An historical perspective on lignan biosynthesis: Monolignol,allylphenol and hydroxycinnamic acid coupling and downstream metabolism

This review describes discoveries from this laboratory on monolignol, allylphenol and hydroxycinnamic acid coupling, and downstream metabolic conversions, affording various lignan skeleta. Stereoselective 8-8′ coupling (dirigent protein-mediated) of coniferyl alcohol to afford (+)-pinoresinol is comprehensively discussed, as is our current mechanistic/kinetic understanding of the protein’s radical-radical binding, orientation and coupling properties, and insights gained for other coupling modes, e.g. affording (−)-pinoresinol. In a species dependent manner, (+)- or (−)-pinoresinols can also undergo enantiospecific reductions, catalyzed by various bifunctional pinoresinol-lariciresinol reductases (PLR), to afford lariciresinol and then secoisolariciresinol. With X-ray structures giving a molecular basis for differing PLR enantiospecificities, comparisons are made herein to the X-ray structure of the related enzyme, phenylcoumaran benzylic ether reductase, capable of 8-5′ linked lignan regiospecific reductions. Properties of the enantiospecific secoisolariciresinol dehydrogenase (also discovered in our laboratory and generating 8-8′ linked matairesinol) are summarized, as are both in situ hybridization and immunolocalization of lignan pathway mRNA/proteins in vascular tissues. This entire 8-8′ pathway thus overall affords secoisolariciresinol and matairesinol, viewed as cancer preventative agent precursors, as well as intermediates to cancer treating substances, such as podophyllotoxin derivatives. Another emphasis is placed on allylphenol/hydroxycinnamic acid coupling and associated downstream metabolism, e.g. affording the antiviral creosote bush lignan, nordihydroguaiaretic acid (NDGA), and the fern lignans, blechnic/brainic acids. Regiospecific 8-8′ allylphenol coupling is described, as is characterization of the first enantiospecific membrane-bound polyphenol oxidase, (+)-larreatricin hydroxylase, involved in NDGA formation. Specific [¹³C]-labeling also indicated that Blechnum lignans arise from stereoselective 8-2′ hydroxycinnamic acid coupling. Abbreviations: CD – circular dichroism; e.e. – enantiomeric excess; DP – dirigent protein; ESI-MS – electrospray ionization mass spectrometry; MALDI -TOF – matrix assisted laser desorption ionization-time of flight; MALLS – multiangle laser light scattering; PLR – pinoresinol lariciresinol reductase; SDH – secoisolariciresinol dehydrogenase. This revised version was published online in June 2006 with corrections to the Cover Date.     

Biocatalyzed esterification of oleic acid using cell suspension and dried biomass of Aspergillus sp. RBD01

Esterification is an industrially important reaction in the field of food and fuel industries. In biofuel and allied industries, long-chain alkyl esters are generally produced from different fat rich feedstocks including non-edible oils, acid oils, and tallow, using a variety of catalysts. Amongst these, whole cell systems have prominently been explored in recent past. The present study focused on the use of Aspergillus sp. RBD01 as a whole cell catalyst, in dry and whole cell suspension, to esterify oleic acid with different alcohols as acyl acceptors. Esterification with dried biomass resulted in better conversion of oleic acid to its respective ester as compared to cell suspension. Further, increase in chain length of alcohol resulted in decrease in the yield from ethyl oleate (98% EO) to decyl oleate (77% DO) with alcohols having an even number of carbon atoms giving better yield of esters over alcohols with odd numbers.     

Degradation of lignin β‐aryl ether units in Arabidopsis thaliana expressing LigD,LigF and LigG from Sphingomonas paucimobilis SYK‐6

Lignin is a major polymer in the secondary plant cell wall and composed of hydrophobic interlinked hydroxyphenylpropanoid units. The presence of lignin hampers conversion of plant biomass into biofuels; plants with modified lignin are therefore being investigated for increased digestibility. The bacterium Sphingomonas paucimobilis produces lignin‐degrading enzymes including LigD, LigF and LigG involved in cleaving the most abundant lignin interunit linkage, the β‐aryl ether bond. In this study, we expressed the LigD, LigF and LigG (LigDFG) genes in Arabidopsis thaliana to introduce postlignification modifications into the lignin structure. The three enzymes were targeted to the secretory pathway. Phenolic metabolite profiling and 2D HSQC NMR of the transgenic lines showed an increase in oxidized guaiacyl and syringyl units without concomitant increase in oxidized β‐aryl ether units, showing lignin bond cleavage. Saccharification yield increased significantly in transgenic lines expressing LigDFG, showing the applicability of our approach. Additional new information on substrate specificity of the LigDFG enzymes is also provided.     

Novel amphiphilic fluoroalkylated derivatives of xylitol,D-glucose and D-galactose for medical applications: hemocompatibility and co-emulsifying properties

1-O-(4,4,5,5,6,6,7,7,8,8,9,9,9-Tridecafluorononyl)xylitol 6 was synthesized as a novel standard compound for the assessment of hemocompatibility and co-emulsifying properties in microemulsions for biomedical uses. 3-O-(1,1,2,4,4,5,7,7,8,8,9,9,9-Tridecafluoro-5-trifluoro-methyl-3,6-dioxanonyl)-D-glucose 9 and 6-O-(1,1,2,4,4,5,7,7,8,8,9,9,9-tridecafluoro-5-trifluoromethyl-3,6-dioxanonyl)-D-galactose 12 were synthesized by nucleophilic addition of protected carbohydrates to perfluorinated vinyl oligoether. Biological tests revealed very good hemocompatibility and co-emulsifying properties for the amphiphiles 6, 9 and 12.     

Stereocontrolled synthesis of 1-acetylen-2,3-di-o-benzyl-tetrahydrofurans, 1,4-anhydro-arabinitol,and alpha,beta-dihydroxy-gamma-alkyl-butyrolactones

This article describes a concise and efficient synthesis of 1-acetylen-2,3-di-O-benzyl-tetrahydrofurans from tartaric acid esters using as the key step the stereocontrolled cyclization of Co(2)(CO)(6)-complexed propargylic diols. This molecule led to enantiomerically pure 1,4-anhydro-arabinitol and alpha,beta-dihydroxy-gamma-alkyl-butyrolactones. In the latter case, the critical methylene oxidation at the oxygen vicinal position was performed by RuO(4).     

Enantiomer effects of huperzine A on the aryl acylamidase activity of human cholinesterases

1. Acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BuChE, EC 3.1.1.8) are serine hydrolase enzymes that catalyze the hydrolysis of acetylcholine.2. (–) Huperzine A is an inhibitor of AChE and is being considered for the treatment of Alzheimer's disease.3. In addition to esterase activity, AChE and BuChE have intrinsic aryl acylamidase activity.4. The function of aryl acylamidase is unknown but has been speculated to be important in Alzheimer pathology.5. Kinetic effects of (–) huperzine A and ( ±)$ huperzine A on the aryl acylamidase activity of human cholinesterases were examined.6. (–) Huperzine A inhibited the aryl acylamidase activities of both AChE and BuChE.7. (±) Huperzine A inhibited this function in AChE but stimulated BuChE aryl acylamidase suggesting that the (+) enantiomer is a powerful activator of this enzyme activity.8. The two huperzine enantiomers may prove to be useful tools to examine the function of aryl acylamidase activity, including its role in Alzheimer pathology.     

Regulation of cytochrome P450 gene expression in the olfactory mucosa

The mammalian olfactory mucosa (OM) is unique among extrahepatic tissues in having high levels, and tissue-selective forms, of cytochrome P450 (CYP) enzymes. These enzymes may have important toxicological implications, as well as biological functions, in this chemosensory organ. In addition to a tissue-selective, abundant expression of CYP1A2, CYP2A, and CYP2G1, some of the OM CYPs are also known to have an early developmental expression, a resistance to xenobiotic inducers, and a lack of responsiveness to circadian rhythm. Efforts to fully characterize the regulation of CYP expression in the OM, and to identify the underlying mechanisms, are important for our understanding of the physiological functions and toxicological significance of these biotransformation enzymes, and may also shed unique light on the general mechanisms of CYP regulation. The aim of this mini-review is to provide a summary of current knowledge of the various modes of regulation of CYPs expressed in the OM, an update on our mechanistic studies on tissue-selective CYP expression, and a review of the literature on xenobiotic inducibility of OM CYPs. Our goal is to stimulate further studies in this exciting research area, which is of considerable importance, in view of the constant exposure of the human nasal tissues to inhaled, as well as systemically derived, chemicals, the prevalence of olfactory system damage in individuals with neurodegenerative diseases, and the current uncertainty in risk assessments for potential olfactory toxicants.