momordicine I

Momordicine I is one of the cucurbitane-type triterpenoids contained in bitter gourd. Under acidic conditions, the aldehyde group at C-19 and the hydroxyl group at C-7 undergo nucleophilic addition within the molecule, yielding 5,19-hemiacetal. When methanol is present, 5,19-acetal and methyl ether are also generated. These derivatives were not detected in non-methanol-based extraction of bitter gourd fruit, but were detected upon extraction with methanol after heat treatment, and were also found in 2 out of 8 commercial bitter-gourd dietary supplement brands. This is a report within the scope of the abstract.

Similar intramolecular cyclization has been confirmed in a related cucurbitane-type glycoside, momordicoside K, in a 1982 study. When momordicoside K (bearing an aldehyde group at C-19) was left in a 0.01 N hydrochloric acid methanol solution at room temperature for 1.5 hours, 5,19-methyl acetal was formed, and when the reduction derivative was further treated in a 50% acetic acid-methanol solution at 38 degrees Celsius, it underwent cyclization to 5,19-epoxy ether. Although the starting compound and linkage site differ from the case of momordicine I, this backbone demonstrates that the aldehyde or oxygen at C-19 undergoes cyclization readily under mild acidic alcohol conditions.

Momordicine I and II are positioned by the authors as the bitter-tasting triterpenoids that contribute most to the bitterness of bitter gourd. When bitter-gourd juice was fermented using the lactic acid bacterium Lactiplantibacillus plantarum dy-1, the content of momordicine I and II became significantly lower than in the non-fermented juice, whereas the content of momordicine U, a deglycosylated precursor of momordicine I, increased more than 2-fold. This is a result showing relative differences in composition, and absolute content values are not presented.