The mechanism of formation of N-formylkynurenine by heme dioxygenases.

Heme dioxygenases catalyze the oxidation of L-tryptophan to N-formylkynurenine (NFK), the first and rate-limiting step in tryptophan catabolism. Although recent progress has been made on early stages in the mechanism, there is currently no experimental data on the mechanism of product (NFK) formation. In this work, we have used mass spectrometry to examine product formation in a number of dioxygenases. In addition to NFK formation (m/z = 237), the data identify a species (m/z = 221) that is consistent with insertion of a single atom of oxygen into the substrate during O(2)-driven turnover. The fragmentation pattern for this m/z = 221 species is consistent with a cyclic amino acetal structure; independent chemical synthesis of the 3a-hydroxypyrroloindole-2-carboxylic acid compound is in agreement with this assignment. Labeling experiments with (18)O(2) confirm the origin of the oxygen atom as arising from O(2)-dependent turnover. These data suggest that the dioxygenases use a ring-opening mechanism during NFK formation, rather than Criegee or dioxetane mechanisms as previously proposed.

Evidence for heme oxygenase activity in a heme peroxidase.

The heme peroxidase and heme oxygenase enzymes share a common heme prosthetic group but catalyze fundamentally different reactions, the first being H(2)O(2)-dependent oxidation of substrate using an oxidized Compound I intermediate, and the second O(2)-dependent degradation of heme. It has been proposed that these enzymes utilize a common reaction intermediate, a ferric hydroperoxide species, that sits at a crossroads in the mechanism and beyond which there are two mutually exclusive mechanistic pathways. Here, we present evidence to support this proposal in a heme peroxidase. Hence, we describe kinetic data for a variant of ascorbate peroxidase (W41A) which reacts slowly with tert-butyl hydroperoxide and does not form the usual peroxidase Compound I intermediate; instead, structural data show that a product is formed in which the heme has been cleaved at the alpha-meso position, analogous to the heme oxygenase mechanism. We interpret this to mean that the Compound I (peroxidase) pathway is shut down, so that instead the reaction intermediate diverts through the alternative (heme oxygenase) route. A mechanism for formation of the product is proposed and discussed in the light of what is known about the heme oxygenase reaction mechanism.

Reassessment of the reaction mechanism in the heme dioxygenases.

Indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO) are heme enzymes that catalyze the O(2)-dependent oxidation of L-tryptophan to N-formyl-kynurenine. Previous proposals for the mechanism of this reaction have suggested that deprotonation of the indole NH group, either by an active-site base or by oxygen bound to the heme iron, as the initial step. In this work, we have examined the activity of 1-Me-L-Trp with three different heme dioxygenases and their site-directed variants. We find, in contrast to previous work, that 1-Me-L-Trp is a substrate for the heme dioxygenase enzymes. These observations suggest that deprotonation of the indole N(1) is not essential for catalysis, and an alternative reaction mechanism, based on the known chemistry of indoles, is presented.

Cytotoxicity of prenylated xanthones and other constituents from the wood of Garcinia merguensis.

One new prenylated xanthone 5-farnesyltoxyloxanthone B ( 4), three known xanthones alpha-mangostin ( 1), rubraxanthone ( 2) and isocowanol ( 3) as well as (2 E,6 E,10 E)-4beta-hydroxy-3-methyl-5beta-(3,7,11,15-tetramethylhexadeca-2,6,10-tetraenyl)-cyclohex-2-en-1-one ( 5) and 3,3',4- O-trimethylellagic acid were isolated from the wood of GARCINIA MERGUENSIS Wight. The cytotoxic activities of compounds 1 - 5 were evaluated against the MCF-7, MDA-MB-231, NCI-H-460 and SF-268 cell lines with rubraxanthone 2 and 5 exhibiting the highest activity at 9.0 and 12.1 microM, respectively, against MCF-7 cells.

Dihydroxyxanthones prenylated derivatives: synthesis, structure elucidation, and growth inhibitory activity on human tumor cell lines with improvement of selectivity for MCF-7.

The synthesis, structure elucidation, and antitumor activity of 11 xanthones are reported, being the compounds 3, 4, 6-8, and 9 described for the first time. Xanthones 1 and 2 were used as building blocks to obtain the prenylated derivatives 3-8. Prenylation was carried out using prenyl bromide in alkaline medium. Dihydropyranoxanthones 9-11 were obtained from compounds 4 and 5 by an oxidative ring closure. The structure of the compounds was established by IR, UV, MS, and NMR ((1)H, (13)C, COSY, HSQC, and HMBC) techniques and for compounds 4, 6, and 11 the structure was confirmed by X-ray crystallographic analysis. The effect of the 11 xanthones on the in vitro growth of four human tumor cell lines, MCF-7 (breast adenocarcinoma), NCI-H460 (non-small cell lung cancer), SF-268 (central nervous system cancer), and UACC-62 (melanoma) is also described.

Merodrimanes and other constituents from Talaromyces thailandiasis.

Chemical study of a previously undescribed fungus, Talaromyces thailandiasis, furnished the two new merodrimanes thailandolides A (1) and B (2), an O-methylated derivative (3) of the aromatic fragment incorporated in thailandolide B, and three known closely related 1(3H)-isobenzofuran derivatives, penisimplicissin (4a), vermistatin (4b), and hydroxydihydrovermistatin (4c). Structures were established by spectroscopic measurements and confirmed by X-ray analyses of compounds 1 and 4b. The unusual peptide analogue N-benzoylphenylalanyl-N-benzoylphenyl alaninate (5) isolated earlier from a higher plant was also found.

Bacillisporins D and E, new oxyphenalenone dimers from Talaromyces bacillisporus.

The oligophenalenone dimer duclauxin and two new analogues, bacillisporins D and E, were isolated from the fungus TALAROMYCES BACILLISPORUS in addition to the previously reported bacillisporins A, B and C. Structures were established by spectroscopic studies. Duclauxin and bacillisporins A, B, C and E were evaluated for cytotoxicity against three human cancer cell lines. Bacillisporin A was strongly active against MCF-7 and NCI-H460 and moderately active against SF-268 while bacillisporins B, C and duclauxin were moderately active against all three cell lines.

Clerodanes and other constituents of Cleidion spiciflorum.

The polyoxygenated clerodane, spiciflorin (1a), was isolated from Cleidion spiciflorum (Burm. f.) Merr. (Euphorbiaceae). Other constituents were the glucoside of anol (2), columbin, scopoletin, 3,3',4-O-trimethylellagic acid, acetylaleuritolic acid, common triterpenes and phenols.

An unusual glucoside from Cleistanthus gracilis.

Extraction of roots and stems of Cleistanthus gracilis furnished common triterpenes, plant sterols and the unusual glucoside (+) gracicleistanthoside, the glucoside of 2-beta-hydroxy-8-azabicyclo-(5,2,0)-4beta,9beta-epoxynona-5,7-diene.

Cytotoxicities of xanthones and cinnamate esters from Hypericum hookerianum.

5-Hydroxy-2-methoxyxanthone (1), 2-hydroxy-3-methoxyxanthone (2), trans-kielcorin (3), 4-hydroxy-3-methoxyphenyl ferulate (4) and 3beta-O-caffeoylbetulinic acid (5) were isolated from Hypericum hookerianum. Compounds 1-5 were tested against the growth of three human tumor cell lines, MCF-7, NCI-H460 and SF-268. Compounds 4 and 5 exhibited significant inhibitory activity effects against all three; GI50 values for 4 were 15.1 +/- 1.6, 18.7 +/- 2.3 and 15.9 +/- 2.7 and for 5 12.2 +/- 2.4, 19.6 +/- 2.3 and 24.3 +/- 2.5. Compound 3 was less active with GI50 values of 55.1 +/- 2.3, 49.7 +/- 3.0 and 40.5 +/- 1.5, while 1 and 2 exhibited only weak effects. Compounds 4 and 5 were moderately effective in influencing the mitogenic response to human lymphocytes to hemoagglutinin, with IC values of 26.1 +/- 3.6 and 40.8 +/- 4.9, respectively.

5,7-Dihydroxychromones and 8-hydroxytetrahydrochromones from Horsfieldia irya.

Wood of Horsfieldia irya contained 2-n-nonyl- and 2-(6-phenylhexyl)-5,7-dihydroxychromone, 2-n-nonyl-8-hydroxy- and 2-(6-phenylhexyl)-8-hydroxy-5,6,7,8-tetrahydrochromone as well as dihydrocubebin.

Biosynthesis of podophyllotoxin in Linum album cell cultures.

Cell cultures of Linum album Kotschy ex Boiss. (Linaceae) showing high accumulation of the lignan podophyllotoxin (PTOX) were established. Enzymological studies revealed highest activities of phenylalanine ammonia-lyase, cinnamyl alcohol dehydrogenase, 4-hydroxycinnamate:CoA ligase and cinnamoyl-CoA:NADP oxidoreductase immediately prior to PTOX accumulation. To investigate PTOX biosynthesis, feeding experiments were performed with [2-(13)C]3',4'-dimethoxycinnamic acid, [2-(13)C]3',4'-methylenedioxycinnamic acid (MDCA), [2-(13)C]3',4',5'-trimethoxycinnamic acid, [2-(13)C]sinapic acid, [2-(13)C]- and [2,3-(13)C(2)]ferulic acid. Analysis of the metabolites by HPLC coupled to tandem mass spectrometry revealed incorporation of label from ferulic acid into PTOX and deoxypodophyllotoxin (DOP). In addition, MDCA was also unambiguously incorporated intact into PTOX. These observations suggest that in L. album both ferulic acid and methylenedioxy-substituted cinnamic acid can be incorporated into lignans. Furthermore, it appears that, in this species, the hydroxylation of DOP is a rate-limiting point in the pathway leading to PTOX. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/wo.1007/s00425-002-0834-1.

Further halotyrosine derivatives from the marine sponge Suberea aff. praetensa.

Reexamination of the marine sponge Suberea aff. praetensa, (Row) from the Gulf of Thailand furnished in addition to bromotyrosine derivatives found previously 5-bromo- and 5-chlorocavernicolin, cavernicolins 1 and 2, two other brominated tyrosine metabolites, a known bisoxazolidone and a new unusual rearranged tyrosine metabolite subereatensin. Several of the metabolites exhibited significant inhibitory effects against five human cancer cell lines.

Polyoxygenated cyclohexene derivatives from Ellipeiopsis cherrevensis.

Aerial parts of Ellipeiopsis cherrevensis contained the polyoxygenated cyclohexenes zeylenol, ferrudiol and three analogs, ellipeiopsols A, B and C. The C-1 stereochemistry of ferrudiol has been revised.