Biosynthesis of monoterpene hydrocarbons from [1-3H]neryl pyrophosphate and [1-3H]geranyl pyrophosphate by soluble enzymes from Citrus limonum

Inhibition of tumor progression by naturally occurring terpenoids

CONTEXT

Cancer is a major public health problem in India and many other parts of the world. Its two main characteristics are uncontrolled cell growth and metastasis. Natural products represent a rich source of compounds that have found many applications in various fields of medicines and therapy including cancer therapy. Effective ingredients in several plant-derived medicinal extracts are terpenoid compounds and many terpenes have biological activities and are used for the treatment of human diseases.

OBJECTIVES

This review attempted to collect all available published scientific literature of eight naturally occurring terpenoids and their effect on inhibition of tumor progression.

METHODS

The present review is about eight potent naturally occurring terpenoids that have been studied for their pharmacological properties in our lab and this review includes 130 references compiled from all major databases.

RESULTS

Literature survey revealed that triterpenoids, such as glycyrrhizic acid, ursolic acid, oleanolic acid, and nomilin, the diterpene andrographolide, and the monoterpenoids like limonene and perillic acid had shown immunomodulatory and antitumor activities. All of them could induce apoptosis in various cancer cells by activating various proapoptotic signaling cascades. Many of these terpenoids found to inhibit metastatic progression and tumor-induced angiogenesis. The molecular mechanisms that involved in these activities include inhibition of various oncogenic and anti-apoptotic signaling pathways and suppression or nuclear translocation of various transcription factors including nuclear factor kappa B (NF-κB).

CONCLUSION

The chemopreventive and chemoprotective effects of these compounds point toward their possible role in modern anticancer therapies.

Monoterpene biosynthesis in lemon (Citrus limon). cDNA isolation and functional analysis of four monoterpene synthases

Citrus limon possesses a high content and large variety of monoterpenoids, especially in the glands of the fruit flavedo. The genes responsible for the production of these monoterpenes have never been isolated. By applying a random sequencing approach to a cDNA library from mRNA isolated from the peel of young developing fruit, four monoterpene synthase cDNAs were isolated that appear to be new members of the previously reported tpsb family. Based on sequence homology and phylogenetic analysis, these sequences cluster in two separate groups. All four cDNAs could be functionally expressed in Escherichia coli after removal of their plastid targeting signals. The main products of the enzymes in assays with geranyl diphosphate as substrate were (+)-limonene (two cDNAs) (-)-beta-pinene and gamma-terpinene. All enzymes exhibited a pH optimum around 7; addition of Mn(2+) as bivalent metal ion cofactor resulted in higher activity than Mg(2+), with an optimum concentration of 0.6 mm. K(m) values ranged from 0.7 to 3.1 microm. The four enzymes account for the production of 10 out of the 17 monoterpene skeletons commonly observed in lemon peel oil, corresponding to more than 90% of the main components present.

Prevention and therapy of cancer by dietary monoterpenes

Monoterpenes are nonnutritive dietary components found in the essential oils of citrus fruits and other plants. A number of these dietary monoterpenes have antitumor activity. For example, d-limonene, which comprises >90% of orange peel oil, has chemopreventive activity against rodent mammary, skin, liver, lung and forestomach cancers. Similarly, other dietary monoterpenes have chemopreventive activity against rat mammary, lung and forestomach cancers when fed during the initiation phase. In addition, perillyl alcohol has promotion phase chemopreventive activity against rat liver cancer, and geraniol has in vivo antitumor activity against murine leukemia cells. Perillyl alcohol and d-limonene also have chemotherapeutic activity against rodent mammary and pancreatic tumors. As a result, their cancer chemotherapeutic activities are under evaluation in Phase I clinical trials. Several mechanisms of action may account for the antitumor activities of monoterpenes. The blocking chemopreventive effects of limonene and other monoterpenes during the initiation phase of mammary carcinogenesis are likely due to the induction of Phase II carcinogen-metabolizing enzymes, resulting in carcinogen detoxification. The post-initiation phase, tumor suppressive chemopreventive activity of monoterpenes may be due to the induction of apoptosis and/or to inhibition of the post-translational isoprenylation of cell growth-regulating proteins. Chemotherapy of chemically induced mammary tumors with monoterpenes results in tumor redifferentiation concomitant with increased expression of the mannose-6-phosphate/insulin-like growth factor II receptor and transforming growth factor beta1. Thus, monoterpenes would appear to act through multiple mechanisms in the chemoprevention and chemotherapy of cancer.

Monoterpenes in breast cancer chemoprevention

A number of dietary monoterpenes have chemopreventive activity against rat mammary cancer. For example, d-limonene, which comprises over 90% of orange peel oil, has chemopreventive activity against rodent mammary cancer during the initiation phase as well as the promotion/progression phase. Similarly, the monoterpenoids carveol, uroterpenol, and sobrerol have chemopreventive activity against mammary cancer when fed during the initiation phase. d-limonene and perillyl alcohol, a more potent analog of limonene, also have chemotherapeutic activity against rodent mammary and pancreatic tumors. As a result, their cancer chemotherapeutic activities are under evaluation in Phase I clinical trials. Several mechanisms of action may account for the antitumor activities of monoterpenes. The blocking chemopreventive effects of limonene and other monoterpenes during the initiation phase of mammary carcinogenesis are due to the induction of Phase II carcinogen-metabolizing enzymes, resulting in carcinogen detoxification. The post-initiation phase chemopreventive and chemotherapeutic activities of monoterpenes may be due to the induction of tumor cell apoptosis, tumor redifferentiation, and/or inhibition of the post-translational isoprenylation of cell growth-regulating proteins. Thus, monoterpenes act through multiple mechanisms in the chemoprevention of mammary and other cancers.

Characterization and mechanism of (4S)-limonene synthase, a monoterpene cyclase from the glandular trichomes of peppermint (Mentha x piperita)

(4S)-Limonene synthase, a monoterpene cyclase isolated from the secretory cells of the glandular trichomes of Mentha x piperita (peppermint), catalyzes the cyclization of geranyl pyrophosphate to (4S)-limonene, a key intermediate in the biosynthesis of p-menthane monoterpenes in Mentha species. The enzyme synthesizes principally (-)-(4S)-limonene (greater than 94% of the total products), plus several other monoterpene olefins. The general properties of (4S)-limonene synthase resemble those of other monoterpene cyclases. The enzyme shows a pH optimum near 6.7, an isoelectric point of 4.35, and requires a divalent metal ion for catalysis, either Mg2+ or Mn2+, with Mn2+ preferred. The Km value measured for geranyl pyrophosphate was 1.8 microM. The activity of (4S)-limonene synthase was inhibited by sodium phosphate, sodium pyrophosphate, and reagents directed against the amino acids cysteine, methionine, and histidine. In the presence of Mn2+, geranyl pyrophosphate protected against cysteine-directed inhibition, suggesting that at least one cysteine residue is located at or near the active site. Experiments with alternate substrates and substrate analogs confirmed many elements of the proposed reaction mechanism, including the binding of geranyl pyrophosphate in the form of a complex with the divalent metal ion, the preliminary isomerization of geranyl pyrophosphate to linalyl pyrophosphate (a bound intermediate capable of cyclization), and the participation of a series of carbocation:pyrophosphate anion pairs in the reaction sequence.

Wound-inducible pinene cyclase from grand fir: purification, characterization, and renaturation after SDS-PAGE

The major wound-inducible monoterpene synthase (cyclase) of grand fir (Abies grandis) stems transforms geranyl pyrophosphate to both (-)-alpha-pinene (40%) and (-)-beta-pinene (60%). The enzyme was purified to apparent homogeneity by anion-exchange and hydrophobic interaction chromatography, coupled to discontinuous native polyacrylamide gel electrophoresis at neutral pH and polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate (also at neutral pH) followed by renaturation in 1% Tween 20 (polyoxyethylenesorbitan monolaurate). The renatured enzyme produced a mixture of isomeric pinenes from geranyl pyrophosphate identical to that generated by the native form. The protein exhibited a molecular weight of 63,000 by gel permeation chromatography and of 62,000 by denaturing gel electrophoresis, indicating that the monomer is active. The enzyme required Mn2+ (Km = 30 microM) for activity, exhibited a Km value of 6 microM for the substrate geranyl pyrophosphate, showed a pH optimum at 7.8 and temperature optimum at 42 degrees C, and was inhibited by pyrophosphate (I50 = 0.17 mM), orthophosphate (I50 = 51 mM), and alpha-pinene, as well as by the histidine-directed reagent diethylpyrocarbonate (I50 = 0.64 mM) and the cysteine-directed reagent p-hydroxymercuribenzoate (I50 = 1.9 microM). Although similar in many respects to constitutive monoterpene cyclases of herbaceous species, this inducible cyclase, the first enzyme of this type to be purified to homogeneity from a conifer, is distinguished by the relatively high pH optimum, and the strict specificity and high affinity for the divalent metal ion cofactor.

Partial purification and characterization of two sesquiterpene cyclases from sage (Salvia officinalis) which catalyze the respective conversion of farnesyl pyrophosphate to humulene and caryophyllene

Humulene cyclase and caryophyllene cyclase, two enzymes which catalyze the cyclization of farnesyl pyrophosphate to the respective sesquiterpene olefins, have been partially purified from the supernatant fraction of a sage (Salvia officinalis) leaf epidermis extract and separated from each other by a combination of hydrophobic interaction, gel filtration, and ion-exchange chromatography. The molecular weight of both cyclases was estimated by gel filtration to be 57,000 and both cyclases exhibited a pH optimum of 6.5 and preferred Mg2+ (Km approximately 1.5 mM) as the required divalent metal cation. Both enzymes possessed a Km of about 1.7 microM for farnesyl pyrophosphate, were strongly inhibited by p-hydroxymercuribenzoate, and exhibited comparable sensitivities to a variety of other potential inhibitors. The properties of the two sesquiterpene olefin cyclases, which are the first from a higher plant source to be examined in detail, were very similar to each other and to other monoterpene, sesquiterpene, and diterpene cyclases previously described.

Partial purification and characterization of pentalenene synthase

Pentalenene synthase, an enzyme which catalyzes the cyclization of farnesyl pyrophosphate to the sesquiterpene hydrocarbon pentalenene, has been partially purified from the supernatant fraction of Streptomyces UC5319 by a combination of anion-exchange, hydroxylapatite, and gel-filtration chromatography. The molecular weight of the partially purified synthase was estimated by gel filtration chromatography to be 57,000 and the cyclase activity was shown to be associated with a major protein band among eight visible by nondenaturing polyacrylamide disc gel electrophoresis. The Km for farnesyl pyrophosphate was 0.77 +/- 0.21 microM and the Vmax for the partially purified synthase was 287 +/- 21 nmol of pentalenene/mg protein per hour. Cyclase activity required the presence of a divalent metal cation. Although either Mg2+ or Mn2+ could be used, Mn2+ was inhibitory at concentrations above 2.5 mM. No other cofactors were required. Whereas neither product, pentalenene nor inorganic pyrophosphate, showed significant inhibition of cyclase activity at concentrations of ca 10 microM, the combination of the two resulted in an approximate sevenfold increase in the apparent Km for farnesyl pyrophosphate, suggesting that both products can bind cooperatively at the active site to inhibit pentalenene synthase competitively.

Monoterpene formation by leucoplasts of Citrofortunella mitis and Citrus unshiu : Steps and conditions of biosynthesis

Leucoplasts of immature calamondin and satsuma fruits were incubated with [1-(14)C] isopentenyl pyrophosphate under various conditions. Optimal incorporation of the tracer into geranyl pyrophosphate and monoterpene hydrocarbons occurred in the presence of exogenous dimethylallyl pyrophosphate and Mn(2+) which was more effective than Mg(2+). The dependence of dimethylallyl pyrophosphate showed that about 10 moles were required for 1 mole of isopentenyl pyrophosphate for the best recovery in monoterpene hydrocarbon biosynthesis. A time-course incorporation of isopentenyl pyrophosphate revealed that the C10 hydrocarbon elaboration was dependent on the geranyl pyrophosphate production and at no time neryl pyrophosphate was synthesized by leucoplasts. The amount of labelled farnesyl pyrophosphate was rather low whatever the conditions used in the experiments and sesquiterpene hydrocarbon biosynthesis was never observed.

Monoterpene hydrocarbon biosynthesis by isolated leucoplasts of Citrofortunella mitis

A plastid vesicle preparation isolated from exocarpium of young Citrofortunella mitis (calamondin) fruits was able to synthesise monoterpene hydrocarbons when incubated with isopentenyl pyrophosphate. The electron-microscope comparison between this organelle fraction and the various plastid classes present in the peel tissues has shown the structural identity between these plastid vesicles and the leucoplasts of the epithelial cells lining the secretory pockets. The monoterpene biosynthesis required the presence of dimethylallyl pyrophosphate, Mn(2+) or Mg(2+) and was increased by addition of 2-mercaptoethanol. Evidence is provided that the leucoplast vesicles act as a complete system in which occur all the successive steps involved in monoterpene hydrocarbon elaboration from isopentenyl pyrophosphate.

Substrate and metal specificity in the enzymic synthesis of cyclic monoterpenes from geranyl and neryl pyrophosphate

A partially purified enzyme (carbocyclase) from the flavedo of Citrus limonum formed alpha-pinene, beta-pinene, limonene, and gamma-terpinene from geranyl pyrophosphate (GPP) and neryl pyrophosphate. The maximum specific activities obtained were 7.0 and 3.6 nmol/min/mg, respectively. Cross-inhibition by the two substrates were observed and the ability to utilize neryl pyrophosphate was almost completely lost with aging. Citronellyl pyrophosphate and dimethylallyl pyrophosphate were the most effective inhibitors of carbocyclase. Isopentenyl pyrophosphate, the monophosphate esters of nerol and geraniol, as well as inorganic pyrophosphate were much less effective inhibitors. The enzyme had an absolute requirement for Mn2+. It could be replaced with about 2% effectiveness by Mg2+ and Co2+. Kinetic studies showed that the observed reaction rate correlates with the calculated concentration of the GPP (Mn2+)2 species. Previous evidence with nonenzymatic reactions and the results presented support the view that the mechanism of carbocyclase may be the intramolecular analog of prenyltransferase.

Demonstration that limonene is the first cyclic intermediate in the biosynthesis of oxygenated p-menthane monoterpenes in Mentha piperita and other Mentha species

The volatile oil of mature Mentha piperita (peppermint) leaves contains as major components the oxygenated p-menthane monoterpenes l-menthol (47%) and l-menthone (24%) as well as very low levels of the monoterpene olefins limonene (1%) and terpinolene (0.1%), which are considered to be probable precursors of the oxygenated derivatives. Immature leaves, which are actively synthesizing monoterpenes, produce an oil with comparatively higher levels of limonene (approximately 3%), and isolation of the pure olefin showed this compound to consist of approximately 80% of the l-(4S)-enantiomer and approximately 20% of the d-(4R)-enantiomer. The time course of incorporation of [U-14C]sucrose into the monoterpenes of M. piperita shoot tips was consistent with the initial formation of limonene and its subsequent conversion to menthone via pulegone. d,l-[9-3H]Limonene and [9,10-3H]terpinolene were prepared and tested directly as precursors of oxygenated p-menthane monoterpenes in M. piperita shoot tips. Limonene was readily incorporated into pulegone, menthone, and other oxygenated derivatives, whereas terpinolene was not appreciably incorporated into these compounds. Similarly, d,l-[9-3H]limonene was specifically incorporated into pulegone in Mentha pulegium and into the C-2-oxygenated derivative carvone in Mentha spicata, confirming the role of this olefin as the essential precursor of oxygenated p-menthane monoterpenes. Soluble enzyme preparations from the epidermis of immature M. piperita leaves converted the acyclic terpenoid precursor [1-3H]geranyl pyrophosphate to limonene as the major cyclic product, providing a further indication that this olefin plays a central role in the formation of oxygenated monoterpenes in Mentha. No free intermediates were detected in the cyclization of geranyl pyrophosphate to limonene, suggesting that the olefin is the first cyclic intermediate to arise in the pathway, and resolution of the biosynthetic limonene, by crystallization of the derived d- and l-carvoximes, indicated an enantiomer mixture nearly identical to that isolated from the leaf oil.