The role of apolipophorin III in in vivo lipoprotein interconversions in adult Manduca sexta

Sustained flight in the moth, Manduca sexta, necessitates lipid mobilization and transport to flight muscle, a process mediated by the adipokinetic hormone. An adult specific high density lipophorin (lipoprotein, HDLp-A, Mr = 7.68 X 10(5)) accepts diacylglycerol from the fat body, increasing in size and decreasing in density, to give a low density lipophorin (lipoprotein, LDLp, Mr = 1.56 X 10(6)). During this process, several molecules of the small apolipoprotein, apolipophorin III (apoLp-III), are added to the two molecules originally present in HDLp-A. A study of the time course of adipokinetic hormone-induced loading of diacylglycerol onto HDLp-A, using the analytical ultracentrifuge and gel filtration, suggests that a lipoprotein of density intermediate between HDLp-A and LDLp was formed transiently. Analysis of lipoproteins separated by density gradient ultracentrifugation in the course of the loading process indicates that apoLp-III is added more rapidly than diacylglycerol and that it changes its conformation on the surface as more diacylglycerol is added. Taken together with the known properties of apoLp-III, a prolate ellipsoid with an axial ratio of 3, we suggest that initially apoLp-III adds to the expanded hydrophobic surface of the lipoprotein with its short axis parallel to the surface and that apoLp-III subsequently unfolds to cover a greater area of hydrophobic surface. Exchange experiments with labeled apoLp-III showed that the two apoLp-III molecules in HDLp-A do not exchange with free apoLp-III, even when the lipoprotein passed through a loading and unloading cycle, suggesting a structural role for apoLp-III in HDLp-A.

Regulation of ecdysone biosynthesis in the tobacco hornworm, Manduca sexta: titre of the haemolymph stimulatory factor during the last larval instar

A recently isolated haemolymph protein appears to be an important regulator of ecdysone biosynthesis by prothoracic glands in Manduca sexta. Using a dose-response titration protocol, the haemolymph titre of this stimulatory factor was determined during the last larval instar. The titre was high (greater than 2.0 U ml-1) on days 0 and 1, then dropped significantly to 0.55 U ml-1 on day 2, and remained depressed until day 4. The titre of the stimulatory factor then increased to a peak of 1.62 U ml-1 on day 7, and remained elevated (approx. 1.1 U ml-1) until the end of the instar. A set of physical and biochemical criteria was used to confirm that the stimulatory activity present in haemolymph on different days of the instar represented the presence of the factor. The data are consistent with the hypothesis that fluctuations in the titre of the haemolymph stimulatory factor play a critical role in regulating ecdysone biosynthesis during larval-pupal development.

In vitro synthesis, release and uptake of storage proteins by the fat body of Manduca sexta: putative hormonal control

1. Two major proteins (P1 and P2) are synthesized by the fifth instar larval fat body of Manduca sexta and then released into the hemolymph. 2. These proteins are later sequestered by the pre-pupal fat body. 20-Hydroxyecdysone does not appear to affect the synthesis of either protein. 3. When day 2 fifth instar larvae are neck-ligated there is an excessive synthesis (supersynthesis) of P2 (arylphorin). 4. Juvenile hormone I (JH I) applications to ligated animals had no effect, but brain homogenate injections resulted in the inhibition of P2 synthesis. 5. Neck ligations of larvae between days 5 and 6 revealed a head critical period between day 5 + 12 hr and day 5 + 18 hr, after which the head is unnecessary for the sequestration of either protein by the fat body. 6. JH I and JH III applications to ligated larvae before the head critical period do not restore the ability of the fat body to sequester the storage proteins. 7. P1 and P2 appear to be synthesized differentially and P2 is sequestered by the fat body to a much lesser extent than P1. 8. P2 is the hemolymph storage protein of both larval and pupal stages, whereas P1 appears to be the storage protein of the pupal fat body. 9. The data indicate that the synthesis of arylphorin and the resorption of both proteins are controlled by a putative head factor(s).

Lipoprotein biosynthesis in the larvae of the tobacco hornworm, Manduca sexta

Lipoprotein biosynthesis in larvae of the tobacco hornworm (Manduca sexta) was investigated. By immunoblotting, it was shown that the apoproteins are present in the fat body, but not in the midgut. Fat body incubated in vitro with [35S]methionine secreted labeled apoproteins. However, when the density of the secreted particle was determined, it was found at 1.24-1.28 g/ml instead of 1.15 g/ml, which is the density of the circulating lipoprotein. Lipid analysis of immunoprecipitated lipoprotein secreted by the fat body showed a phospholipid/diacylglycerol ratio of 8.3 rather than 0.9, the ratio found in the circulating lipoprotein. When labeled oleic acid or triolein was fed to larvae, it was found that greater than 98% of the label in the circulating lipoprotein was in diacylglycerol. In studies using animals raised on a fat-free diet, it was shown that the circulating lipoprotein has properties comparable to those of the material secreted in vitro by the fat body and that this diacylglycerol-poor particle can be converted to the normal lipoprotein by feeding a bolus of triolein. These data support the hypothesis that the fat body makes and secretes a "nascent" lipoprotein which contains apoproteins and phospholipid, but is devoid of diacylglycerol. The diacylglycerol is then picked up from the midgut to complete assembly of the mature circulating lipoprotein.

Development of the prepupal Verson's gland of the tobacco hornworm, Manduca sexta, and its hormonal control

The segmentally arranged Verson's glands are epidermal derivatives comprised of three cells: the duct, saccule, and secretory cells. The development of these glands was followed through the 5th instar and larval-pupal transition of Manduca sexta. The glands are relatively small during the feeding stage, begin to grow at wandering, and undergo about a 50-fold increase in size during the prepupal period. The increase in size is due mainly to the hypertrophy of the secretory cell which synthesizes a heterogeneous set of proteinaceous secretory products. Three prominent 11 to 12 kiloDalton (kD) polypeptides are made by the pharate fifth larval gland, whereas the pupal gland produces polypeptides ranging from 14 to 75 kD with a major complex at 30 to 34 kD. The secretory product is poured out onto the surface of the new cuticle at the time of ecdysis and contains all of the major proteins detected in extracts of the whole gland. The accumulation of secretory products by the gland occurs during the prepupal peak of ecdysteroid and is blocked if this rise is prevented by abdominal isolation. Infusion of 30 micrograms 20-hydroxyecdysone (20-HE) into such isolated abdomens caused synthesis of the pupal products. Treatment with the juvenile hormone mimic, methoprene, during the fifth instar showed that the commitment of the glands to produce the pupal proteins is independent of and occurs before the overlying epidermis becomes committed to make pupal cuticle.

Preliminary characterization of crystals of the protein insecticyanin from the tobacco hornworm Manduca sexta L

Crystals of insecticyanin, a blue protein from the haemolymph of the tobacco hornworm Manduca sexta L. have been obtained. They are hexagonal, space group P6(1)22 (or P6(5)22), with unit cell dimensions a = b = 79.1 A, c = 312.1 A. The crystals diffract to 2.5 A resolution, and they are suitable for X-ray diffraction analysis.

Effects of chitin synthesis inhibitors on incorporation of nucleosides into DNA and RNA in a cell line from Manduca sexta (L)

Five putative chitin synthesis inhibitors (CSI) were tested to determine if they inhibited nucleoside incorporation into acid precipitable material in a cell line from Manduca sexta (L.). The results varied. Diflubenzuron (DFB) (100 micron) inhibited cytidine incorporation by 38%; EL-494 (100 micron) inhibited adenosine incorporation by 43%; Bay Sir 8514 (100 micron) inhibited uridine incorporation by 24%. Superdiflubenzuron (100 micron) was the worst inhibitor overall (18-22%) for the benzoylphenyl urea CSI. The triazine CSI, CGA 19255, was the best inhibitor tested with 60% inhibition for cytidine and 49% for adenosine incorporation into DNA and RNA. Examination of cells incubated with diflubenzuron by scanning electron microscopy revealed distinct external morphological changes. Transmission electron microscopy showed that crystalline structures accumulated in the cytoplasm of cells treated with DFB. The crystalline structures were assumed to be diflubenzuron and they persisted even after diflubenzuron was removed from the medium.

Developmental changes in choline uptake and acetylcholine metabolism in the larval brain of the tobacco hornworm, Manduca sexta

The larval brain of the tobacco hornworm, Manduca sexta, was maintained in vitro and the uptake of labelled and unlabelled choline as well as their subsequent metabolism were measured by high-voltage paper electrophoresis. Significant levels of choline lipid metabolites, phosphorylcholine and acetylcholine (ACh) were noted. Unbound choline reached equilibrium after 6-8 h of incubation, while ACh accumulation continued to increase after 24 h indicating that the rate of synthesis exceeded the rate of breakdown. An apparent Km could not be determined for these whole-organ studies; however, the Vmax for ACh accumulation for days 5 (70 pmol/brain/h) and 6 (105 pmol/brain/h) of the last larval instar did vary significantly while the level of unbound choline in the brain did not change. The level of choline uptake was dependent upon the presence of Na+ and Ca2+, while the amount of ACh accumulated was affected specifically by the presence of Mg2+, the latter ion activating acetylcholinesterase. The determination of levels of unbound choline and ACh accumulation in the developing brain during the last two larval instars demonstrated increases in acetylcholine accumulation at previously reported times of the release of the hormone that initiates the molting process, prothoracicotropic hormone. These changes in the patterns of ACh accumulation occur during 4-8 h time intervals; this is the first report of such short-range changes in neurotransmitter metabolism in whole brains. The intensity of the ACh accumulation shift is equivalent to the intensity of the hormone burst. Other fluctuations in the levels of ACh accumulation and free choline correlate with the development of the brain.

Chelonus sp.: suppression of host ecdysteroids and developmentally stationary pseudoparasitized prepupae

When eggs of the caterpillar Trichoplusia ni are stung by Chelonus sp. (near C. curvimaculatus) (Braconidae), the developing host larvae precociously spin a cocoon but then remain developmentally stationary in the prepupal stage. The latter event happens even in hosts which were stung and precociously spin cocoons but which, upon dissection, contain no obvious parasite. Injection of radiolabeled ecdysone into either pseudoparasitized or allatectomized larvae demonstrates suppressed rates of conversion of ecdysone to 20-hydroxyecdysone when compared with controls. The data indicate that the occurrence of developmentally stationary pseudoparasitized prepupae is due to less production of ecdysteroid and less conversion of ecdysone to 20-hydroxyecdysone, both probably as a result of suppressed juvenile hormone titer.

L-Canavanine and protein synthesis in the tobacco hornworm Manduca sexta

L-Canavanine, a nonprotein amino acid of certain leguminous plants, manifests potent insecticidal properties in a canavanine-sensitive insect such as the tobacco hornworm Manduca sexta (L.) (Sphingidae). This arginine analog is activated and aminoacylated by arginyl-tRNA synthetase and incorporated into nascent polypeptide chains to create structurally aberrant, canavanine-containing proteins. Analysis of incorporation of [3H]leucine into protein in M. sexta larvae that had been injected with canavanine revealed that this arginine analog stimulates protein synthesis. During the first 3 hr after injection of canavanine, canavanine-mediated net stimulation of protein formation was readily discerned. Thereafter, the stimulation of protein synthesis appeared to be offset by the preferential degradation of anomalous proteins. Double-label protein-turnover experiments with larvae injected with [14C]canavanine- and [3H]arginine-containing hemolymph proteins showed that canavanine-containing proteins were degraded preferentially.

Delta endotoxin inhibits Rb+ uptake, lowers cytoplasmic pH and inhibits a K+-ATPase in Manduca sexta CHE cells

Delta endotoxin, a 68 kilodalton protein isolated from Bacillus thuringiensis spp. Kurstaki, is a potent entomocidal agent that alters a K+ current across midgut tissue of many phytophagous insects. This toxin completely inhibited the vanadate-sensitive 86Rb+ uptake and mimicked the vanadate-induced decrease in cytosolic pH in a cell line (CHE) originating from Manduca sexta embryonic tissue. The toxin also inhibited a K+-sensitive-ATPase in the plasma membranes isolated from these cells. Using the K+-sensitive-ATPase substrate p-nitrophenyl phosphate, delta endotoxin was found to have a Ki of 0.4 microM. These data suggest that the toxin inhibits a K+-ATPase responsible for 86Rb+ uptake in the CHE cells. The relationship between the toxin inhibition of K+-ATPase and toxin-altered K+ current is discussed.

High specific activity enantiomerically enriched juvenile hormones: synthesis and binding assay

A stereoselective total synthesis of chiral juvenile hormone I is described that allows stoichiometric introduction of two tritium atoms in the final step. Both optical antipodes of the pivotal epoxy alcohol intermediate were prepared in 95% enantiomeric excess by the Sharpless epoxidation of a (Z)-allylic alcohol. Elaboration of the hydroxy-methyl group to a vinyl group followed by selective homogeneous tritiation affords optically active juvenile hormone I analogs at 58 Ci/mmol. Competitive binding of the labeled 10R, 11S and 10S,11R enantiomers with unlabeled enantiomers to the hemolymph binding protein of Manduca sexta larvae was determined by using a dextran-coated charcoal assay. The natural 10R,11S enantiomer has twice the relative binding affinity of the 10S,11R enantiomer. The availability of such high specific activity optically pure hormones will contribute substantially to the search for high-affinity receptors for juvenile hormones in the nuclei of cells. Moreover, the chiral 12-hydroxy-(10R,11S)-epoxy intermediate allows modification of juvenile hormone for solid-phase biochemical and radioimmunochemical work without altering either the biologically important carbomethoxy or epoxy recognition sites.

Barium modifies the concentration dependence of active potassium transport by insect midgut

The rate of active K+ transport by the isolated lepidopteran midgut shows a rectangular hyperbolic relation to [K+] over the range 20 to 70 mM K+ in the absence of any divalent cation. Addition of Ba++ to the hemolymph (K+ uptake) side introduces a linear component to the concentration dependence, such that active K+ transport is decreased at [K+] of 55 mM or less, but increased transiently at higher [K+]. As [Ba++] is increased over the range 2 to 8 mM the linear component increases and the saturating component decreases; in 8 mM Ba++ the concentration dependence is dominated by the linear component. The effect of Ba++ cannot easily be accounted for by simple competition with K+ for basal membrane uptake sites. Similar effects might be exercised by other alkali earth cations, since the concentration dependence of active K+ transport possesses a substantial linear component in solutions containing 5 mM Ca++ and 5 mM Mg++ (the alkali earth metal concentrations of standard lepidopteran saline).

Intracellular stimulation of an identified neuron evokes cardioacceleratory peptide release

The central nervous system of the tobacco hawkmoth, Manduca sexta, is known to contain two cardioacceleratory peptides (CAP's), both of which function in vivo as cardioregulatory neurohormones. Intracellular electrical stimulation of a single abdominal ganglion neuron evokes the release of CAP-like bioactivity. This stimulation-evoked bioactivity is destroyed by prior treatment with protease. The possibility that intracellular stimulation of a CAP-containing neuron synaptically activated additional spiking neurons is eliminated.

Identification of neurones containing cardioacceleratory peptides (CAPs) in the ventral nerve cord of the tobacco hawkmoth, Manduca sexta

The abdominal ganglion neurosecretory cells responsible for the synthesis and release of two insect neurohormones, cardioacceleratory peptides 1 and 2 (CAP1 and CAP2), from the perivisceral organs (PVOs) have been identified in the tobacco hawkmoth, Manduca sexta. Previous work established the existence of two groups of abdominal ganglion cell bodies with axons projecting to the PVO: four laterally-situated pairs and five pairs lying on the midline (Taghert & Truman, 1982b). Micro-dissection and bioassay of various parts of an abdominal ganglion revealed that CAP activity was greatest in the medial portion of the ganglion, the portion containing the 10 midline neurones. Six of the 10 midline neurosecretory cells, the new midline bilateral (MB) cells, appeared to differentiate post-embryonically, commencing differentiation late in the last larval instar and reaching maturity midway through adult development. The development of the new MB cells was mirrored by the accumulation of CAP activity in the abdominal nerve cord. Not present in measurable amounts in larvae, CAP activity was first detectable a few days after pupation and reached maximal levels midway through adult development. CAP-like bioactivity was collected from the PVO in response to antidromic stimulation of the nerve containing the new MB axons. No CAP-like bioactivity was detected in those preparations in which the new MB axons were severed or in which other nerves were stimulated. Intracellular stimulation of a new MB neurone evoked the release from the PVO of measurable levels of CAP bioactivity. It was shown that this stimulation-evoked, cardioacceleratory activity was sensitive to protease treatment, and was released only from the cell that was stimulated. On the basis of these experiments, it was concluded that the CAPs are synthesized and secreted from the new MB cells.

Oxygen supply and limiting oxygen pressures in an insect larva

Larvae of the moth, Carpocapsa saltitans, demonstrate a diurnal activity pattern of rhythmic twitching which, under conditions of controlled light and temperature, is characterized by a predictable frequency and regularity. The twitching activity is shown to be sensitive to the partial pressure of environmental oxygen, and it ceases altogether at a particular PO2 called 'critical'. Use is made of the 'critical' PO2 in normobaric and hypobaric conditions to deduce the roles of diffusion and convection in the larval oxygen transport mechanisms; and also as a value for the total decrement of PO2 from ambient air to mitochondria, in order to evaluate predicted values based on calculations of resistance to oxygen flow. For this latter study 'porosity' of the larva and the seed pod in which it is normally housed was inferred from measured rates of water vapor loss, and oxygen uptake rates of the larvae were measured by the manometric technique of Warburg. Applying these data to a model system the conclusion was reached that almost the total resistance to oxygen flow is at the spiracle.

Purification and characterization of a biliverdin-associated protein from the hemolymph of Manduca sexta

A biliverdin binding protein, insecticyanin, has been isolated from the hemolymph of the fourth instar tobacco hornworm Manduca sexta. The protein has been purified to apparent homogeneity by conventional chromatography with a cumulative yield of 40-50%. The protein (Mw 71 600) is composed of three subunits (Mr 23 000). Each subunit binds one biliverdin molecule. Proton magnetic resonance spectroscopy and absorption spectroscopy demonstrate that the bilin is the biliverdin IX gamma isomer.

Chloride transport across the integumentary epithelium of Manduca sexta (Lepidoptera: Sphingidae)

Moulting fluid of Manduca sexta contains high concentrations of potassium and bicarbonate (100 mM) and low concentrations of chloride (5 mM). This fluid begins to disappear from the exuvial space approximately 9-10 h before the actual shedding of the integument. During this time, the integument can be isolated in an Ussing cell and electrical properties measured in vitro. In a normal 32 mM KHCO3 saline, potential difference (PD) is around 10 mV, exuvial side positive, and short-circuit current (SCC) is 15-20 microA cm-2. Substitution of chloride slightly reduces both PD and SCC, although resistance does not change significantly. Measurement of chloride transport in the absence of K+ indicates that 100% of the SCC can be accounted for by the net chloride flux (approximately 2 microA cm-2). The Km and Jmax for transepithelial chloride transport are 14 mM and 0.1 microEq cm-2 h-1. Bilateral potassium addition stimulates chloride transport, doubling net chloride flux as potassium concentration increases from 2 to 5 mM. Chloride net flux is not inhibited by the presence of furosemide (1 mM), nor in HCO3- -free saline by thiocyanate (1 or 10 mM) or acetazolamide (0.1 mM), but is inhibited by 100% N2. The pattern of chloride transport in M. sexta is similar to that previously reported for the rectum of locusts. As chloride is normally at low concentrations in the moulting fluid, it is suggested that this transport system acts to maintain low intracellular concentrations which may be necessary for enzymatic functions in the epidermal cells and has little importance in fluid transport.

Cuticular cavities: storage chambers for cyanoglucoside-containing defensive secretions in larvae of a Zygaenid moth

As a defensive reaction against predators the larvae of Zygaena trifolii Esper. 1783, release highly viscous fluid droplets out of cuticular cavities. The fluid appears on the cuticular surface upon contraction of the irritated segments, with no specialized muscles being involved. Two morphologically different types of cavities have been found: the larger ones are located beneath pigment spots, the smaller ones occupy the remaining surface except in the ventral region. Both types have complicated cuticular opening structures. The defensive fluid contains the cyanoglucosides linamarin and lotaustralin, the amino acid beta-cyano-L-alanine, proteins and water. Although a considerable amount of fluid (3-6 microliter per sixth-instar larva) is stored in the cuticle, fine structural examinations of the epidermis do not show any specific cells or cell areas with morphological adaptations for secretion. Further, there do not exist any major cytological differences between the cells below the cavities and in the ventral region, where those cavities are absent.

Soluble peptidoglycan fragments stimulate antibacterial protein synthesis by fat body from larvae of Manduca sexta

Both hemocytes and fat body from larvae of Manduca sexta, which have been injected with inducers of antibacterial protein synthesis, contain immunoreactive lysozyme. However, fat body is a richer source and has been demonstrated to synthesize and release lysozyme and cecropin-like peptides (bactericidins) in vitro. Fat body secretion of lysozyme and bactericidins is stimulated by addition of soluble peptidoglycan fragments to culture medium. The rate of lysozyme secretion by fat body varies as a function of peptidoglycan inducer concentration. These data are consistent with the hypothesis that, in vivo, bacteria must be phagocytized and partially degraded (processed) by hemocytes to generate a signal (peptidoglycan) that subsequently induces antibacterial protein synthesis by fat body.

The antibacterial effect of attacins from the silk moth Hyalophora cecropia is directed against the outer membrane of Escherichia coli

The attacins are antibacterial proteins which accumulate in the hemolymph of the giant silk moth, Hyalophora cecropia, in response to a bacterial infection. Here we show that the permeability barrier function of the outer membrane is affected shortly after addition of attacin to growing cultures of Escherichia coli. Specifically, the penetration through the outer membrane of beta-lactam antibiotics, chicken egg white lysozyme and the detergent Triton X-100 was found to be facilitated. The sensitivity of E. coli to cecropin B, another antibacterial protein present in the hemolymph of H. cecropia, was also found to be increased after treatment with attacin. The results suggest that the target of the attacins in E. coli is the outer membrane. Other effects of the attacins which have been observed are likely to be indirect consequences of the alteration in the properties of the outer membrane. These effects include changes in the cell shape, irregular patterns of cell division and lysis. The minimal concentration at which the attacins affected the growth of E. coli was 1 and 0.5 microM for the neutral (pI 7) and basic (pI 9) attacins, respectively, which corresponds to less than 2% of the concentration of the attacins in the hemolymph of infected pupae.

Characterization of monovalent ion transport systems in an insect cell line (Manduca sexta embryonic cell line CHE)

The monovalent ion transport systems of an immortalized insect cell line (CHE) have been investigated. These cells are unusual in that unlike most vertebrate cells, their normal extracellular environment consists of high potassium and low sodium concentrations. CHE cells maintained high intracellular [K+] through both a furosemide-inhibitable and a vanadate-inhibitable transport system. Intracellular exchangeable [Na+] was slightly lower than the extracellular [Na+] and was maintained at this level through a vanadate-sensitive transport system. Na+ uptake was also inhibited by furosemide: however, the stoichiometry of furosemide-sensitive Na+ uptake when compared with furosemide-sensitive K+ uptake indicated that these cations are not cotransported. 4,4'-Diisothiocyano-2,2'-disulfonic acid stilbene (DIDS) inhibited Na+, K+, and Cl- uptake. Vanadate and furosemide decreased cytoplasmimic pH, while cytoplasmic pH increased in the presence of DIDS. A model is presented explaining how Na+, K+, Cl-, H+ and HCO3-fluxes are regulated in these cells.

Isolation of 2-fluorocitrate produced by in vivo dealkylation of 29-fluorostigmasterol in an insect

A novel pro-insecticide, 29-fluorostigmasterol, is proposed to cause mortality due to release of fluoroacetate during side chain dealkylation. The 29-3H-labeled substrate was fed to third instar tobacco hornworms (Manduca sexta) and erythro-2-fluoro-[2-3H] citrate was isolated in 0.012% yield by ion-exchange, silica gel, and reverse-phase chromatography of the tricarboxylic acid, trimethyl ester, and trimethyl ester benzoate, respectively. The less toxic 29-fluoro-[29-3H]sitosterol did not provide sufficient labeled fluorocitrate to allow isolation, while a more toxic 16-3H-labeled 16-fluorofatty acid gave nearly 1% conversion to labeled fluorocitrate. This is the first direct chemical evidence for the fate of the two carbons removed during phytosterol dealkylation in an insect. It is also the first use of labeled fluoroacetate precursors to identify labeled 2-fluorocitrate as an in vivo metabolite of these precursors.

Stereochemical course of pheromone biosynthesis in the arctiid moth, Creatonotos transiens

The biosynthetic conversion of a pyrrolizidine alkaloid (heliotrine, IV) to a male moth pheromone (hydroxydanaidal, III) is found to proceed with inversion of configuration at the remaining asymmetric center (C-7).