Allium telomeres unmasked: the unusual telomeric sequence (CTCGGTTATGGG)n is synthesized by telomerase.

Phylogenetic divergence in Asparagales plants is associated with switches in telomere sequences. The last switch occurred with divergence of the genus Allium (Amaryllidaceae) from the other Allioideae (formerly Alliaceae) genera, resulting in uncharacterized telomeres maintained by an unknown mechanism. To characterize the unknown Allium telomeres, we applied a combination of bioinformatic processing of transcriptomic and genomic data with standard approaches in telomere biology such as BAL31 sensitivity tests, terminal restriction fragment analysis, the telomere repeat amplification protocol (TRAP), and fluorescence in situ hybridization (FISH). Using these methods, we characterize the unusual telomeric sequence (CTCGGTTATGGG)n present in Allium species, demonstrate its synthesis by telomerase, and characterize the telomerase reverse transcriptase (TERT) subunit of Allium cepa. Our findings open up the possibility of studying the molecular details of the evolutionary genetic change in Allium telomeres and its possible role in speciation. Experimental studies addressing the implications of this change in terms of the interplay of telomere components may now be designed to shed more light on telomere functions and evolution in general.

A tissue homogenate method to prepare gram-scale Allium thiosulfinates and their disulfide conjugates with cysteine and glutathione.

The health benefits of Allium vegetables are widely attributed to the enzyme-derived organosulfur compounds called thiosulfinates (TS). However, the lack of a suitable method to prepare TS in good yields has hampered the evaluation of their biological activities. This paper describe a simple enzymatic method using Allium tissue homogenates as a reaction system to prepare gram-scale TS, including those enriched in 1-propenyl groups, which are particularly difficult to obtain. This method is simple, easy to scale up, and requires no column purification step, making it suitable for practical large-scale production of Allium TS. The prepared TS were further utilized to prepare the disulfide conjugates with cysteine and glutathione (CySSR and GSSR, R = methyl, ethyl, propyl, 1-propenyl, and allyl), which are the presumptive metabolites of TS. Among all of the Allium CySSR and GSSR conjugates, the newly prepared glutathione conjugate with 1-propenyl TS, GSSPe, showed the most potent effect to induce quinone reductase (QR, a representative phase II enzyme) in murine hepatoma cells (Hepa 1c1c7) and inhibit nitric oxide (NO) production in lipopolysaccharide (LPS)-stimulated macrophage cells (RAW 264.7).

Characterization of callase (ß-1,3-D-glucanase) activity during microsporogenesis in the sterile anthers of Allium sativum L. and the fertile anthers of A. atropurpureum.

We examined callase activity in anthers of sterile Allium sativum (garlic) and fertile Allium atropurpureum. In A. sativum, a species that produces sterile pollen and propagates only vegetatively, callase was extracted from the thick walls of A. sativum microspore tetrads exhibited maximum activity at pH 4.8, and the corresponding in vivo values ranged from 4.5 to 5.0. Once microspores were released, in vitro callase activity peaked at three distinct pH values, reflecting the presence of three callase isoforms. One isoform, which was previously identified in the tetrad stage, displayed maximum activity at pH 4.8, and the remaining two isoforms, which were novel, were most active at pH 6.0 and 7.3. The corresponding in vivo values ranged from pH 4.75 to 6.0. In contrast, in A. atropurpureum, a sexually propagating species, three callase isoforms, active at pH 4.8-5.2, 6.1, and 7.3, were identified in samples of microsporangia that had released their microspores. The corresponding in vivo value for this plant was 5.9. The callose wall persists around A. sativum meiotic cells, whereas only one callase isoform, with an optimum activity of pH 4.8, is active in the acidic environment of the microsporangium. However, this isoform is degraded when the pH rises to 6.0 and two other callase isoforms, maximally active at pH 6.0 and 7.3, appear. Thus, factors that alter the pH of the microsporangium may indirectly affect the male gametophyte development by modulating the activity of callase and thereby regulating the degradation of the callose wall.

Identification of amino acid residues essential for onion lachrymatory factor synthase activity.

Lachrymatory factor synthase (LFS), an enzyme essential for the synthesis of the onion lachrymatory factor (propanethial S-oxide), was identified in 2002. This was the first reported enzyme involved in the production of thioaldehyde S-oxides via an intra-molecular H(+) substitution reaction, and we therefore attempted to identify the catalytic amino acid residues of LFS as the first step in elucidating the unique catalytic reaction mechanism of this enzyme. A comparison of the LFS cDNA sequences among lachrymatory Allium plants, a deletion analysis and site-directed mutagenesis enabled us to identify two amino acids (Arg71 and Glu88) that were indispensable to the LFS activity. Homology modeling was performed for LFS/23-169 on the basis of the template structure of a pyrabactin resistance 1-like protein (PYL) which had been selected from a BLASTP search on SWISS-MODEL against LFS/23-169. We identified in the modeled structure of LFS a pocket corresponding to the ligand-binding site in PYL, and Arg71 and Glu88 were located in this pocket.

Degradation of proteins by enzymes exuded by Allium porrum roots - a potentially important strategy for acquiring organic nitrogen by plants.

Nitrogen is one of the crucial elements that regulate plant growth and development. It is well-established that plants can acquire nitrogen from soil in the form of low-molecular-mass compounds, namely nitrate and ammonium, but also as amino acids. Nevertheless, nitrogen in the soil occurs mainly as proteins or proteins complexed with other organic compounds. Proteins are believed not to be available to plants. However, there is increasing evidence to suggest that plants can actively participate in proteolysis by exudation of proteases by roots and can obtain nitrogen from digested proteins. To gain insight into the process of organic nitrogen acquisition from proteins by leek roots (Allium porrum L. cv. Bartek), casein, bovine serum albumin and oxidized B-chain of insulin were used; their degradation products, after exposure to plant culture medium, were studied using liquid chromatography-mass spectrometry (LC-MS). Casein was degraded to a great extent, but the level of degradation of bovine serum albumin and the B-chain of insulin was lower. Proteases exuded by roots cleaved proteins, releasing low-molecular-mass peptides that can be taken up by roots. Various peptide fragments produced by digestion of the oxidized B-chain of insulin suggested that endopeptidase, but also exopeptidase activity was present. After identification, proteases were similar to cysteine protease from Arabidopsis thaliana. In conclusion, proteases exuded by roots may have great potential in the plant nitrogen nutrition.

The phospholipase A inhibitor, aristolochic acid, disrupts cortical microtubule arrays and root growth in Arabidopsis.

The role of phospholipase A(2) in Arabidopsis root growth and microtubule organisation was investigated using a specific inhibitor, aristolochic acid. At 0.5-1.5 microm concentrations, this inhibitor reduced root elongation and caused radial swelling of the root tip. The normally transverse cortical microtubules in root tip cells became progressively more disorganised with increasing concentrations of the inhibitor. Microtubule disorganisation also occurred in leaf epidermal cells of Allium porrum. We propose that phospholipase A(2) is involved in microtubule organisation and anisotropic growth in a manner similar to that reported previously for phospholipase D, thus broadening the significance of phospholipid signalling in microtubule organisation in plants.

Biochemical and genetic analysis of carbohydrate accumulation in Allium cepa L.

Onion and shallot (Allium cepa L.) exhibit wide variation in bulb fructan content, and the Frc locus on chromosome 8 conditions much of this variation. To understand the biochemical basis of Frc, we conducted biochemical and genetic analyses of Allium fistulosum (FF)-shallot (A. cepa Aggregatum group) alien monosomic addition lines (AALs; FF+1A-FF+8A) and onion mapping populations. Sucrose and fructan levels in leaves of FF+2A were significantly lower than in FF throughout the year, and the springtime activity of acid invertase was also lower. FF+8A showed significantly higher winter sucrose accumulation and sucrose phosphate synthase (SPS) activity. Inbred high fructan (Frc_) lines from the 'W202Ax Texas Grano 438' onion population exhibited significantly higher sucrose levels prior to bulbing than low fructan (frcfrc) lines. Sucrose synthase (SuSy) activity in these lines was correlated with leaf hexose content but not with Frc phenotype. Markers for additional candidate genes for sucrose metabolism were obtained by cloning a major SPS expressed in onion leaf and exhaustively mining onion expressed sequence tag resources. SPS and SuSy loci were assigned to chromosome 8 and 6, respectively, using AALs and linkage mapping. Further loci were assigned, using AALs, to chromosomes 1 (sucrose phosphate phosphatase), 2 (SuSy and three invertases) and 8 (neutral invertase). The concordance between chromosome 8 localization of SPS and elevated leaf sucrose levels conditioned by high fructan alleles at the Frc locus in bulb onion or alien monosomic additions of chromosome 8 in A. fistulosum suggest that the Frc locus may condition variation in SPS activity.

Comparative study of antioxidant properties of wild growing and cultivated Allium species.

Allium species are cultivated for the edible bulb, which is used mainly as flavoring in foods. Besides that, they could prevent tumor promotion and some processes that are associated with free radicals, such as cardiovascular diseases and aging. Therefore, different Allium species, both cultivated (Allium nutans L., A. fistulosum L., A. vineale L., A. pskemense B. Fedtsch, A. schoenoprasum L., A. cepa L. and A. sativum L.) and wild (A. flavum L., A. sphaerocephalum L., A. atroviolaceum Boiss, A. vineale L., A. ursinum L., A. scorodoprasum L., A. roseum L. and A. subhirsutum L.), were investigated in order to evaluate the antioxidant properties of their bulbs. This study reports on the results obtained for the bulb antioxidant enzyme activities (superoxide dismutase, catalase, guaiacol peroxidase, glutathione peroxidase), the quantities of non-enzymatic plant antioxidants (reduced glutathione and total flavonoids), the contents of soluble proteins, vitamin C, carotenoids, chlorophylls a and b, as well as for the quantities of malonyldialdehyde and .OH and O2.- radicals.

Direct comparison between genomic constitution and flavonoid contents in Allium multiple alien addition lines reveals chromosomal locations of genes related to biosynthesis from dihydrokaempferol to quercetin glucosides in scaly leaf of shallot (Allium cepa L.).

The extrachromosome 5A of shallot (Allium cepa L., genomes AA) has an important role in flavonoid biosynthesis in the scaly leaf of Allium fistulosum-shallot monosomic addition lines (FF+nA). This study deals with the production and biochemical characterisation of A. fistulosum-shallot multiple alien addition lines carrying at least 5A to determine the chromosomal locations of genes for quercetin formation. The multiple alien additions were selected from the crossing between allotriploid FFA (female symbol) and A. fistulosum (male symbol). The 113 plants obtained from this cross were analysed by a chromosome 5A-specific PGI isozyme marker of shallot. Thirty plants were preliminarily selected for an alien addition carrying 5A. The chromosome numbers of the 30 plants varied from 18 to 23. The other extrachromosomes in 19 plants were completely identified by using seven other chromosome markers of shallot. High-performance liquid chromatography analyses of the 19 multiple additions were conducted to identify the flavonoid compounds produced in the scaly leaves. Direct comparisons between the chromosomal constitution and the flavonoid contents of the multiple alien additions revealed that a flavonoid 3'-hydroxylase (F3'H) gene for the synthesis of quercetin from kaempferol was located on 7A and that an anonymous gene involved in the glucosidation of quercetin was on 3A or 4A. As a result of supplemental SCAR analyses by using genomic DNAs from two complete sets of A. fistulosum-shallot monosomic additions, we have assigned F3'H to 7A and flavonol synthase to 4A.

Study of 2-(2-pyridyl)benzothiazoline as a novel fluorescent probe for the identification of superoxide anion radicals and the determination of superoxide dismutase activity in scallion genus foods.

This paper presents a novel spectrofluorometric method using the novel fluorescent probe 2-(2-pyridyl)benzothiazoline for the determination of superoxide dismutase (SOD) activity. The fluorescent probe was synthesized in house and fully characterized by elemental analysis and by IR and (1)H NMR spectra. It could specially identify and trap superoxide anion radicals (O2(.-)), and then was oxidized by O2(.-) to form a strong fluorescence product. On the basis of this reaction, the spectrofluorometric method was proposed and successfully used to determine SOD activity. The proposed method has a better selectivity in the determination of reactive oxygen species, because the probe can be oxidized to afford a highly fluorescent product only by O2(.-) excluding hydrogen peroxide and hydroxyl radical. As a kind of simple, rapid, precise, and sensitive technique, it could avoid the errors caused by detection time and was applied to the measurement of SOD activity in scallion genus foods with satisfactory results.

Aminotransferase, L-amino acid oxidase and beta-lyase reactions involving L-cysteine S-conjugates found in allium extracts. Relevance to biological activity?

Several cysteine S-conjugates that occur in extracts of garlic and other plants of the allium family possess anti-oxidant properties, and many, including S-allyl-L-cysteine (SAC) and S-allylmercapto-L-cysteine (SAMC), are promising anti-cancer agents. To understand possible biochemical mechanisms contributing to the protective effects, the ability of selected allium-derived L-cysteine S-conjugates to undergo various enzyme-catalyzed transformations was investigated. SAC, SAMC, S-propylmercapto-L-cysteine and S-penta-1,3-dienylmercapto-L-cysteine were shown to be substrates of: (a) highly purified rat kidney glutamine transaminase K (GTK); (b) purified snake venom L-amino acid oxidase; and (c) a cysteine S-conjugate beta-lyase present in rat liver cytosol. S-Methylmercapto-L-cysteine was shown to be a substrate of GTK and L-amino acid oxidase, but not of the cysteine S-conjugate beta-lyase. Evidence is presented that a major enzyme responsible for the cysteine S-conjugate beta-lyase reactions in the rat liver cytosol is gamma-cystathionase. The possible role of gamma-cystathionase in generating sulfane sulfur from the disulfide-containing cysteine S-conjugates present in allium extracts, and the possible role of this sulfane sulfur in enzyme regulation, targeting of cancer cells and detoxification reactions is discussed. An interesting side finding of the present work is that rat liver mitochondria are more active than rat liver cytosol in catalyzing a cysteine S-conjugate beta-lyase reaction with the mitochondrial protoxicant S-(1,1,2,2-tetrafluoroethyl)-L-cysteine (TFEC) at physiological pH and at low substrate concentration.

Production and characterization of alien chromosome additions in shallot (Allium cepa L. Aggregatum group) carrying extra chromosome(s) of Japanese bunching onion (A. fistulosum L.).

First and second backcrosses of amphidiploid hybrids (2n = 4x = 32, genomes AAFF) between shallot (Allium cepa Aggregatum group) and A. fistulosum were conducted to produce A. cepa - A. fistulosum alien addition lines. When shallot (A. cepa Aggregatum group) was used as a pollinator, the amphidiploids and allotriploids set germinable BC(1) and BC(2) seeds, respectively. The 237 BC(1) plants mainly consisted of 170 allotriploids (2n = 3x = 24, AAF) and 42 hypo-allotriploids possessing 23 chromosomes, i.e., single-alien deletions (2n = 3x-1 = 23, AAF-nF). The single-alien deletions in the BC(1) progeny showed dwarfing characteristics and were discriminated from the allotriploids (2n = 24) and hyper-allotriploids (2n = 25) by means of flow cytometric analysis. The chromosome numbers of 46 BC(2) seedlings varied from 16 to 24. Eight monosomic additions (2n = 2x+1 = 17, AA+nF) and 20 single-alien deletions were found in these BC(2) seedlings. Consequently, six kinds of A. cepa - A. fistulosum alien chromosome additions possessing different chromosome numbers (2n = 17, 18, 20, 21, 22, 23) were recognized in the BC(1) and BC(2) populations. A total of 79 aneuploids, including 62 single-alien deletions, were analyzed by a chromosome 6F-specific isozyme marker (Got-2) in order to recognize its existence in their chromosome complements. This analysis revealed that two out of 62 single-alien deletions did not possess 6F. One (AAF-6F) out of the possible eight single-alien deletions could be identified at first. The present study is a first step toward the development of a useful tool, such as a complete set of eight different single-alien deletions, for the rapid chromosomal assignment of genes and genetic markers in A. fistulosum.

Formation of volatile compounds in model experiments with crude leek (Allium ampeloprasum Var. Lancelot) enzyme extract and linoleic acid or linolenic acid.

Three continuous assays are described for lipoxygenase (LOX), hydroperoxide lyase (HPL) and alcohol dehydrogenase (ADH) in leek tissue. The catalytic activity of LOX showed significant difference (significance level 5%) between linolenic acid (9.43 x 10(-)(4) katals per kg protein) and linoleic acid (2.53 x 10(-)(4) katals per kg protein), and the pH-optimum of LOX was 4.5-5.5 against linoleic acid. The catalytic activity of HPL was statistically the same for 9-(S)-hydroperoxy-(10E,12Z)-octadecadienoic acid (1.01 x 10(-)(2) katals per kg protein) and 13-(S)-hydroperoxy-(9Z,11E)-octadecadienoic acid (7.69 x 10(-)(3) katals per kg protein). ADH showed a catalytic activity of 5.01 x 10(-)(4) katals/kg of protein toward hexanal. Model experiments with crude enzyme extract from leek mixed with linoleic acid or linolenic acid demonstrated differences in the amount of produced aroma compounds. Linoleic acid resulted in significantly most hexanal, heptanal, (E)-2-heptenal, (E)-2-octenal, (E,E)-2,4-decadienal, pentanol, and hexanol, whereas linolenic acid resulted in significantly most (E)-2-pentenal, (E)-2-hexenal, (E,Z)-2,4-heptadienal, (E,E)-2,4-heptadienal, and butanol. Leek LOX produced only the 13-hydroperoxide of linoleic acid and linolenic acid.

Formation of aroma compounds during long-term frozen storage of unblanched leek (Allium ampeloprasum Var. Bulga) as affected by packaging atmosphere and slice thickness.

Content of aroma compounds and catalytic activity of lipoxygenase (LOX), hydroperoxide lyase (HPL), and alcohol dehydrogenase (ADH) were analyzed in 4- and 15-mm unblanched leek slices packed in atmospheric air (4- and 15-mm) or 100% nitrogen (N) (only 15-mm) seven times during 12 months of frozen storage (12M). Total amount of sulfur compounds was influenced by storage time, slice thickness, and atmosphere (concentration in fresh 4-mm slices = 17.8 mg/L, 4-mm 12M = 3.48 mg/L, fresh 15-mm slices = 2.48 mg/L, 15-mm 12M = 0.418 mg/L and 15-mm N 12M = 1.81 mg/L). The 4-mm slices significantly developed the most aldehydes after 12M (total amount = 9.28 mg/L) compared to 15-mm 12M (6.49 mg/L) and 15-mm N 12M (4.33 mg/L). LOX activity is positively influenced by nitrogen packaging, and HPL activity is influenced by slice thickness, whereas ADH is unaffected by both parameters.

Transmission of alien chromosomes from selfed progenies of a complete set of Allium monosomic additions: the development of a reliable method for the maintenance of a monosomic addition set.

Selfed progeny of a complete set of Allium fistulosum - Allium cepa monosomic addition lines (2n = 2x + 1 = 17, FF+1A-FF+8A) were produced to examine the transmission rates of respective alien chromosomes. All eight types of the selfed monosomic additions set germinable seeds. The numbers of chromosomes (2n) in the seedlings were 16, 17, or 18. The eight extra chromosomes varied in transmission rate (%) from 9 (FF+2A) to 49 (FF+8A). The complete set of monosomic additions was reproduced successfully by self-pollination. A reliable way to maintain a set of Allium monosomic additions was developed using a combination of two crossing methods, selfing and female transmission. FF+8A produced two seedlings with 18 chromosomes. Cytogenetical analyses, including GISH, showed that the seedlings were disomic addition plants carrying two entire homologous chromosomes from A. cepa in an integral diploid background of A. fistulosum. Flow cytometry analysis showed that a double dose of the alien 8A chromosome caused fluorescence intensity values spurring in DNA content, and isozyme analysis showed increased glutamate dehydrogenase activity at the gene locus Gdh-1.

Formation of aroma compounds and lipoxygenase (EC 1.13.11.12) activity in unblanched leek (Allium ampeloprasum Var. Bulga) slices during long-term frozen storage.

The content of aroma compounds (dynamic headspace) and catalytic activity of lipoxygenase (LOX) (EC. 1.13.11.12) were analyzed in 15 mm unblanched leek slices seven times during 12 months of frozen storage. The aroma profile changed from consisting of almost only sulfur compounds such as dipropyl disulfide [concentration in fresh leek (FL) = 0.197 mg/L, concentration after 12 months of frozen storage (12M) = 0.0409 mg/L] and propyl (E)-propenyl disulfide (FL = 0.0437 mg/L, 12M = 0.00452 mg/L) in the fresh leeks to being dominated by numerous saturated and unsaturated aldehydes, such as hexanal (FL = 1.53 mg/L, 12M = 3.63 mg/L), (E,E)-2,4-nonadienal (FL = 0.000 mg/L, 12M = 0.0647 mg/L), and (E,E)-2,4-decadienal (FL = 0.129 mg/L, 12M = 0.594 mg/L) at the end of the storage period. The catalytic activity of LOX diminished throughout frozen storage, but approximately 25% of the original activity was present after 12 months of storage.

Characterization of some Allium hybrids by aroma precursors, aroma profiles, and alliinase activity.

Various Allium hybrids, obtained by the crossbreeding of Allium cepa (onion) as the mother plant and six taxonomically distant wild species obtained by embryo rescue, were investigated with special respect to their individual profiles of cysteine sulfoxides as well as enzymically and nonenzymically formed aroma substances. Alliinase (EC 4.4.1.4) catalyzes the conversion of odorless (+)-S-alk(en)yl-L-cysteine sulfoxides into volatile thiosulfinates. These thiosulfinates were converted to a variety of sulfides by steam distillation. SPME-gas chromatography (GC) and high-performance liquid chromatography (HPLC) used for the analysis of aroma components and their precursors permitted a high sample throughput, so that numerous gene bank accessions and Allium breeding materials were analyzed within a comparatively short time. Cysteine sulfoxides as well as alliinase activity were found in all investigated samples at different levels, but (+)-S-methyl-L-cysteine sulfoxide (methiin) was the most abundant sulfoxide present. (+)-S-(trans-1-Propenyl)-L-cysteine sulfoxide (isoalliin) is typical for onion and was found in all investigated hybrids. The pattern of the other cysteine sulfoxides depended strongly on the parent plants used. The profile of aroma components corresponded with the related pattern of aroma precursors (cysteine sulfoxides). Successful hybridization was proven by randomly amplified polymorphic DNA analysis. Together with these established marker techniques, HPLC and SPME-GC analysis provide support to breeding projects designed to improve the health and aroma properties of Allium hybrids.

Comparison of C-S lyase in Lentinus edodes and Allium sativum.

The characteristics of C-S lyase in Lentinus edodes (shiitake) were compared with those in Allium sativum (garlic). C-S lyase mRNA from shiitake was hybridized with the garlic C-S lyase cDNA fragment, being almost the same length as that from garlic. The isoelectric point of the C-S lyase from shiitake was between pH 4 and 5, while that from garlic was over a wider range between pH 4 and 8. Different from the C-S lyase from garlic, that from shiitake was not a glycoprotein without being stained by PAS, and was not bound to the anti-garlic C-S lyase antibody. Similar to garlic C-S lyase, shiitake C-S lyase comprised a homodimer, and its molecular mass was 84 kDa. However, the N-terminal amino acid sequences of each subunit of shiitake C-S lyase were totally different from those of garlic C-S lyase.

Molecular cloning and functional characterization of cDNAs encoding cysteine synthase and serine acetyltransferase that may be responsible for high cellular cysteine content in Allium tuberosum.

The plants belonging to the genus Allium are known to accumulate sulfur-containing secondary compounds that are derived from cysteine. Here, we report on molecular cloning and functional characterization of two cDNAs that encode serine acetyltransferase and cysteine synthase from A. tuberosum (Chinese chive). The cDNA for serine acetyltransferase encodes an open reading frame of 289 amino acids, of which expression could complement the lacking of cysE gene for endogenous serine acetyltransferase in Escherichia coli. The cDNA for cysteine synthase encodes an open reading frame of 325 amino acids, of which expression in the E. coli lacking endogenous cysteine synthase genes could functionally rescue the growth without addition of cysteine. Both deduced proteins seem to be localized in cytosol, judging from their primary structures. Northern blot analysis indicated that both transcripts accumulated in almost equal levels in leaves and root of green and etiolated seedlings of A. tuberosum. The activity of recombinant serine acetyltransferase produced from the cDNA was inhibited by L-cysteine, which is the end-product of the pathway; however, the sensitivity to cysteine (48.7 microM of the concentration for 50% inhibition, IC(50)) was fairly low compared with that of previously reported serine acetyltransferases ( approximately 5 microM IC(50)) from various plants. In A. tuberosum, the cellular content of cysteine was several-fold higher than those in Arabidopsis thaliana and tobacco. This higher concentration of cysteine in A. tuberosum is likely due to the lower sensitivity of feedback inhibition of serine acetyltransferase to cysteine.

Cysteine sulfoxides and alliinase activity of some Allium species.

The flavor precursors of 17 species belonging to the Alliaceae family were analyzed by HPLC, and results were evaluated with respect to the classification of species into their genus, subgenus, and section. Identification and quantification of these precursors were carried out by synthetic and natural reference materials. In addition, nine of these species were investigated in terms of their alliinase activity. Alliinase (EC 4.4.1.4) catalyzes the conversion of odorless (+)-S-alk(en)yl-L-cysteine sulfoxides into volatile thiosulfinates. Cysteine sulfoxides as well as alliinase activity were found in all investigated samples, and (+)-S-methyl-L-cysteine sulfoxide was most abundant. (+)-S-Propyl-L-cysteine sulfoxide was detected in only a few, not closely related, species. Analysis of the crude protein extract of nine species gave evidence that alliinase activities of samples were similar in terms of pH and temperature optimum, K(M) value, and substrate specificity. For all investigated protein extracts, the highest specific alliinase activity was found for (+)-S-(2-propenyl)-L-cysteine sulfoxide (alliin). The substrate specificity of these enzymes was not related to relative abundance of the cysteine sulfoxides. However, SDS-PAGE yielded some significant differences among species in terms of their total protein compositions. Species belonging to different subgenera exhibited a specific protein pattern with molecular masses between 13 and 35 kDa.