Substrate specificity of tuliposide-converting enzyme, a unique non-ester-hydrolyzing carboxylesterase in tulip: Effects of the alcohol moiety of substrate on the enzyme activity

Identification of tuliposides K-M in tulip bulbs via an enzyme reaction-based screening method using a tuliposide-converting enzyme

Tuliposides (Pos) are major defense-related secondary metabolites in tulip, having 4-hydroxy-2-methylenebutanoyl and/or (3S)-3,4-dihydroxy-2-methylenebutanoyl groups at the C-1 and/or C-6 positions of d-glucose. The acyl group at the C-6 position is converted to antimicrobial lactones (tulipalins) by an endogenous Pos-converting enzyme. Based on this enzyme activity, we examined tulip bulb extracts and detected HPLC peaks that disappeared following the reaction by the Pos-converting enzyme. Spectroscopic analyses of the three purified compounds revealed that one of them was a glucose ester-type Pos, while the other two were identified as a glucoside ester-type Pos. These compounds were designated as PosK, L, and M. They were specific to bulbs, with the highest content in the outermost layer, but they were markedly less abundant than PosG, the minor bulb Pos we identified earlier. The study results suggest that tulip bulbs contain at least four minor Pos in addition to the major 6-PosA. Although PosK-M were present in almost all of the tested tulip cultivars, they were detected in only a few wild species, indicative of their potential utility as chemotaxonomic markers in tulip. Identification of PosK-M as 6-PosA derivatives unveils the biosynthetic diversity of Pos, the well-known group of secondary metabolites in tulip.

Occurrence of Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid and its regioisomer 4-methylene-2-oxo-glutaric acid in tulip tissues

Although Z-2-oxo-4-methyl-3-pentene-1,5-dioic acid (Z-OMPD) has been identified as a major dicarboxylic acid in tulip tissues, its biosynthetic pathway has not been elucidated. Herein, Z-OMPD was isolated from tulip leaves and chemically synthesized. Comparisons of these samples revealed that Z-OMPD exists as a tautomeric mixture at physiological pH. As a regioisomer of Z-OMPD, we enzymatically and chemically prepared 4-methylene-2-oxo-glutaric acid (4-MEOG) for the first time. Using these compounds as standards, the occurrence of Z-OMPD and 4-MEOG in various tissues of the tulip cultivar "Murasakizuisho" was evaluated directly and by 2,4-dinitrophenylhydrazone derivatization. Z-OMPD was found to be abundant in the aerial tissues, whereas 4-MEOG was almost absent from all tissues. Stability analyses of Z-OMPD and 4-MEOG revealed that no double bond isomerization occurred at physiological pH, suggesting that enzyme systems are responsible for Z-OMPD biosynthesis in tulip tissues.

Identification of tuliposide G, a novel glucoside ester-type tuliposide, and its distribution in tulip

Tuliposides (Pos) are major defensive secondary metabolites in tulip (genus Tulipa), having 4-hydroxy-2-methylenebutanoyl and/or (3S)-3,4-dihydroxy-2-methylenebutanoyl groups at the C-1 and/or C-6 positions of d-glucose. The acyl group at the C-6 position is converted to antimicrobial lactones, tulipalins, by tuliposide-converting enzymes (TCEs). In the course of a survey of tulip tissue extracts to identify novel Pos, we found a minute high-performance liquid chromatography peak that disappeared following the action of a TCE, and whose retention time differed from those of known Pos. Spectroscopic analyses of the purified compound, as well as its enzymatic degradation products, revealed its structure as 5″-O-(6-O-(4'-hydroxy-2'-methylenebutanoyl))-β-d-glucopyranosyl-(2″R)-2″-hydroxymethyl-4″-butyrolactone, which is a novel glucoside ester-type Pos. We gave this compound the trivial name 'tuliposide G' (PosG). PosG accumulated in bulbs, at markedly lower levels than 6-PosA (the major Pos in bulbs), but was not found in any other tissues. Quantification of PosG in bulbs of 52 types of tulip, including 30 cultivars (Tulipa gesneriana) and 22 wild Tulipa spp., resulted in the detection of PosG in 28 cultivars, while PosG was present only in three wild species belonging to the subgenus Tulipa, the same subgenus to which tulip cultivars belong, suggesting the potential usefulness of PosG as a chemotaxonomic marker in tulip.

Isolation and identification of tuliposides D and F from tulip cultivars

6-Tuliposides A (6-PosA) and B (6-PosB) are major defensive secondary metabolites in tulip cultivars (Tulipa gesneriana), having an acyl group at the C-6 position of d-glucose. Although some wild tulip species produce 1,6-diacyl-glucose type of Pos (PosD and PosF), as well as 6-PosA/B, they have not yet been isolated from tulip cultivars. Here, aiming at verifying the presence of PosD and PosF in tulip cultivars, tissue extracts of 25 cultivars were analyzed by high-performance liquid chromatography (HPLC). Although no HPLC peaks for PosD nor PosF were detected in most cultivars, we found two cultivars giving a minute HPLC peak for PosD and the other two cultivars giving that for PosF. PosD and PosF were then purified from petals of cultivar 'Orca' and from pistils of cultivar 'Murasakizuisho', respectively, and their identities were verified by spectroscopic analyses. This is the first report that substantiates the presence of 1,6-diacyl-glucose type of Pos in tulip cultivars.