Isopentenoid synthesis in isolated embryonic Drosophila cells. Possible regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity by shunted mevalonate carbon

Coenzyme Q plays opposing roles on bacteria/fungi and viruses in Drosophila innate immunity

Coenzyme Q (CoQ or ubiquinone) is a lipid-soluble component of virtually all types of cell membranes and has been shown to play multiple metabolic functions. Several clinical diseases including encephalomyopathy, cerebellar ataxia and isolated myopathy were shown to be associated with CoQ deficiency. However, the role of CoQ in immunity has not been defined. In the present study, we showed that flies defective in CoQ biosynthetic gene coq2 were more susceptible to bacterial and fungal infections, while were more resistant to viruses. We found that Drosophila contained both CoQ9 and CoQ10, and food supplement of CoQ10 could partially rescue the impaired immune functions of coq2 mutants. Surprisingly, wild-type flies fed CoQ10 became more susceptible to viral infection, which suggested that extra caution should be taken when using CoQ10 as a food supplement. We further showed that CoQ was essential for normal induction of anti-microbial peptides and amplification of viruses. Our work determined CoQ content in Drosophila and described its function in immunity for the first time.

Farnesylcysteine lyase is involved in negative regulation of abscisic acid signaling in Arabidopsis

The Arabidopsis FCLY gene encodes a specific farnesylcysteine (FC) lyase, which is responsible for the oxidative metabolism of FC to farnesal and cysteine. In addition, fcly mutants with quantitative decreases in FC lyase activity exhibit an enhanced response to ABA. However, the enzymological properties of the FCLY-encoded enzyme and its precise role in ABA signaling remain unclear. Here, we show that recombinant Arabidopsis FC lyase expressed in insect cells exhibits high selectivity for FC as a substrate and requires FAD and molecular oxygen for activity. Arabidopsis FC lyase is also shown to undergo post-translational N-glycosylation. FC, which is a competitive inhibitor of isoprenylcysteine methyltransferase (ICMT), accumulates in fcly mutants. Moreover, the enhanced response of fcly mutants to ABA is reversed by ICMT overexpression. These observations support the hypothesis that the ABA hypersensitive phenotype of fcly plants is the result of FC accumulation and inhibition of ICMT.

Arabidopsis thaliana plants possess a specific farnesylcysteine lyase that is involved in detoxification and recycling of farnesylcysteine

In plants, prenylated proteins are involved in actin organization, calcium-mediated signal transduction, and many other biological processes. Arabidopsis thaliana mutants lacking functional protein prenyltransferase genes have also revealed roles for prenylated proteins in phytohormone signaling and meristem development. However, to date, the turnover of prenylated plant proteins and the fate of the prenylcysteine (PC) residue have not been described. We have detected an enzyme activity in Arabidopsis plants that metabolizes farnesylcysteine (FC) to farnesal, which is subsequently reduced to farnesol. Unlike its mammalian ortholog, Arabidopsis FC lyase exhibits specificity for FC over geranylgeranylcysteine (GGC), and recognizes N-acetyl-FC (AFC). FC lyase is encoded by a gene on chromosome 5 of the Arabidopsis genome (FCLY, At5g63910) and is ubiquitously expressed in Arabidopsis tissues and organs. Furthermore, T-DNA insertions into the FCLY gene cause significant decreases in FC lyase activity and an enhanced response to abscisic acid (ABA) in seed germination assays. The effects of FCLY mutations on ABA sensitivity are even greater in the presence of exogenous FC. These data suggest that plants possess a specific FC detoxification and recycling pathway.

The mevalonate pathway and the synthesis of juvenile hormone in insects

The mevalonate pathway in insects has two important peculiarities, the absence of the sterol branch and the synthesis of juvenile hormone (JH), that may have influenced the mechanisms of regulation. The data available on these mechanisms indicate that cholesterol does not play a regulatory role and that JH modulates transcript levels of a number of genes of the mevalonate pathway or can influence the translatability and/or stability of the transcripts themselves. These data suggest that the mevalonate pathway in insects can best be interpreted in terms of coordinated regulation, in which regulators act in parallel to a number of enzymes, as occurs in the cholesterol-driven pathway in vertebrates.

Differential accumulation of dimethylallyl diphosphate in leaves and needles of isoprene- and methylbutenol-emitting and nonemitting species

The biosynthesis and emission of volatile plant terpenoids, such as isoprene and methylbutenol (MBO), depend on the chloroplastic production of dimethylallyl diphosphate (DMAPP). To date, it has been difficult to study the relationship of cellular DMAPP levels to emission of these volatiles because of the lack of a sensitive assay for DMAPP in plant tissues. Using a recent DMAPP assay developed in our laboratories, we report that species with the highest potential for isoprene and MBO production also exhibit elevated light-dependent DMAPP production, ranging from 110% to 1,063%. Even species that do not produce significant amounts of volatile terpenoids, however, exhibit some potential for light-dependent production of DMAPP. We used a nonaqueous fractionation technique to determine the intracellular distribution of DMAPP in isoprene-emitting cottonwood (Populus deltoides) leaves; approximately 65% to 70% of the DMAPP recovered at midday occurred in the chloroplasts, indicating that most of the light-dependent production of DMAPP was chloroplastic in origin. The midday concentration of chloroplastic DMAPP in cottonwood leaves is estimated to be 0.13 to 3.0 mM, which is consistent with the relatively high K(m)s that have been reported for isoprene synthases (0.5-8 mM). The results provide support for the hypothesis that the light dependence of isoprene and MBO emissions is in part due to controls over DMAPP production.

Nonradioactive assay for cellular dimethylallyl diphosphate

A sensitive, nonradioactive method was developed to measure cellular levels of dimethylallyl diphosphate (DMAPP), a central intermediate of isoprenoid metabolism in nature. The assay is based on the hydrolysis of DMAPP in acid to the volatile hydrocarbon isoprene (2-methyl-1,3-butadiene), with subsequent analysis of isoprene by headspace gas chromatography with reduction gas detection. In the assay, cell samples are directly acidified with 4 M H(2)SO(4) in sealed reaction vials. Therefore, there is no need to extract metabolites, purify them, and keep them stable prior to analysis, and degradative enzymatic activities are destroyed. DMAPP levels of 23 +/- 4 nmol (g fresh weight)(-1) [ca. 85 nmol (g dry weight)(-1)] and 80 +/- 14 nmol (g fresh weight)(-1) [ca. 296 nmol (g dry weight)(-1)] were measured in dark- and light-adapted leaves of Populus deltoides (Eastern cottonwood), respectively. Evidence is presented to show that DMAPP is the major leaf metabolite giving rise to isoprene following acid hydrolysis. DMAPP levels in Bacillus subtilis and Saccharomyces cerevisiae were determined to be 40.8 +/- 16.7 pmol (OD(600))(-1) [ca. 638 pmol (mg dry weight)(-1)] and 6.3 +/- 3.7 pmol (OD(600))(-1) [ca. 139 pmol (mg dry weight)(-1)], respectively. The method should be suitable for any cell or tissue type and isolated cellular organelles.

Farnesol oxidation in insects: evidence that the biosynthesis of insect juvenile hormone is mediated by a specific alcohol oxidase

The oxidation of farnesol to farnesoic acid is a key step in insect juvenile hormone biosynthesis. We herein present preliminary characterization of the enzyme-catalyzed oxidation of farnesol to farnesal in larval corpora allata homogenates of the tobacco hornworm, Manduca sexta. This conversion, which is highly substrate specific, has a K(m) apparent of 1 microM and a pH optimum between 6 and 7. Results from chemical modification experiments indicate that the enzyme possesses an active site tyrosine residue. Although farnesol oxidation in adult M. sexta corpora allata homogenates was previously identified as being catalyzed by a dehydrogenase, the corresponding conversion in larvae is not effected by the addition of nicotinamide cofactors. Instead, enzymatic activity is slightly enhanced by the addition of FAD, decreases when incubations are performed anaerobically, and is completely inhibited when either sodium dithionite or glucose oxidase is added. Although the effect of various additives suggests that the oxidation of farnesol to farnesal does not require a metal redox center, 1,10-phenanthroline (but not 4,7-phenanthroline) is a weak irreversible inhibitor of farnesol oxidation (IC(50)=11 mM). The addition of exogenous metals (Fe2+, Cu2+, Ni2+, and Co2+) caused differential effects on farnesol metabolism, with Cu2+ being highly inhibitory. Taken together, this data suggests that the oxidation of farnesol to farnesal in larval corpora allata is mediated by a specific oxygen-dependent enzyme, perhaps a flavin and/or iron-dependent oxidase.

Isopentenoid synthesis in isolated embryonic Drosophila cells: absolute mevalonic acid utilization and 3-hydroxy-3-methylglutaryl-coenzyme A reductase modulation

The relationship between absolute isopentenoidogenesis (total and specific) and 3-hydroxy-3-methylglutaryl-coenzyme A suppression in response to increased mevalonate availability is unknown. We determined absolute isopentenoidogenesis values for the nonsterologenic Drosophila Kc cell incubated (2 h) with increasing [3H]mevalonate concentrations. At least 80% of the maximum suppression of 3-hydroxy-3-methyl glutaryl-co-enzyme A activity was achieved when total isopentenoidogenesis was increased only 2-fold. However, a 12-fold increase in total isopentenoidogenesis was achieved at higher exogenous [3H]mevalonate concentrations. Thus, modulation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity was coupled to physiological changes in mevalonate/nonsterol isopentenoid availability. In contrast, isopentenoid accumulation, oxidation, and secretion were enhanced with pharmacological increases in mevalonate availability. Furthermore, an apparent constancy of total isopentenoidogenesis values plus increased metabolism of exogenous mevalonate and a significant (35-45%) suppression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase activity, in response to exogenous substrate concentrations (less than 150 microM), supported a partial or complete compensatory dimunition in endogenous substrate synthesis. Since these responses occurred within the 2-h study, earlier time periods must be assessed to (i) define the initial nonsterol-mediated regulatory response and (ii) to trap the nonsterol isopentenoid regulatory signal molecule(s).

Isopentenoid synthesis in isolated embryonic Drosophila cells: absolute, basal mevalonate synthesis rate determination

Embryonic Drosophila cells (Kc cells) and [5-3H]mevalonate (less than or equal to 10 microM) were used to determine the absolute basal in vivo rate of total mevalonic acid synthesis/utilization. An absolute in vivo mevalonic acid synthesis rate of 0.69 nmol/h/mg total cell protein was measured. Absolute mevalonate utilization was obtained by correcting for the extent of endogenous dilution of exogenous [3H]mevalonate at isotopic equilibrium. Cellular [3H]farnesol specific radioactivity was used as representative of a rapidly turning over isopentenoid pool. Although our previous Kc cell study (Havel, C. M., Rector, E. R. II, Watson, J. A., 1986, J. Biol. Chem. 261, 10,150-10,156) demonstrated that greater than or equal to 40% of the metabolized [3H]mevalonate appeared as 3H-labeled media water, this report established that t,t-3,7,11-[3H]trimethyl-2,6,10-dodecatriene-1,12 dioic acid was also secreted. Media accumulation of the C15-alpha,omega-prenyl dioic acid and 3H2O was related directly to [3H]mevalonic acid availability. This is the first mevalonate carbon balance study reported for a eukaryotic organism. It was concluded that (i) Kc cells synthesized more mevalonate than needed for normal growth and essential isopentenoids and (ii) excess mevalonate carbon accumulated intra- and extracellularly as isopentenoid compounds distal to C5 products. Finally, this study emphasized the need to measure total mevalonate utilization and not mevalonate conversion to a single isopentenoid end product in carbon balance investigations.

Isoprenoid metabolism in Plasmodium falciparum during the intraerythrocytic phase of malaria

Products of the isoprenoid metabolism were identified upon incubations of extracts from Plasmodium falciparum infected red blood cells with [14C] mevalonate. Uninfected erythrocytes and wild type yeast Saccharomyces cerevisiae extracts were used as controls. In parasitized red blood cells as well as in yeast extracts, mevalonate was converted into the biosynthetic isoprenoid precursors of sterol pathway until farnesyl pyrophosphate. In contrast, no mevalonate conversion was observed in uninfected erythrocyte extracts. The isoprenoid metabolism appeared stage-dependent as shown by the increase of radiolabelled farnesyl pyrophosphate amount at the beginning of the schizogonic phase (30-36 hours).

Utilization of mevalonate by aorta for the synthesis of medium-chain n-fatty acids (C8, C10) and acylglycerols

[2-14C]Mevalonate was incorporated into acylglycerols by bovine aorta. The majority of the radioactivity was found in diacylglycerols. Hydrolysis, followed by analysis by thin-layer chromatography, paper chromatography, gas chromatography, and gas chromatography-mass spectrometry showed that the radioactivity was greatest in the medium-chain n-fatty acids, caprylic (C8) and capric (C10) acids. Triacylglycerols and diacylglycerols have important functions in the maintenance of the metabolic integrity of organs and their constituent cells. Therefore, medium-chain fatty acids may play significant roles, such as solubilization and transport of cholesterol and regulation of metabolism, as constituents of diacylglycerols. The results suggest that the trans-methylglutaconate shunt of mevalonate metabolism functions in the aorta.

Characterization and regulation of HMG-CoA reductase during a cycle of juvenile hormone synthesis

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase was characterized in cockroach corpora allata which produce insect juvenile hormone III (methyl-(10R)10,11-epoxy-3,7,11-tri-methyl-2E,6E-dodecadienoate ). HMG-CoA reductase is a microsomal enzyme dependent on NADPH and dithiothreitol (or glutathione) for activity. The enzyme selectively reduced (3S)-HMG-CoA to (3R)-mevalonate with an apparent KM of 7.6 microM. Mevinolin was a competitive inhibitor of HMG-CoA reductase with a KI of 2.4 nM. No evidence for a modulation of enzyme activity by phosphorylation was obtained. Levels of HMG-CoA reductase were not altered after incubation of the corpora allata with either mevinolin (to decrease isoprenoid flux) or with mevalonate or farnesol (to increase isoprenoid flux). Split pairs of corpora allata were used to compare JH III synthetic activity with HMG-CoA reductase activity during the cycle of JH III synthesis that controls vitellogenesis and oocyte growth in adult females. Both activities changed over 10-fold and peaked on day 5 after emergence/mating, but JH III synthesis did not parallel HMG-CoA reductase activity precisely thereafter. The half-life of HMG-CoA reductase measured in the presence of cycloheximide was significantly different between low and high activity glands and was not related to the half-life of JH III synthesis. The results suggest that HMG-CoA reductase should not be considered 'the rate-limiting enzyme' in juvenile hormone synthesis by Diploptera punctata corpora allata.