Purification and characterization of prophenoloxidases from pupae of Drosophila melanogaster

Insect phenoloxidase and its diverse roles: melanogenesis and beyond

Insect life on earth is greatly diversified despite being exposed to several infectious agents due to their diverse habitats and ecological niche. One of the major factors responsible for their successful establishment is having a powerful innate immune system. The most common and effective method used by insects in recognizing pathogen and non-self-substances is the melanization process among others. The key enzyme involved in melanin biosynthesis is the copper containing humoral defense enzyme, phenoloxidase (PO). This review focused on understanding about PO and that had been in research for nearly a century. The review elaborates about evolutionary significance of PO in arthropods, its relationship with mammalian tyrosinases, various substrates, activators and inhibitors involved in the activation of phenoloxidase cascade, as it requires an integrated system of activation that vary among insect species. The enzyme also plays a vital role in insect immunity by involving in several other immune functions like sclerotization, wound healing, opsonization, encapsulation and nodule formation. Further, gene knock down or knock out of PO genes and inhibition of PO-melanization cascade by several mechanisms can also be considered as promising future alternative to control serious pests by making them highly susceptible to any targeted attack.

Effect of the insect phenoloxidase on the metabolism of l-DOPA

Insect prophenoloxidase (PPO) induces melanization around pathogens. Before melanization, PPO is cleaved into phenoloxidase (PO) by serine proteases. Insect PPO can also be activated by exogenous proteases secreted by pathogens as well as by other compounds, such as ethanol and cetylpyridinium chloride (CPC). However, the effect of these activators on the activity of PO is unclear. In this study, the insect endogenous serine protease AMM1, α-chymotrypsin, and ethanol were used to activate recombinant Drosophila PPO1 (rPPO1), and the PO activity differed depending on the activator applied. The PO-induced intermediates during melanization also varied markedly in their numbers and abundances. Therefore, this study indicates that the mechanism of PPO activation influences PO activity. It also suggests that PO-induced different intermediates may affect the antibacterial activity during melanization due to their toxicity.

Cloning, Expression, and Characterization of Prophenoloxidases from Asian Corn Borer, Ostrinia furnacalis (Gunée)

Insect phenoloxidase (PO) belongs to the type 3 copper protein family and possesses oxidoreductase activities. PO is typically synthesized as a zymogen called prophenoloxidase (PPO) and requires the proteolytic activation to function. We here cloned full-length cDNA for 3 previously unidentified PPOs, which we named OfPPO1a, OfPPO1b, and OfPPO3, from Asian corn borer, Ostrinia furnacalis (Gunée), in addition to the previously known OfPPO2. These conceptual PPOs and OfPPO2 all contain two common copper-binding regions, two potential proteolytic activation sites, a plausible thiol-ester site, and a conserved C-terminal region but lack a secretion signal peptide sequence at the N-terminus. O. furnacalis PPOs were highly similar to other insect PPOs (42% to 79% identity) and clustered well with other lepidopteran PPOs. RT-PCR assay showed the transcripts of the 4 OfPPOs were all detected at the highest level in hemocytes and at the increased amounts after exposure to infection by bacteria and fungi. Additionally, we established an Escherichia coli (E. coli) expression system to produce recombinant O. furnacalis PPO proteins for future use in investigating their functions. These insights could provide valuable information for better understanding the activation and functioning mechanisms of O. furnacalis PPOs.

Cloning, expression, and characterization of prophenoloxidase from Antheraea pernyi

Prophenoloxidase (PPO) is an essential enzyme in insect innate immunity because of its role in humoral defense. In this study, we have cloned a full-length cDNA of Antheraea pernyi prophenoloxidase (ApPPO) with an open-reading frame encoding 683 amino acids, and the deduced amino acid sequence of ApPPO exhibited a high similarity with those of lepidoptera. The expression of ApPPO was inducible so that the mRNA level was significantly upregulated in the microbial challenged tissues, including fat body, hemocytes, and midgut. To better investigate the enzymatic and immunological properties of ApPPO, recombinant ApPPO (rApPPO) was produced in Escherichia coli. Several functional verification experiments were performed after studying the enzymatic properties. It was found that rApPPO could be stimulated by the microbial challenged larvae hemolymph and then killed bacteria in the radial diffusion assay. Furthermore, rApPPO also induced the transcription of cecropins after injected into the larvae 24 h later.

Identification and characterization of a QM protein as a possible peptidoglycan recognition protein (PGRP) from the giant tiger shrimp Penaeus monodon

In an attempt to identify a peptidoglycan recognition protein (PGRP) in Penaeus (Penaeus) monodon, in vitro pull-down binding assays were used between shrimp proteins and purified peptidoglycan (PG). By gel electrophoresis and mass spectrometry followed by Mascot program analysis, proteins from shrimp hemocyte peripheral membrane proteins showed significant homology to records for a QM protein, actin and prophenoloxidase 2 precursor (proPO2), while proteins from cell-free plasma showed significant homology to records for a vitellogenin, a fibrinogen related protein (FREP) and a C-type lectin. Due to time and resource limitations, specific binding to PG was examined only for recombinant PmQM protein and PmLec that were synthesized based on sequences reported in the Genbank database (accession numbers FJ766846 and DQ078266, respectively). An in vitro assay revealed that hemocytes would bind with and encapsulate agarose beads coated with recombinant PmQM (rPmQM) or rPmLec and that melanization followed 2h post-encapsulation. ELISA tests confirmed specific binding of rPmQM protein to PG. This is the first time that PmQM has been reported as a potential PGRP in shrimp or any other crustacean. The two other potential PGRP identified (FREP and the vitellin-like protein present in male P. monodon, unlike other vitellin subunits) should also be expressed heterologously and tested for their ability to activate shrimp hemocytes.

Formation of disulfide bonds in insect prophenoloxidase enhances immunity through improving enzyme activity and stability

Type 3 copper proteins, including insect prophenoloxidase (PPO), contain two copper atoms in the active site pocket and can oxidize phenols. Insect PPO plays an important role in immunity. Insects and other invertebrates show limited recovery from pathogen invasion and wounds if phenoloxidase (PO) activity is low. In most insect PPOs, two disulfide bonds are present near the C-terminus. However, in Pimpla hypochondriaca (a parasitoid wasp), each PPO contains one disulfide bond. We thus questioned whether the formation of two sulfide bonds in insect PPOs improved protein stability and/or increased insect innate immunity over time. Using Drosophila melanogaster PPO1 as a model, one or two disulfide bonds were deleted to evaluate the importance of disulfide bonds in insect immunity. rPPO1 and mutants lacking disulfide bonds could be expressed and showed PO activity. However, the PO activities of mutants lacking one or two disulfide bonds significantly decreased. Deletion of disulfide bonds also reduced PPO thermostability. Furthermore, antibacterial activities against Escherichia coli and Bacillus subtilis significantly decreased when disulfide bonds were deleted. Therefore, the formation of two disulfide bond(s) in insect PPO enhances antibacterial activity by increasing PO activity and stability.

Activity of fusion prophenoloxidase-GFP and its potential applications for innate immunity study

Insect prophenoloxidase (PPO) is essential for physiological functions such as melanization of invading pathogens, wound healing and cuticle sclerotization. The insect PPO activation pathway is well understood. However, it is not very clear how PPO is released from hemocytes and how PPO takes part in cellular immunity. To begin to assess this, three Drosophila melanogaster PPO genes were separately fused with GFP at the C-terminus (rPPO-GFP) and were over-expressed in S2 cells. The results of staining and morphological observation show that rPPO-GFP expressed in S2 cells has green fluorescence and enzyme activity if Cu(2+) was added during transfection. Each rPPO-GFP has similar properties as the corresponding rPPO. However, cells with rPPO-GFP over-expressed are easier to trace without PO activation and staining. Further experiments show that rPPO1-GFP is cleaved and activated by Drosophila serine protease, and rPPO1-GFP binds to Micrococcus luteus and Beauveria bassiana spores as silkworm plasma PPO. The above research indicates that the GFP-tag has no influence on the fusion enzyme activation and PPO-involved innate immunity action in vitro. Thus, rPPO-GFP may be a convenient tool for innate immunity study in the future if it can be expressed in vivo.

Properties of Drosophila melanogaster prophenoloxidases expressed in Escherichia coli

Insect prophenoloxidases (PPOs) are a group of important innate immunity proteins. Although there have been numerous studies dealing with the PPO activation cascade, the detailed biochemical behaviors of the PPO family proteins remain to be clearly established. This is due primarily to the difficulty in obtaining adequate amounts of PPO proteins for comprehensive characterization. In this study, we expressed three Drosophila melanogaster PPO genes in Escherichia coli, and extensively evaluated expression conditions for obtaining soluble proteins. Through the manipulation of expression conditions, particularly the culture temperature of PPO-transformed E. coli cells, we were able to obtain large quantities of soluble recombinant PPO proteins. Additional Cu(2+), either added into the culture medium during PPO induction or directly mixed with the purified rPPO preparations, was necessary to produce Cu(2+) associated proenzymes. Cu(2+) associated PPOs showed obvious enzyme activities after activation by either ethanol or cetylpyridinium chloride, or by AMM1 (a pupal protein fraction containing native serine proteases for PPO activation). Dose responses for association of individual purified Drosophila rPPOs with Cu(2+) showed that Drosophila rPPO1 and rPPO3 had relatively higher affinity for Cu(2+) than rPPO2 did. Surprisingly, however, high concentration of Cu(2+) (2 mM) completely inhibited PPO activity. Each rPPO had similar activity when dopamine or l-DOPA was the substrate. However, rPPO1 alone had very high activity if l-tyrosine was used as a substrate. After activation by ethanol or 2-propanol, Km and Vmax of the three rPPOs changed as shown in the following: rPPO2<rPPO3<rPPO1. If activated by ethanol, the Km and Vmax of each rPPO were lower than by 2-propanol. Due to the difficulty in obtaining functional PPOs via traditional purification methods, the method established in this study will be helpful to produce active insect recombinant PPOs for the study of PPO properties and functions in the future.

Drosophila melanogaster prophenoloxidases respond inconsistently to Cu2+ and have different activity in vitro

Dipteran insects, like mosquitoes, possess more than two prophenoloxidase (PPO) genes, but it is unclear whether their gene products differ in biochemical properties and physiological functions. Here, we used three Drosophila melanogaster PPOs as models to study their properties through expression in S2 cells. Our data revealed that the PPOs were expressed in the ethanol-activatable conformation: rPPO1 and rPPO2 needed additional Cu(2+) in the medium, but rPPO3 did not. rPPO1 bound Cu(2+) within minutes; rPPO2 did that in hours when Cu(2+) were present at a higher concentration. Thus, rPPO1 and rPPO2 were expressed as apo-rPPO and became holo-PPO upon Cu(2+) binding; rPPO3 was holo-PPO immediately after expression. Surprisingly, in the absence of ethanol, the apparently intact rPPO3 catalyzed dopamine oxidation and melanization. The successful method for rPPO expression in S2 cells described in this paper will provide us with an opportunity to study the properties of a specific PPO gene in a small insect like mosquitoes in the future.

Selectively enhanced expression of prophenoloxidase activating enzyme 1 (PPAE1) at a bacteria clearance site in the white shrimp, Litopenaeus vannamei

BACKGROUND

The prophenoloxidase-activating (PO activating) system plays an important role in the crustacean innate immunity, particularly in wound healing and pathogen defense. A key member of this system is prophenoloxidase-activating enzyme (PPAE), which is the direct activator of prophenoloxidase (proPO). Despite their importance in crustacean PO activating system, the studies on them remain limited.

RESULTS

Here we report on a PPAE of white shrimp, Litopenaeus vannamei (lvPPAE1), which showed 94% similarity to PPAE1 of Penaeus monodon. We found that lvPPAE1 in fluid hemocytes was down regulated after challenge by Vibrio harveyi but was enhanced when shrimps were exposed to a bacteria-rich environment for long-term. In vivo gene silence of lvPPAE1 by RNAi can significantly reduce the phenoloxidase activity (PO) and increase the susceptibility of shrimps to V. harveyi. Although lvPPAE1 was down-regulated in fluid hemocytes by Vibrio challenge, its expression increased significantly in gill after bacteria injection, which is the primary bacteria-clearance tissue.

CONCLUSION

Suppressed expression in fluid hemocytes and enhanced expression in gill indicates selectively enhanced expression at the bacterial clearance site. This is a novel feature for PPAE expression. The results will contribute to our understanding of the PO activating system in crustaceans.

Purification, characterization and immunolocalization of a novel protease inhibitor from hemolymph of tasar silkworm, Antheraea mylitta

A novel serine protease inhibitor (AmPI) was purified from larval hemolymph of tasar silkworm, Antheraea mylitta by two-step process of trypsin-affinity and gel-filtration (FPLC) chromatography. AmPI was active against larval midgut and commercial bovine trypsin and chymotrypsin. The extent of purification was determined by SDS and Native PAGE. The protease inhibitor had an apparent molecular weight of approximately 14.5 kDa as determined by SDS-PAGE. Its activity was stable over a pH range of 4.5-9 and temperatures range of 4-65 degrees C. Molecular weight as determined by MALDITOF-MS was between 13241.63 and 13261.66 Da. MS profile of AmPI also suggests two isoforms of AmPI because of glycosylation by heptose (C(7)H(14)O(7)). This confirmed the result of Native PAGE showing two bands. N-terminal amino acid sequence of this protein did not show similarity to any known protease inhibitor. To study the functional implications of AmPI in insect, it was localized in insect body tissue of different larval instars by immunogold labeling technique using GAR-gold conjugate as secondary antibody. The pattern of localization suggests constitutive nature of AmPI, which may have role in insect's defense mechanism.

Identification of the gene encoding pro-phenoloxidase A(3) in the fruitfly, Drosophila melanogaster

The fruitfly, Drosophila melanogaster, has three proPO genes (DoxA1, CG8193 and CG2952). DoxA1 has been shown to encode proPO A(1), one of the two proPO isoforms (A(1) and A(3)). However, which of CG8193 or CG2952 encodes proPO A(3) has so far remained elusive. In Northern analysis, CG8193 expression was strong during the larval stage, yet expression of CG2952 was not detected at any stage. Immunoblot analyses with specific antibodies detected CG8193 in the larval hemolymph at the mobility of the endogenous proPOA(3), though no signal for CG2952. These results indicate that the expression of CG2952 is very low and that CG8193 is the gene that encodes proPO A3. Processing of A(1)and A(3) isoforms in adult homogenate and activity of recombinant proPOs were also investigated.

Armigeres subalbatus prophenoloxidase III: Cloning, characterization and potential role in morphogenesis

It has long been suggested that phenoloxidases (POs) play key roles in various physiological functions in insects, e.g., cuticular sclerotization, wound healing, egg tanning and melanotic encapsulation of pathogens. Here we report that a mosquito PO, designated Armigeres subalbatus prophenoloxidase III (As-pro-PO III), is likely involved in the morphogenesis in mosquito. Expression profile analysis found that As-pro-PO III mRNA is persistently expressed in adult mosquitoes and is not significantly affected by blood feeding, microfilariae inoculation, or Escherichia coli inoculation, but expression levels of As-pro-PO III fluctuated in larval and pupal stages. Knockdown of As-pro-PO III expression in pupae using double-stranded RNA resulted in high pupal mortality and deformed adults that subsequently died following emergence. Promoter activity analyses by electrophoretic mobility-shift assays and transfection assays suggest that the As-pro-PO III gene is positively regulated by a putative Zeste motif, a developmental regulatory element. These results suggest that As-pro-PO III is associated with morphogenesis of mosquitoes.

Similar enzyme activation and catalysis in hemocyanins and tyrosinases

This review presents the common features and differences of the type 3 copper proteins with respect to their structure and function. In spite of these differences a common mechanism of activation and catalysis seems to have been preserved throughout evolution. In all cases the inactive proenzymes such as tyrosinase and catecholoxidase are activated by removal of an amino acid blocking the entrance channel to the active site. No other modification at the active site seems to be necessary to enable catalytic activity. Hemocyanins, the oxygen carriers in many invertebrates, also behave as silent inactive enzymes and can be activated in the same way. The molecular basis of the catalytic process is presented based on recent crystal structures of tyrosinase and hemocyanin. Minor conformational differences at the active site seem to decide about whether the active site is only able to oxidize diphenols as in catecholoxidase or if it is also able to o-hydroxylate monophenols as in tyrosinase.

Evidence for proteins involved in prophenoloxidase cascade Eisenia fetida earthworms

The prophenoloxidase cascade represents one of the most important defense mechanisms in many invertebrates. Following the recognition of microbial saccharides by pattern recognition molecules, proteinases cleave inactive prophenoloxidase to its active form, phenoloxidase. Phenoloxidase is a key enzyme responsible for the catalysis of the melanization reaction. Final product melanin is involved in wound healing and immune responses. Prophenoloxidase cascade has been widely described in arthropods; data in other invertebrate groups are less frequent. Here we show detectable phenoloxidase activity in 90-kDa fraction of the coelomic fluid of earthworms Eisenia fetida. Amino acid sequencing of peptides from the active fraction revealed a partial homology with invertebrate phenoloxidases and hemocyanins. Moreover, the level of phenoloxidase activity is lower and the activation slower as compared to other invertebrates.

Thermolabile variant, PHOX-S, of prophenol oxidase in Drosophila melanogaster

The Phox(S) strain of Drosophila melanogaster is an electrophoretically slow variant found in a wild population at Victoria, Australia. Prophenol oxidase isoform A(1) from PHOX-S was purified and characterized biochemically and genetically. The purified fraction of A(1) from PHOX-S showed a homodimer with a molecular weight of the subunit of approximately 77 kDa. The Phox(S) strain was temperature sensitive in vivo in culture, and the purified protein was thermolabile in vitro. By the deletion mapping method, the Phox(S) locus was cytologically estimated to be at the location 55-A on the right arm of the second chromosome and 79.6 genetically. These data show that PHOX-S is an electrophoretic variant of MOX and that PHOX-S is the first thermolabile protein found in invertebrate prophenol oxidase.

Phenoloxidase in larvae of Plodia interpunctella (Lepidoptera: Pyralidae): molecular cloning of the proenzyme cDNA and enzyme activity in larvae paralyzed and parasitized by Habrobracon hebetor (Hymenoptera: Braconidae)

Phenoloxidase (PO) is a major component of the insect immune system. The enzyme is involved in encapsulation and melanization processes as well as wound healing and cuticle sclerotization. PO is present as an inactive proenzyme, prophenoloxidase (PPO), which is activated via a protease cascade. In this study, we have cloned a full-length PPO1 cDNA and a partial PPO2 cDNA from the Indianmeal moth, Plodia interpunctella (Hubner) (Lepidoptera: Pyralidae) and documented changes in PO activity in larvae paralyzed and parasitized by the ectoparasitoid Habrobracon hebetor (Say) (Hymenoptera: Braconidae). The cDNA for PPO1 is 2,748 bp and encodes a protein of 681 amino acids with a calculated molecular weight of 78,328 and pI of 6.41 containing a conserved proteolytic cleavage site found in other PPOs. P. interpunctella PPO1 ranges from 71-78% identical to other known lepidopteran PPO-1 sequences. Percent identity decreases as comparisons are made to PPO-1 of more divergent species in the orders Diptera (Aa-48; As-49; and Sb-60%) and Coleoptera (Tm-58; Hd-50%). Paralyzation of host larvae of P. interpunctella by the idiobiont H. hebetor results in an increase in phenoloxidase activity in host hemolymph, a process that may protect the host from microbial infection during self-provisioning by this wasp. Subsequent parasitization by H. hebetor larvae causes a decrease in hemolymph PO activity, which suggests that the larval parasitoid may be secreting an immunosuppressant into the host larva during feeding.

Phenoloxidase activity in Apis mellifera honey bee pupae, and ecdysteroid-dependent expression of the prophenoloxidase mRNA

Phenoloxidase (monophenol, l-dopa: oxygen oxidoreductase, EC 1.14.18.1) is a multicopper oxidase, which plays an important role in melanin synthesis, necessary for defense against intruding microorganisms and parasites, wound healing and cuticle pigmentation. A phenoloxidase from the hemolymph of honey bee pupae exhibited an apparent molecular mass of 70 kDa, as estimated by gel filtration and SDS-PAGE. Optimal pH and temperature were 6.5 and 20 degrees C, respectively. Activity was fully stable for 30 min at 50 degrees C. Like phenoloxidases from the hemolymph of other insects, the honey bee enzyme was activated by trypsin and inhibited by protease inhibitors and phenylthiourea. Only high concentrations of sodium azide effectively inhibited the detected activity. A low concentration (5 microM) of Ca2+, Mg2+, and Mn2+ had a stimulatory effect on the activity. Single Michaelis-Menten curves were observed for l-dopa and dopamine oxidation, but the affinity of the enzyme for dopamine was greater than for L-dopa. Semiquantitative RT-PCR and Southern blot analysis using a 359 bp labeled probe, and quantification of the prophenoloxidase mRNA levels by real-time PCR showed increased amounts of transcripts in hemocytes and integument from young pupae injected with 20-hydroxyecdysone.

Circular dichroism of prophenol oxidase in relation to the structural stability in Drosophila melanogaster

Circular dichroism (CD) of purified Drosophila melanogaster prophenol oxidase has been measured in the range of 195-245 nm. So far, few investigations about the interaction on higher-order structures have been performed. CD spectra of Drosophila prophenol oxidase with 2-propanol activator showed fluctuation of alpha-helices. At a high temperature of 80 degrees C, prophenol oxidase was partially denatured. However, it showed reversible recovery by renaturation after returning to low temperature at 30 degrees C. The conformational changes and reversible denaturation-renaturation interaction of the prophenol oxidase protein are discussed.

The prophenoloxidase-activating system in invertebrates

A major innate defense system in invertebrates is the melanization of pathogens and damaged tissues. This important process is controlled by the enzyme phenoloxidase (PO) that in turn is regulated in a highly elaborate manner for avoiding unnecessary production of highly toxic and reactive compounds. Recent progress, especially in arthropods, in the elucidation of mechanisms controlling the activation of zymogenic proPO into active PO by a cascade of serine proteinases and other factors is reviewed. The proPO-activating system (proPO system) is triggered by the presence of minute amounts of compounds of microbial origins, such as beta-1,3-glucans, lipopolysaccharides, and peptidoglycans, which ensures that the system will become active in the presence of potential pathogens. The presence of specific proteinase inhibitors prevents superfluous activation. Concomitant with proPO activation, many other immune reactions will be produced, such as the generation of factors with anti-microbial, cytotoxic, opsonic, or encapsulation-promoting activities.

Tyrosinases from crustaceans form hexamers

Tyrosinases, which are widely distributed among animals, plants and fungi, are involved in many biologically essential functions, including pigmentation, sclerotization, primary immune response and host defence. In the present study, we present a structural and physicochemical characterization of two new tyrosinases from the crustaceans Palinurus elephas (European spiny lobster) and Astacus leptodactylus (freshwater crayfish). In vivo, the purified crustacean tyrosinases occur as hexamers composed of one subunit type with a molecular mass of approx. 71 kDa. The tyrosinase hexamers appear to be similar to the haemocyanins, based on electron microscopy. Thus a careful purification protocol was developed to discriminate clearly between tyrosinases and the closely related haemocyanins. The physicochemical properties of haemocyanins and tyrosinases are different with respect to electronegativity and hydrophobicity. The hexameric nature of arthropod tyrosinases suggests that these proteins were the ideal predecessors from which to develop the oxygen-carrier protein haemocyanin, with its allosteric and co-operative properties, later on.

Regulation of antibacterial and antifungal innate immunity in fruitflies and humans

Insects have been very successful in adapting to their environment, and the ability of the insect immune system to detect and elicit the appropriate response against various invading pathogens has helped in this success. Unlike the vertebrate immune system, which consists of both innate and adaptive components, insect immunity probably consists entirely of an innate immune response, as no evidence of an adaptive response has been found. The innate immune response is described as either a reaction against "lack of self," or the interaction between host germline-encoded receptors and molecules unique to a particular class of invading organisms. Once the invading organism is recognized, the host immune response can be activated via signaling pathways that lead to the appropriate reaction. This review endeavors to put forth how through genetic, molecular, and biochemical studies of the fruit fly Drosophila melanogaster, as well as other insects, it is now understood that aspects of the insect and vertebrate innate immune system are very similar.

Purification, characterization and molecular cloning of prophenoloxidases from Sarcophaga bullata

Prophenoloxidase (PPO) is a key enzyme associated with both melanin biosynthesis and sclerotization in insects. This enzyme is involved in three physiologically important processes viz., cuticular hardening, defense reactions and wound healing in insects. It was isolated from the larval hemolymph of Sarcophaga bullata and purified by employing ammonium sulfate precipitation, Phenyl Sepharose chromatography, DEAE-Sepharose chromatography, and Sephacryl S-200 column chromatography. The purified enzyme exhibited two closely moving bands on 7.5% SDS-PAGE under denaturing conditions. From the estimates of molecular weight on Sephacryl S-100, TSK-3000 HPLC column and SDS-PAGE, which ranged from 90,000 to 100,000, it was inferred that the enzyme is made up of a single polypeptide chain. Activation of PPO (K(a)=40 microM) was achieved by the cationic detergent, cetyl pyridinium chloride below its critical micellar concentration (0.8 mM) indicating that the detergent molecules are binding specifically to the PPO and causing the activation. Neither anionic, nor nonionic (or zwitterionic) detergents activated the PPO. The active enzyme exhibited wide substrate specificity and marked thermal unstability. Using primers designed to conserved amino acid sequences from known PPOs, we PCR amplified and cloned two PPO genes from the sarcophagid larvae. The clones encoded polypeptides of 685 and 691 amino acids. They contained two distinct copper binding regions and lacked the signal peptide sequence. They showed a high degree of homology to dipteran PPOs. Both contained putative thiol ester site, two proteolytic activation sites and a conserved C-terminal region common to all known PPOs.

Molecular characterization of a prophenoloxidase cDNA from the malaria mosquito Anopheles stephensi

Some refractory anopheline mosquitoes are capable of killing Plasmodium, the causative agent of malaria, by melanotic encapsulation of invading ookinetes. Phenoloxidase (PO) appears to be involved in the formation of melanin and toxic metabolites in the surrounding capsule. A cDNA encoding Anopheles stephensi prophenoloxidase (Ans-proPO) was isolated from a cDNA library screened with an amplimer produced by reverse transcriptase polymerase chain reaction (RT-PCR) with degenerate primers designed against conserved proPO sequences. The 2.4-kb-long cDNA has a 2058 bp open reading frame encoding Ans-proPO of 686 amino acids. The deduced amino acid sequence shows significant homology to other insect proPO sequences especially at the two putative copper-binding domains. In A. stephensi, Ans-proPO expression was detected in larval, pupal and adult stages. The Ans-proPO mRNA was detected by RT-PCR and in situ hybridization in haemocytes, fat body and epidermis of adult female mosquitoes. A low level of expression was detected in the ovaries, whereas no expression was detected in the midguts. Semi-quantitative RT-PCR analysis of Ans-proPO mRNA showed that its expression was similar in adult female heads, thoraxes and abdomens. No change in the level of Ans-proPO expression was found in adult females after blood feeding, bacterial challenge or Plasmodium berghei infection. However, elevated PO activity was detected in P. berghei-infected mosquitoes, suggesting that in non-selected permissive mosquitoes PO may be involved in limiting parasite infection. Genomic Southern blot and immunoblots suggest the presence of more than one proPO gene in the A. stephensi genome, which is consistent with the findings in other Diptera and Lepidoptera species. The greatest similarity in sequence and expression profile between Ans-proPO and A. gambiae proPO6 suggests that they might be homologues. Our results demonstrate that Ans-proPO is constitutively expressed through different developmental stages and under different physiological conditions, implying that other factors in the proPO activation cascade regulate melanotic encapsulation.

Deleterious effect of null phenoloxidase mutation on the survival rate in Drosophila melanogaster

The effect of null activity of phenoloxidase on the survival rate was investigated in mutants of Drosophila melanogaster. MoxGM95 and Dox-3KD95, structural genes for prophenoloxidase A1 and A3, were found in natural populations in the former Soviet Union, and affected the phenoloxidase activity in active A1 or A3, respectively. After linking the visible markers located on the second chromosome together with the variants, cross experiments were performed to make homozygote, rdo Dox-3KD95 pr C MoxGM95 wt. No double mutant had emerged. In the mutant, c MoxGM95 wt Pu2, the viability was greatly reduced. These results suggested that phenoloxidase and tyrosine-3-hydroxylase act as indispensable proteins to maintain life in Drosophila.

Prophenoloxidase from brown shrimp (Penaeus californiensis) hemocytes

Prophenoloxidase (proPO) was purified from blood cells of the brown shrimp Penaeus californiensis by ultracentrifugation and dye affinity chromatography. The isolated proPO is a 114-kDa monomeric protein as determined by SDS-PAGE. This protein can be hydrolyzed by proteinases, producing a 107-kDa active phenoloxidase (PO). The isoelectric point for both protein forms was 7.35. The PO reaction using L-DOPA as substrate, has an optimum pH of 8, and was poorly inhibited by sodium azide, thiourea and EDTA, but strongly inhibited by diethyl thiocarbamate. According to the substrate affinity and inhibition characteristics, this phenoloxidase was classified as a tyrosinase-like phenoloxidase. Purified proPO was not activated by bacterial lipopolysaccharides or beta-glucans.

Isolation and characterization of phenoloxidase from egg masses of the gastropod mollusc, Biomphalaria glabrata

Previous studies have shown that phenoloxidase activity is present in the albumen gland and egg masses of Biomphalaria glabrata, and its potential role in egg formation in this snail has been proposed. In the present study, a phenoloxidase enzyme has been isolated from the supernatant of egg mass homogenates using a combination of hydrophobic interaction chromatography and gel filtration high-performance liquid chromatography (GF-HPLC). The isolated phenoloxidase eluted as a single peak of activity upon GF-HPLC (representing a 132-fold purification) and subsequently was detected as a single band with an estimated molecular mass of 35 kDa by SDS-PAGE analysis. Phenylthiourea-inhibitable mono- and diphenoloxidase activities were demonstrated for the isolated enzyme suggesting that both enzyme activities are associated with a single, tyrosinase-type molecule.

Activation of prophenoloxidase A1 by an activating enzyme in Drosophila melanogaster

An activating enzyme for prophenoloxidase A1 was isolated from pupae of Drosophila melanogaster, and the activation of purified prophenoloxidase A1 with this enzyme was analyzed. The purification included ammonium sulfate fractionation, DEAE-cellulose, Superdex 75, arginine-Sepharose and hydroxyapatite column chromatography. The prophenoloxidase activating enzyme was determined to be a 28.5-kDa protein consisting of a single polypeptide. The kinetics of the activation reactions was unusual in that the final levels of phenoloxidase activity varied depending on the initial concentrations of the activating enzyme, not those of the prophenoloxidase. The activation was effectively suppressed by the inhibitors of trypsin-type serine protease. The protein has amidolytic activity, and Boc-Val-Pro-Arg-MCA was the best substrate among the synthetic substrates examined. The molecular mass of the activated phenoloxidase was smaller than that of the prophenoloxidase, indicating that a 5-kDa peptide was released from the prophenoloxidase by limited proteolysis with the activating enzyme. The cleavage site of prophenoloxidase A1 was shown to be between Arg and Phe at positions 52 and 53.

Biological mediators of insect immunity

Infection in insects stimulates a complex defensive response. Recognition of pathogens may be accomplished by plasma or hemocyte b1p4eins that bind specifically to bacterial or fungal polysaccharides. Several morphologically distinct hemocyte cell types cooperate in the immune response. Hemocytes attach to invading organisms and then isolate them by phagocytosis, by trapping them in hemocyte aggregates called nodules, or by forming an organized multicellular capsule around large parasites. These responses are often accompanied by proteolytic activation of the phenoloxidase zymogen that is present in the hemolymph. A component of insect immune responses to bacteria is the synthesis by fat body and hemocytes of a variety of antibacterial proteins and peptides, which are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been recently discovered in insect hemolymph. The promoters for several antibacterial protein genes in insects are regulated by transcription factors similar to those involved in mammalian acute phase responses.

The prophenoloxidase activating system and associated proteins in invertebrates

In this review, we present arguments indicating that prophenoloxidase (proPO) activating system acts as a pattern recognition and defence system in invertebrate blood. Phenoloxidase (PO) activity has been found in the blood of many invertebrates. At least in arthropods, echinoderms and urochordates, the inactive pro-form, proPO has been found to be elicited by the microbial cell-wall components beta-1, 3-glucans, lipopolysaccharide and/or peptidoglycan. This activation seems to involve elicitor-binding proteins and serine protease(s). ProPO, the proPO-activating enzyme (ppA) and plasma elicitor-binding proteins, have been purified from some arthropods, and proPO and the beta-1, 3-glucan binding protein (beta GBP) have been cloned and sequenced from crayfish. Arthropod proPO has a molecular mass of 70-90 kDa and PO has a molecular mass of 60-70 kDa. The beta GBP also stimulates phagocytosis of fungal cells and, after reacting with beta-1, 3-glucan, blood-cell degranulation (and release of the proPO system). In addition, a cell-adhesion protein (of 70-100 kDa), apparently associated with the proPO system, has been purified from arthropods. This mediates blood-cell adhesion, degranulation, phagocytosis and encapsulation. The cell-adhesion protein and beta GBP bind to a common blood-cell membrane receptor. It would be interesting to see the sequences of more proPO system components and investigate whether the scheme for cellular communication and defence, involving the cell-adhesion protein, elicitor-binding proteins and the membrane receptor described in arthropods, applies to invertebrates in general.

The prophenoloxidase from the wax moth Galleria mellonella: purification and characterization of the proenzyme

A prophenoloxidase (PPO) was purified from the hemolymph of the larvae of Galleria mellonella. A 135-fold purification of the proenzyme with 25% yield was achieved by a combination of different chromatographic methods. An alternative micropreparation of pure PPO by a novel method for native electrophoresis in polyacrylamide gel is also described. The molecular mass of the native PPO was estimated to be 300 kDa by the pore-limit gradient electrophoresis in polyacrylamide gel. In the presence of sodium dodecyl sulphate, two closely migrating subunits of 80 and 83 kDa were detected under non-reducing conditions. The PPO was shown to be a glycoprotein and its isoelectric point was 6.2. The amino-acid composition of the purified protein was similar to the PPO from Bombyx mori. The monospecific antibody raised against the purified PPO crossreacted with the (pro)phenoloxidase in hemolymph of Manduca sexta. The activation of the PPO with chymotrypsin was investigated and two proteins of 67 and 50 kDa were found to be products of the proteolytic cleavage. The N-terminus of the G. mellonella PPO was blocked, but eleven partial internal sequences were determined after fragmentation of the purified PPO with trypsin. Three of these peptides exhibited significant homology with highly conserved sequences found in arthopod hemocyanins and insect storage proteins, which indicates that the PPO belongs to this family.

Nucleotide sequence of the cDNA encoding the proenzyme of phenol oxidase A1 of Drosophila melanogaster

Clones encoding pro-phenol oxidase [pro-PO; zymogen of phenol oxidase (monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1)] A1 were isolated from a lambda gt10 library that originated from Drosophila melanogaster strain Oregon-R male adults. The 2294 bp of the cDNA included a 13-bp 5'-noncoding region, a 2070-bp encoding open reading frame of 690 amino acids, and a 211-bp 3'-noncoding region. A hydrophobic NH2-terminal sequence for a signal peptide is absent in the protein. Furthermore, there are six potential N-glycosylation sites in the sequence, but no amino sugar was detected in the purified protein by amino acid analysis, indicating the lack of an N-linked sugar chain. The potential copper-binding sites, amino acids 200-248 and 359-414, are highly homologous to the corresponding sites of hemocyanin of the tarantula Eurypelma californicum, the horseshoe crab Limulus polyphemus, and the spiny lobster Panulirus interruptus. On the basis of the phylogenetic tree constructed by the neighbor-joining method, vertebrate tyrosinases and molluscan hemocyanins constitute one family, whereas pro-POs and arthropod hemocyanins group with another family. It seems, therefore, likely that pro-PO originates from a common ancestor with arthropod hemocyanins, independently to the vertebrate and microbial tyrosinases.

Partial purification of plasma phenoloxidase of the yellow fever mosquito Aedes aegypti L. (Diptera: Culicidae)

A nitrocellulose-based assay was developed using a dot-blot apparatus to detect phenoloxidase activity in column fractions. Using this assay, plasma phenoloxidase was partially purified from Aedes aegypti larvae using hydrophobic interaction chromatography, gel filtration, and ion-exchange chromatography. The molecular weight (M(r)) native enzyme was 130,000, and it contained subunits of 76,000, 62,000, and 58,000. Two phenoloxidase peaks were observed by ion exchange chromatography, and these fractions had distinct polypeptide profiles as detected by SDS-PAGE.

Activation of prophenoloxidase with 2-propanol and other organic compounds in Drosophila melanogaster

Activation with 2-propanol and other organic compounds of prophenoloxidase purified from pupae of Drosophila melanogaster was analyzed. A1, one of the two isozymes of the prophenoloxidase, could be activated with both an endogenous activating system and artificial organic compounds including alcohols. A1 was activated within 2 min after addition of 2-propanol. The phenoloxidase activity of A1, which had been activated with 2-propanol, decreased gradually by lowering the concentration of 2-propanol taking c 60 min to attain a low level, and the activity could be re-elevated at the re-introduction of 2-propanol. Thus the reversibility of the activation of A1 in response to the change of the concentration of 2-propanol in the activating mixture could be observed. Optimum concentration of 2-propanol for the rate of activation was 50%, optimum temperature was 30 degrees C and optimum pH was 7.5. The final level of the phenoloxidase activity, which had been activated with 2-propanol, was higher than that activated with the endogenous activating system. The activated state of A1 showed properties of a tyrosinase-type phenoloxidase. The results suggested that the activation of A1 with 2-propanol is caused by the reversible conformational change of the prophenoloxidase molecule.