Purification and characterization of Klebsiella pneumoniae cytotoxins.

Isolation, purification and characterization of 3 new cytotoxins of a K. pneumoniae strain isolated from ready to eat pork sausage are reported. Purification process involved extraction of cytotoxins with polymyxin B sulphate, salt precipitation, gel filtration and anion exchange chromatography. Klebsiella cytotoxin (KCT) I, a glycoprotein of about 65 kDa was verocytotoxic, enterotoxic and dermonerotic. KCT II was erythemogenic, verocytotoxic and enterotoxic protein of co 55 kDa, while KCT III was about double in MW (110 kDa) hadverocytotoxicity but neither enterotoxicity nor dermatotoxicity. KCT I and II caused granulation, conglomeration, shrinkage, detachment and lysis of MDBK and Vero cells, while KCT III induced enlargement, vacuolation, granulation, multinucleolation and syncytia formation in exposed cells. All the three cytotoxins induced specific neutralizing antibodies and cytotoxins were detectable in nanogram quantities with enzyme-linked immunosorbant assay using homologous antibodies. None of the anticytotoxin cross-reacted with either heterologous Klebsiella cytotoxins or with verocytotoxic preparations of Shigella dysenteriae.     

Selective lysis of virus-infected cells by cobra snake cytotoxins: A sendai virus, human erythrocytes, and cytotoxin model.

By using a Sendai virus-human erythrocyte model, this work found that virus-infected cells were 10-fold more susceptible to lysis in two of five examined cobra venoms. Four cytotoxins were isolated from the venom of the cobra Naja nigricollis that also showed 10-fold higher cytotoxicity toward virus-infected cells than to untreated cells. As selective destruction of virus-infected cells is of immense importance in clinical practice, this work demonstrates the potential of cobra cytotoxins to serve as leading compounds for the generation of derivatives or fractions with high cytotoxic specificity toward virus-infected cells.     

The structural evolution of cobra venom cytotoxins

Summary In order to analyze the evolutionary behavior of the cobra venom cytotoxins, their probable tertiary structure was predicted using computer graphics. The 41 amino acid sequences known show that the major evolutionary changes have taken place in two particularly exposed areas of the molecular surface. In each area, neighboring residue positions seem to have evolved interdependently, but there is no obvious interdependence between the two areas. Indeed, the relative evolution of these two areas prompts a subdivision of the sequence set into four groups. According to the known cytotoxin circular dichroism spectra, one of these four groups could be characterized by a difference in molecular secondary structure. Sine the two variable areas have functional associations, it is suggested that their evolution may be governed by a target with several similar binding sites.     

Amino-acid sequence of human and bovine brain myelin proteolipid protein (lipophilin) is completely conserved

Proteolipid protein (PLP) was isolated from white matter of human brain by chloroform/methanol extraction and further purified by chromatography. Performic acid oxidation yielded a product homogeneous in NaDodSO4-polyacrylamide electrophoresis with a molecular mass of 30 kDa. The carboxymethylated PLP was chemically cleaved with cyanogen bromide into four fragments: CNBr I 22-24 kDa, CNBr II 5 kDa, CNBr III 1.4 kDa and CNBr IV 0.7 kDa. HBr/dimethylsulfoxide cleavage at tryptophan residues released four fragments: Trp I 14-16 kDa, Trp II 2.0 kDa, Trp III 5 kDa and Trp IV 7 kDa. Hydrophilic fragments were enriched in 50% formic acid (CNBr II, III, IV and Trp II and III), whereas hydrophobic peptides precipitated from this solvent were CNBr I, Trp I and IV. The fragments were separated by gel filtration with 90% formic acid as solvent and finally purified by gel permeation HPLC (Si 60 and Si 100) for automated liquid and solid-phase Edman degradation. Large fragments were further cleaved with different proteinases (trypsin, V8-proteinase, endoproteinase Lys-C and thermolysin). We used an improved strategy in the sequencing of the human proteolipid protein compared with our approach to the structural elucidation of bovine brain PLP. The amino-acid sequence of human PLP contains 276 residues, the same as found in bovine proteolipid protein. The two sequences proved to be identical. The possible importance of the conservative structure of this integral membrane protein is discussed.     

Sequence characterization of venom toxins from Thailand cobra

Several toxins with distinct pharmacological properties were isolated from the venom of Thailand cobra (Naja naja siamensis) by cation-exchange chromatography. Two neurotoxins and one basic toxin with cardiotoxic activity were further purified and sequenced. The neurotoxins characterized were closely similar to the previously reported long- and short-chain neutrotoxins. The complete sequences of one minor neurotoxin and one cardiotoxin analogue were determined with the automatic protein sequencer in non-stop single runs of Edman degradation coupled with C-terminal sequence determination with carboxypeptidase digestion. The minor neurotoxin consists of 62 amino-acid residues with 8 cysteine residues and is found to be almost identical to cobrotoxin, a major toxic component of Formosa cobra (Naja naja atra). The sequence comparison of the 60-residue cardiotoxin with other reported cytotoxins of snake venoms indicates that 8 cysteine residues at the positions 3, 14, 21, 38, 42, 53, 54, and 59 are invariant among all sequences, with only two conservative changes at other positions along the sequence. The upshot of this report exemplified the facile sequence analysis of venom toxins by the application of pulsed-liquid phase protein sequencer and also revealed new analogues of a minor neurotoxin and one major cardiotoxin reported previously on the same species of Thailand cobra.     

Quick Identification of the Members of the Glutamine Synthetase Gene Family from Sunflower by Simultaneous Amplification of cDNA with Degenerate Primers

A single degenerate glutamine synthetase (GS)-specific primer was used to amplify the 3′ end of cDNAs derived from different GS genes that are expressed in leaves and roots of sunflower (Helianthus annuus L. cv. Peredovic). Four types of GS cDNA (I, II, III and IV) were simultaneously amplified from leaves and five types (I, II, V, VI, VII) from roots with a minimum investment of time and experimental work. cDNAs II, III and IV encode chloroplastic isoforms as deduced by the presence of chloroplastic GS-specific features in their sequences. The rest of cDNAs codifies cytosolic isoforms. Using cDNA-specific probes and primers, homologous sequences to all GS cDNAs amplified from cv. Peredovic, except to cDNAs III and IV, were detected in the inbred line R41. This result strongly suggests that the three cDNAs for chloroplastic isoform are allelic sequences from the same locus, and since cDNA type IV contains sequences derived from cDNAs II and III, it indicates a recombinational origin. The results presented are consistent with the existence of a GS gene family in sunflower with at least five members. Four of them, named ggs1.1 to ggs1.4, codify for the cytosolic isoforms (cDNAs I, V, VI and VII). A fifth member, named ggs2, from which three allelic sequences (cDNAs II, III and IV) have been cloned, encodes the chloroplastic isoform. This revised version was published online in July 2006 with corrections to the Cover Date.     

Comparison of the nucleotide sequences at the 3′-terminal region of RNAs from RNA coliphages

In order to study the genealogical relationships among four groups (I to IV) of RNA coliphages, we sequenced 200 to 260 nucleotides from the 3′ termini of 14 phage RNAs according to the method of Sanger et al. (1977), and compared the results. It was found that the sequences of phage RNAs in the same group were extremely homologous (about 90%). On the other hand, when the sequences were compared with those from other groups, they were seen to be only about 50 to 60% homologous between group I and group II, and about 50% homologous between group III and group IV. In other combinations, such as groups I (or II) and III, and groups I (or II) and IV, however, the extent of homology was small. Furthermore, the sequences up to 30 residues from the 3′ end were found to be about 90% homologous between groups I and II, and between groups III and IV.These results confirm our previous findings, that the sequences located in the proximity of the 3′ end of phage RNA in the same group were well-conserved (Inokuchi et al., 1979), and that close relationships exist between groups I and II, and between groups III and IV (Furuse et al., 1979).     

The fusogenic properties of the cytotoxins of cobra venom in a model membrane system

By the methods of EPR spinal probes, energy migration of triplet excitation and NMR spectroscopy, the structural changes on hydrocarbon region of membranes, the changes in dynamic state of water of lipid hydrate jacket, the intermembrane lipid material exchange and fusion of membranes induced by cytotoxins of cobra venom have been studied. The sequence of events preceded the membrane fusion is suggested. The probability of membrane fusion has been shown not to be determined by fusogenic agent structure only, but much it depends on lipid composition of membranes.     

Elucidation of the solution structure of cardiotoxin analogue V from the Taiwan cobra (Naja naja atra)--identification of structural features important for the lethal action of snake venom cardiotoxins

The aim of the present study is to understand the structural features responsible for the lethal activity of snake venom cardiotoxins. Comparison of the lethal potency of the five cardiotoxin isoforms isolated from the venom of Taiwan cobra (Naja naja atra) reveals that the lethal potency of CTX I and CTX V are about twice of that exhibited by CTX II, CTX III, and CTX IV. In the present study, the solution structure of CTX V has been determined at high resolution using multidimensional proton NMR spectroscopy and dynamical simulated annealing techniques. Comparison of the high resolution solution structures of CTX V with that of CTX IV reveals that the secondary structural elements in both the toxin isoforms consist of a triple and double-stranded antiparallel beta-sheet domains. Critical examination of the three-dimensional structure of CTX V shows that the residues at the tip of Loop III form a distinct "finger-shaped" projection comprising of nonpolar residues. The occurrence of the nonpolar "finger-shaped" projection leads to the formation of a prominent cleft between the residues located at the tip of Loops II and III. Interestingly, the occurrence of a backbone hydrogen bonding (Val27CO to Leu48NH) in CTX IV is found to distort the "finger-shaped" projection and consequently diminish the cleft formation at the tip of Loops II and III. Comparison of the solution structures and lethal potencies of other cardiotoxin isoforms isolated from the Taiwan cobra (Naja naja atra) venom shows that a strong correlation exists between the lethal potency and occurrence of the nonpolar "finger-shaped" projection at the tip of Loop III. Critical analysis of the structures of the various CTX isoforms from the Taiwan cobra suggest that the degree of exposure of the cationic charge (to the solvent) contributed by the invariant lysine residue at position 44 on the convex side of the CTX molecules could be another crucial factor governing their lethal potency.     

Amino acid sequence of cytotoxin IIa isolated from the venom of the Indian cobra (Naja naja)

A cytotoxic basic polypeptide, designated as cytotoxin IIa, was purified to homogeneous state from the venom of the Indian cobra (Naja naja) by a combination of gel filtration on Sephadex G-50, CM-cellulose chromatography, and fast protein liquid chromatography. Cytotoxin IIa is a single polypeptide consisting of 60 amino acid residues with four intramolecular disulfide linkages. The toxin showed high cytotoxicity toward Yoshida sarcoma and ascites hepatoma cells as did cytotoxins I and II isolated from the same venom. Analysis of the amino acid sequence revealed that cytotoxin I, IIa, and II are highly homologous in their primary structures and that cytotoxin IIa differs from cytotoxin I only in having Phe 25 and Val 52 in place of Tyr 25 and Glu 52 residues.     

Amino acid sequences of cytotoxin-like basic proteins derived from cobra venoms

Amino acid sequences of cytotoxin-like basic proteins (CLBPs), purified from the venoms of Formosan cobra (Naja naja atra) and Indian cobra (Naja naja), were reinvestigated. The determined sequences differed from those reported previously by Takechi et al. [(1985) Biochem. Int. 11, 795-802; (1987) Biochem. Int. 14, 145-152]. The sequence of CLBPs at residues 25-30 was found to be hydrophilic as compared with those of cytotoxins. The difference in the hydrophobicity of this region might be responsible for the difference in their cytotoxic activities.     

Recharacterization of Pseudomonas fulva Iizuka and Komagata 1963, and proposals of Pseudomonas parafulva sp. nov. and Pseudomonas cremoricolorata sp. nov

Seven Pseudomonas fulva strains obtained from culture collections were taxonomically studied. The seven strains were separated into three clusters (Clusters I to III) on the basis of 16S rRNA gene sequences, and located phylogenetically in the genus Pseudomonas sensu stricto. Further, the strains were classified into 4 groups (Groups I to IV) on the basis of DNA-DNA similarity. As a result, Cluster I was split into Groups I and II. Group I included the type strain of P. fulva and two strains, and levels of DNA-DNA similarity ranged from 88 to 100% among the strains. Group II contained two strains, and the level between the two strains ranged from 91 to 100%. Group III consisted of one strain. Group IV included one strain, and this strain showed a high level of DNA-DNA similarity with the type strain of Pseudomonas straminea NRIC 0164(T). Clusters II and III corresponded to Groups III and IV, respectively. The four groups were separated from one another and from related Pseudomonas species at the level from 3 to 45% of DNA-DNA similarity. The strains of Groups I, II, and III had ubiquinone 9 as the major quinone. According to numerical analysis by the use of 133 phenotypic characteristics, the seven P. fulva strains were split into four phenons (Phenons I to IV). The groups by DNA-DNA similarity corresponded well with the phenons produced by numerical taxonomy, and differential characteristics were recognized. Consequently, Group I was regarded as P. fulva because the type strain (NRIC 0180(T)) of this species was included in this group. Strains in Group II were identified as a new species, Pseudomonas parafulva sp. nov., and the type strain is AJ 2129 (=IFO 16636=JCM 11244=NRIC 0501). NRIC 0181 in Group III was identified as a new species, Pseudomonas cremoricolorata sp. nov., and the type strain is NRIC 0181 (=IFO 16634=JCM 11246). NRIC 0182 in Group IV was identified as P. straminea on the basis of the high level of DNA-DNA similarity with the type strain of this species.     

Characterization of zonal fractions of calf thymus DNA

DNA isolated from calf thymus nuclei is fractionated by zonal centrifugation into 40 sedimentation-rate classes and the reduced viscosity profile determined. This profile is divided into four fractions, I–IV, IV being the fastest sedimenting. The relative concentrations of repetitive DNA sequences in these is determined by hybridization on membrane filters and also hypochromicity by reannealing at 60 °. Repetitive sequences are found in all fractions, although they are slightly more abundant in the order III > II > I. Moreover, fractions I, II, III, act as good competitors in hybridization experiments with each other, implying that a high degree of complementarity exists between repetitive sequences in each of the fractions. Fraction IV had peculiar hydrodynamic properties which have provoked observations on DNA purification.     

A study of sequence homologies in four satellite DNAs of man.

Satellites I, II, III and IV (Corneo et al., 1968,1970,1971) have been purified from human male placental DNA. The sequences present in these four DNA components have been characterized by analytical buoyant density, thermal denaturation, DNA reassociation, DNA hybridization and gel electrophoresis coupled with hybridization following either HaeIII or EcoRI restriction endonuclease digestion. Satellites III and IV were found to be virtually indistinguishable by a variety of criteria. Cross-satellite reassociation showed that 40% of the molecules present in satellite III contain sequences that are homologous to 10% of the molecules of either satellite I or satellite II. Reassociated satellite I melts as a single component, as do the hybrid duplexes between satellite I and satellite III. In contrast, reassociated satellites II, III and IV, and the hybrid duplexes formed between satellites II and III and between satellites II and IV, melt as two distinct components with different thermal stabilities.Digestion of satellite III with HaeIII gives rise to a series of fragments whose sizes are 2, 3, 4, 5, 6, 7, 8 and 11 times the size of the smallest 0.17 × 10³ basepair fragment, in addition to a 3.4 × 10³ base-pair male-specific fragment (Cooke, 1976) and high molecular weight material. The sequences contained in the fragments of the HaeIII ladder are diverged from each other as well as being non-homologous with those of the 3.4 × 10³ base-pair and high molecular weight fragments. The latter contain EcoRI recognition sites. Satellite II has a similar pattern of fragments to satellite III following digestion with HaeIII, although it can be distinguished from satellite III on the basis of the products of EcoRI digestion. Satellite I contains neither HaeIII nor EcoRI recognition sites. The cross-satellite homologies of the sequences present in fragments of differing sizes produced by restriction enzyme digestion have also been studied.     

Mutagenicity of the anticancer drug, caracemide, and related compounds for salmonella

Caracemide, MeCON(CONHMe)(OCONHMe) (I), is a novel anticancer drug. Since it was derived from acetohydroxamic acid (II), a known mutagen, its potential metabolites and related compounds were synthesized and tested for mutagenicities in S. typhimurium TA98 and TA100. These compounds were: MeNHCONH(OCONHMe) (III), MeCONH(OCONHMe) (IV), MeCONOH(CONHMe) (V), MeNHCOONH2 X HCl (VI), MeNHCONHOH (VII), MeNHCOON(CONHMe)2 (VIII), and NOH(CONHMe)2 (IX). The mutagenicities in the absence of rat liver homogenate were: (VI) much greater than (IV) greater than (II), (III), (V). The other compounds were not mutagenic. (I) was mutagenic only in the presence of rat liver homogenate. The doses required to demonstrate mutagenicities of these compounds were from 0.05 to 5 mumoles/plate. The major hydrolytic products at 25 degrees C, pH 7, were (III), (IV), and (V) from (I); (II) and (III) from (IV); and (II), (III), (VII) and MeNHCONH(OCOMe) (X) from (V). (III) was stable at pH 7. Treatment of (IV) with HCl yielded (VI). Hydrolysis of (III) or (V) with ammonia yielded (VII). These results suggest that caracemide may be activated enzymatically or nonenzymatically by deacetylation or decarbamoylation, and its anticancer activity may be related to the reactivity of its metabolites with DNA. The synthetic procedures and characterizations of new compounds (IV), (V) and (X) are described.     

COOH-terminal propeptides of the major human procollagens. Structural, functional and genetic comparisons

The sequences of the carboxy-terminal extensions (COOH-propeptides) of at least one chain of all of the major human procollagens have only recently been deduced, and include those of the interstitial (alpha 1(I), alpha 2(I), alpha 1(II), alpha 1(III)), basement membrane (alpha 1(IV)) and pericellular (alpha 2(V)) procollagens. Comparisons of DNA and protein sequences, corresponding to these COOH-propeptides domains, established the early divergence of the basement membrane alpha 1(IV) COOH-propeptide from the corresponding sequences of the interstitial and pericellular procollagens. The latter are relatively highly conserved and share 58% primary peptide sequence similarities, whereas sequence similarities relative to alpha 1(IV) are limited. Hydropathy profiles and secondary structure potentials further emphasize the clustering of conserved and variable regions among the interstitial and pericellular COOH-propeptides, and provided further evidence for significant structural differences between these sequences and the alpha 1(IV) COOH-propeptide. The most highly conserved sequences of the alpha 1(I), alpha 2(I), alpha 1(II), alpha 1(III) and alpha 2(V) COOH-propeptides include regions surrounding the carbohydrate attachment site, cysteine-containing regions and the COOH-terminal sequences. Cysteinyl, tyrosyl and tryptophanyl residues were found to be highly conserved as were most charged residues. Localization of variable regions, in general, occurs within hydrophilic sequences with high beta-turn potentials that are proximal to intron/exon splice junctions. The most variable sequences are associated with the telopeptides and adjoining NH2-terminal portions of the COOH-propeptides as demonstrated by predictive secondary structure analyses. These results, combined with similar analyses of abnormal alpha 2(I) COOH-propeptide (osteogenesis imperfecta) permitted the identification of subsequences that are likely to be a prerequisite for COOH-propeptide functions, namely procollagen chain recognition and nucleation sites for triple helix formation. These functions are also common to the alpha 1(IV) COOH-propeptide; however, the lack of cleavage of this region and its additional postulated structural role in extracellular matrix interactions likely account for its divergent primary and secondary structure.     

CuZn-Superoxide Dismutases in Rice: Occurrence of an Active, Monomeric Enzyme and Two Types of Isozyme in Leaf and Non-Photosynthetic Tissues

Rice leaves and seed embryos contain four isozymes of CuZn-superoxidedismutase (SOD) and two isozymes of Mn-SOD. CuZn-SOD I is amajor enzyme in leaves, but not in embryos or etiolated seedlings.CuZn-SODs II,III and IV were found in the embryos but were alsofound as minor isozymes in leaves. CuZn-SODs I, II and IV were purified to homogeneity from riceleaves. CuZn-SODs I and II had similar properties with respectto molecular weight, dimeric structure, absorption spectrumand metal content, but their amino acid compositions differedfrom each other. The absorption spectrum of CuZn-SOD IV wassimilar to that of isozymes I and II, but this enzyme was amonomer with a molecular mass of 1.7 kDa. Antibody against CuZn-SODI from rice did not cross-react with isozymes II and IV. Antibodiesagainst CuZn-SOD from spinach leaves cross-reacted with isozymeI but not with isozymes II, III and IV. By contrast, the antibodiesagaist CuZn-SOD from spinach seeds cross-reacted with isozymesII, III and IV but not with isozyme I. Thus, the isozyme thatis expressed mainly in leaves (CuZn-SOD I) and the isozymesexpressed mainly in non-photosynthetic tissues (CuZn-SODs II,III, IV) are immunologically distinct. (Received October 7, 1988; Accepted January 27, 1989)     

A nuclear overhauser study of heme orientational isomerism in monomeric Chironomus hemoglobins

The nuclear Overhauser effect (NOE) was used to investigate heme orientation and to obtain assignments for all resolved resonances in the 1H-NMR spectrum of met-cyano Chironomus thummi thummi monomeric hemoglobins III and IV (Hb III and Hb IV). The only non-heme resolved resonance was found to be from Phe-38 (CD1), and NOE dipolar connectivity between this resonance and the heme 5- and 8-methyls was used to establish the absolute orientation of the heme for each heme-insertion isomer present. The assignments of resonances and heme disorder permitted structural comparisons between the various components, including those due to a point mutation in Hb III. Finally, the characteristic differences of NOE patterns to amino-acid protons from substituents on heme pyrroles I and II formed the basis for assigning resonances and heme orientation relative to native Hb IV for deuterohemin-reconstituted Hb IV, for which there are no X-ray data available.     

Resolving the Conflicts Between Biodiversity Conservation and Socioeconomic Development in China: Fuzzy Clustering Approach

Resolving the conflicts between biodiversity conservation and socioeconomic development is a global pursuit for the long-run prospects of the human species. Based on Wenchuan County, a typical county in southwestern China, a group of 20 indicators quantifying regional biodiversity and socioeconomic development was established to classify and evaluate the county area spatially. A fuzzy c-means clustering (FCM) algorithm was used as the classification method. Three indices including BD, DL and DR characterizing the value of biodiversity, the level and rate of socioeconomic development of the delineated regions were formulated. The results indicated that Wenchuan County was optimally classified into 4 types of regions (region I to IV). The area percentages of the regions vary widely from 4.3 to 65.7%. The sequences of the regions on biodiversity, socioeconomic development level, and socioeconomic development rate were, respectively, IV > II > III > I, I > III > II > IV and III >I >II >IV. The spatial strategy on coordinating biodiversity conservation and regional development is to develop mainly from the east(I, II, III) and to conserve mainly in the west(IV). Eco-industry, such as eco-tourism and eco-agriculture, need to be emphasized in the process of regional development. The quantitative methods used here may have a wide applicability.     

Escherichia coli cytotoxins and enterotoxins.

Vero cell cytotoxins and cytotonic enterotoxins produced by E. coli are toxic proteins, which have been implicated in a number of specific diseases in humans and animals. Nomenclature for these toxins is complicated by the existence of different names for the same toxin. The Vero cell cytotoxins are called verotoxins because they are lethal for Vero cells in culture; they are also known as Shiga-like toxins (SLTs) because they are clearly related to Shiga toxin in structure, amino acid sequence, mechanism of action, and biological activity. SLTs belong to two classes. SLT-I is identical with Shiga toxin and is in a class by itself (class I). The other SLTs are closely related to each other and form a second class (class II). Class II SLTs include SLT-II, SLT-IIv, SLT-IIvha, SLT-IIvhb, and SLT-IIva. All SLTs that have been investigated are A-B subunit protein toxins, whose A subunits possess N-glycosidase activity against 28S rRNA and cause inhibition of protein synthesis in eukaryotic cells. These toxins are enterotoxic as well as cytotoxic. SLTs produced in the intestine are absorbed into the blood stream and affect vascular endothelial cells in target organs. They may also have a direct toxic effect on enterocytes. Diseases in which E. coli SLTs have been implicated include diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome in humans and edema disease in pigs. Variation in receptor specificities among SLTs may be the reason for different disease syndromes in different host species. The E. coli enterotoxins belong to three distinct classes: heat-labile enterotoxin (LT), heat-stable enterotoxin type I or type a (STI, STa), and heat-stable enterotoxin type II or type b (STII, STb). There is clear evidence that these cytotonic enterotoxins play an essential role in diarrheal disease. LT is an A-B subunit protein toxin, closely related to cholera toxin. Following binding of LT to receptors in enterocytes the A subunit is internalized. The enzymatically active A subunit transfers ADP-ribose from NAD to a GTP-dependent adenylate cyclase regulatory protein, thereby elevating intracellular levels of adenylate cyclase. The increased levels of cyclic AMP cause stimulation of A kinase and lead to hypersecretion of electrolytes and fluid. STI is a small peptide of 18 or 19 amino acids. It binds to receptors in enterocytes and stimulates particulate guanyl cyclase. Elevated intracellular cyclic GMP stimulates G kinase, resulting in increased Cl- secretion and impaired absorption of Na+Cl-. STII is a peptide toxin whose mechanism of action is unknown.(ABSTRACT TRUNCATED AT 400 WORDS)