remaining four metabolites are: annulide (457), isoannulide (490), deoxyarteannuin B (477) and artemisinic acid methyl ester (474), which were all isolated in trace amounts.”

Feeding labelled artemisinic acid (473) to intact A. annua plants resulted in slightly fewer labelled metabolites than feeding dihydroartemisinic acid.

“When labeled artemisinic acid (473), the 11,13-dehydro analogue of dihydroartemisinic acid (480), was fed to intact A. annua plants, slightly fewer labeled metabolites were isolated [155].”

In the case of artemisinic acid (473), no allylic hydroperoxide analogous to (481) in Scheme 6 was isolated in vivo as a natural product.

“(However, note that in the case of artemisinic acid (473), no allylic hydroperoxide analogous to (481) in Scheme 6 was isolated in vivo as a natural product”

All seven metabolites from the artemisinic acid (473) feeding experiment are also known as natural products from A. annua, and six of the seven feature in the pairs of metabolites in Table 37.

“However, all seven metabolites from this experiment are also known as natural products from A. annua and six of the seven feature in the pairs of metabolites discussed in Table 37.”

There is no interconversion in either direction between artemisinic acid (473) and dihydroartemisinic acid (480).

“and more recent biosynthetic studies, employing both labeled artemisinic acid [155] and dihydroartemisinic acid [185], have confirmed that there is no interconversion in either direction between dihydroartemisinic acid (473) and artemisinic acid (480)”

There was no evidence for conversion of artemisinic acid to any 11,13-dihydro metabolite, including artemisinin (495).

“furthermore, there was no evidence for conversion to any 11,13-dihydro metabolite, including artemisinin (495)”

In the most recent experiments, artemisinic acid (473) failed to show detectable incorporation into artemisinin but showed significant conversion into arteannuin B (462) and six other highly oxygenated sesquiterpene natural products retaining the 11,13-double bond.

“Although the most recent experiments with artemisinic acid (473) failed to observe any detectable incorporation into artemisinin, a very significant conversion was observed into arteannuin B (462), as well as six other highly oxygenated sesquiterpene natural products, all of which retained the 11,13- double bond, as shown in Scheme 7 in Section 2.6.3 [155]”

Artemisinic acid (473) is not a biosynthetic precursor to artemisinin (495) in A. annua.

“However, the most recent biosynthetic study [155], which employed stable isotope-labeled artemisinic acid [15-[2H313C]-(473)] administered to a whole plant system, seemed to show that artemisinic acid was not a biosynthetic precursor to artemisinin.”

Stable isotope-labeled metabolites detected by NMR represented at least 1% (very often 5-30%) of the label supplied, providing a picture only of the most significant and direct transformations undergone by the precursor.

“By contrast, the stable isotope-labeled metabolites which have been detected by NMR [155] represented at least 1% (very often between 5-30%) of the label that had been supplied and therefore provide a picture only of the most significant and direct transformations which have been undergone by the precursor.”

More recent experiments using stable isotope-labeled dihydro-epi-deoxyarteannuin B (485) fed to an intact plant system showed no detectable incorporation into artemisinin (495), although three novel hydroxylated metabolites were obtained in significant quantities.

“However, once again, more recent experiments using dihydro-epi-deoxyarteannuin B (485) which had been labelled with a stable isotope [169] and fed to an intact plant system have shown no detectable incorporation into artemisinin (495); although three novel hydroxylated metabolites were obtained in significant quantities from this feeding experiment [190].”

Artemisinic acid (473) is not a precursor to dihydroartemisinic acid (480).

“given that it seems now to be proven that artemisinic acid (473) is not a precursor to dihydroartemisinic acid (480) (see crossed arrow in Scheme 11) [155,185]”

Dihydroartemisinic acid (480), rather than artemisinic acid (473), is the true late-stage precursor to artemisinin (495).

“it is consistent with much of the more recent literature (post-1990’s), in which there is now a gathering consensus that dihydroartemisinic acid (480), rather than artemisinic acid (473), is the true late-stage precursor to artemisinin (495).”

epi-Deoxyarteannuin B (478) was obtained as a minor metabolite of artemisinic acid (473), without evidence for accompanying formation of artemisinin (495), in the most recent study.

“epi-Deoxyarteannuin B (478) was obtained as a minor metabolite of artemisinic acid (473), without any evidence for the accompanying formation of artemisinin (495), in the most recent study of the metabolism of artemisinic acid [155].”

In the most recent study of artemisinic acid metabolism, arteannuin B (462) was obtained as the major metabolite of artemisinic acid (473), without evidence for accompanying formation of artemisinin (495).

“As noted above, in the most recent study of the metabolism of artemisinic acid, arteannuin B (462) was obtained as the major metabolite of artemisinic acid (473), without any evidence for the accompanying formation of artemisinin (495) [155].”