Biomedical subjects
Publications and source records attributed to Y Shimohigashi.
A cellulase with activity as a cellobiohydrolase [EC3.2.1.91] was purified from the intestinal juice of the giant snail Achatina fulica. The enzyme (M(r) = about 23,000, and pI = about 5.3) also had a weak beta-glucosidase activity. The amino acid sequence of the N-terminal 20 amino acids was analyzed, and no similarity was noted for sequences of other known cellobiohydrolases.
The nucleotide sequences of four genes encoding Trimeresurus gramineus (green habu snake, crotalinae) venom gland phospholipase A2 (PLA2; phosphatidylcholine 2-acylhydrolase, EC 3.1.1.4) isozymes were compared internally and externally with those of six genes encoding Trimeresurus flavoviridis (habu snake, crotalinae) venom gland PLA2 isozymes. The numbers of nucleotide substitutions per site (KN) for the noncoding regions including introns were one-third to one-eighth of the numbers of nucleotide substitutions per synonymous site (KS) for the protein-coding regions of exons, indicating that the noncoding regions are much more conserved than the protein-coding regions. The KN values for the introns were found to be nearly equivalent to those of introns of T. gramineus and T. flavoviridis TATA box-binding protein genes, which are assumed to be a general (nonvenomous) gene. Thus, it is evident that the introns of venom gland PLA2 isozyme genes have evolved at a similar rate to those of nonvenomous genes. The numbers of nucleotide substitutions per nonsynonymous site (KA) were close to or larger than the KS values for the protein-coding regions in venom gland PLA2 isozyme genes. All of the data combined reveal that Darwinian-type accelerated evolution has universally occurred only in the protein-coding regions of crotalinae snake venom PLA2 isozyme genes.
A cDNA encoding the Trimeresurus gramineus (Tg; green habu snake) TATA-box-binding protein (TgTBP) was cloned and sequenced. The cDNA encodes a 33-kDa protein with an extensive sequence similarity to those derived from other organisms, except for the N-terminal domain. Genes encoding TgTBP and Trimeresurus flavoviridis (Tf; habu snake) TBP (TfTBP) were isolated using a TgTBP cDNA and their nt sequences were determined. They are the first TBP genes entirely sequenced in higher animals. Both genes span over 15 kb and are constructed from eight exons and seven introns. Comparison of the loci of introns on the aligned amino-acid sequences of TBP from six organisms (Tg, Tf, mouse, Arabidopsis thaliana, Schizosaccharomyces pombe and Acanthamoeba castellanii) indicated that there are three highly conserved loci in the C-terminal domain.
Four acidic phospholipase A2 (PLA2) isozymes named PLA2-I, II, III and IV have previously been isolated from Trimeresurus gramineus (green habu snake) venom and sequenced [Oda et al. (1991) Toxicon 29, 157; Fukagawa et al. (1992) Toxicon 30, 1131; Fukagawa et al. (1993) Toxicon 31, 957]. They contain aspartate-49 which is known to bind Ca2+, essential for catalysis. In the present study, a basic PLA2 named PLA2-V containing lysine-49 was newly isolated from the same snake venom. Its isoelectric point was 9.4 and considerably higher than those (c. 4.5) of PLA2-I-IV. PLA2-V was 1.1% as active as PLA2-I toward egg-yolk emulsion but exhibited strong myotoxicity. The amino acid sequence of PLA2-V was determined by sequencing the S-carboxamidomethylated derivative and its peptide fragments produced by enzymatic (clostripain, chymotrypsin, Achromobacter protease I and Staphylococcus aureus V8 protease) cleavages. PLA2-V consists of 122 amino acid residues and is highly homologous (72-78%) to Lys-49 PLA2s so far isolated from Viperidae snake venoms but less homologous (52%) to PLA2-I. The presence of Asn-28, which is characteristic of Lys-49 PLA2s, was confirmed.
Basic proteins I and II (BP-I and BP-II) isolated from the venom of Trimeresurus flavoviridis (Habu snake) are isozymes of highly active Asp-49 phospholipase A2 (Asp-49-PLA2) and classified into the group Lys-49-PLA2. BP-II was found to elicit a strong contraction of guinea pig ileum, and this activity was inhibited completely by 1 microM indomethacin, an inhibitor of the arachidonate cascade. BP-II was inactive in the Ca(2+)-free medium, and p-bromphenacylated His-48-BP-II was also inactive. BP-II exhibited no binding affinity for the cells expressing PLA2 receptors. These results indicated that the contraction elicited by BP-II is due to the hydrolytic action of BP-II, liberating arachidonic acid from the ileum phospholipid biomembranes. In spite of its limited lipolytic activities (av. 0.9% of Asp-49-PLA2) against monomers and micelles of synthetic phospholipids, BP-II hydrolyzed considerably strongly the phospholipids in the artificial bilayer vesicles. Arachidonic acid released from liposomes of beta-arachidonoyl-gamma-stearoyl-L-alpha-phosphatidylcholine was determined by HPLC, and the activity of BP-II was estimated to be about 75% as compared to Asp-49-PLA2. Liposomes encapsulating carboxyfluorescein exhibited a strong dye-leakage induced by BP-II in a concentration-dependent manner, only in the Ca(2+)-containing buffer. The net result from all these observations was that BP-II, a Lys-49-PLA2, is an enzyme that hydrolyzes the membrane phospholipids. In contrast to BP-II, BP-I was found to be considerably weak in hydrolyzing membrane phospholipids, although its activities were distinct. BP-I and BP-II share a common sequence with the sole exception of Asp-67 (BP-I) and Asn-67 (BP-II) in the aligned sequences. This implies that the amino acid at position 67 of Lys-49-PLA2s is the residue required for discriminatory recognition of beta-arachidonoyl-phospholipid membranes.
Thrombin-like snake venoms enzymes, flavoxobin, and okinaxobin I isolated from Trimeresurus flavoviridis and Trimeresurus okinavensis, respectively, were examined in SH-EP cells and evaluated whether or not they can activate human thrombin receptors. Flavoxobin was almost completely inactive in both assays for phosphoinositide turnover and DNA synthesis. In contrast, okinaxobin I stimulated phosphoinositide turnover in a dose dependent manner, but considerably weakly. The EC50 value was about 100 nM, which was 4,000 times larger than that of alpha-thrombin. This stimulation was not inhibited by hirudin, an effective inhibitor of alpha-thrombin. Okinaxobin I also induced a very weak stimulation of DNA synthesis. These results suggest that thrombin-like snake venom enzymes interact with human thrombin receptors in inefficient ways. Weak interactions of the enzymes with thrombin receptor and inhibitor were ascribed to the incomplete formation of a lysine-cation cluster necessary for electrostatic molecular recognition.
A synthetic heptapeptide H-Ser-Phe-Phe-Leu-Arg-Asn-Pro-NH2, which corresponds to the ligand peptide latent in rodent thrombin receptors, was able to activate the thrombin receptor with no thrombin. In order to evaluate the structural requisites of two consecutive phenylalanines, three sets of analogs with substitutions at position either 2 or 3 were synthesized and examined for their stimulatory activity in phosphoinositide turnover in SH-EP epithelial-like cells. The replacement of Phe-2 by Ala completely eliminated the activity, while that of Phe-3 retained about 50% activity with a full stimulation. The Phe/Leu substitution resulted in a large increase (37-fold) in EC50 value for Phe-2, but in insignificant change for Phe-3. Substitution of para-fluorophenylalanine ((p-F)Phe) for Phe-2 enhanced strongly (4-fold) the activity, in contrast to a reduction by the Phe-3/(p-F)Phe substitution. Elimination of either Phe-2 or Phe-3 resulted in a complete loss of activity. These results indicated that Phe-2 and Phe-3 play different roles in the receptor activation. A highly specific aromatic phi-phi interaction was suggested between Phe-2-phenyl and thrombin receptor binding site, while Phe-3 appeared to be important for retaining a bioactive conformation.
Hepatocyte growth factor (HGF) has a strong affinity for heparin. About one fourth of HGF secreted from MRC-5 human embryonic lung fibroblast cells was found to be associated with heparin and heparan sulfate proteoglycan on the cell surface and extracellular matrix. To identify heparin-binding sites within the HGF molecule, we constructed variously deleted mutant HGFs and examined their binding ability to an immobilized heparin column. Native HGF and mutant HGFs, including d-K1 (deletion of the first kringle domain), d-K3 (deletion of the third kringle domain), d-K4 (deletion of the fourth kringle domain), d-beta (deletion of beta-chain), and HK1K2 (consisting of the N-terminal hairpin loop and the first two kringle domains), tightly bound to a heparin column, but d-H (deletion of the N-terminal hairpin loop) and d-K2 (deletion of the second kringle domain) markedly decreased binding ability to the column. These observations suggest that the N-terminal hairpin loop and the second kringle domain are essential for the heparin-binding of HGF. The finding that HK1K2 competed the binding of 125I-HGF to immobilized heparin provided additional evidence that the N-terminal half of HGF alpha-chain is the principal heparin-binding site. The hairpin loop in HGF possesses a cluster of basic amino acid residues and a highly positive net charge, when compared with hairpin loop structures in the other proteins, plasminogen and HGF-like protein. The second kringle domain in HGF has the basic amino acid cluster in the central region. Thus, it is likely that the basic clusters in these domains cooperatively contribute to the binding of HGF to the anionic heparin or heparan sulfate molecule.
A coagulant enzyme, okinaxobin I, which was purified from Trimeresurus okinavenis (himehabu snake) venom, released specifically fibrinopeptide B from fibrinogen to form fibrin clots. In the present study, its isozyme denoted as okinaxobin II has been purified to homogeneity from the same venom by chromatographies on Sephadex G-100, CM-Toyopearl 650M, and FPLC Mono-Q columns. Differently from okinaxobin I, okinaxobin II specifically cleaved fibrinopeptides A and B from fibrinogen similarly as found for alpha-thrombin. The enzyme acted on fibrinogen with specific activity of 42 NIH units/mg at optimum pH of 8.0. Okinaxobin II was a monomeric glycoprotein with a mol. wt of 37,500 on SDS-PAGE, which was reduced to 29,500 after treatment with N-glycanase. Okinaxobin II was much more basic (pI = 8.1) than okinaxobin I (pI = 5.4). The N-terminal sequence was highly similar to those of okinaxobin I and some other snake venom coagulant enzymes such as flavoxobin (Trimeresurus flavoviridis), batroxobin (Bothrops atrox and Bothrops moojeni), and catroxobin (Crotalus atrox). Okinaxobin II hydrolyzed tosyl-L-arginine methyl ester and benzoyl-L-arginine p-nitroanilide. The esterase activity was strongly inhibited by diisopropylfluorophosphate and to a lesser extent by tosyl-L-lysine chloromethyl ketone, indicating that the enzyme is a serine protease like alpha-thrombin. In terms of amino acid composition, okinaxobin II was similar to okinaxobin I and dissimilar to alpha-thrombin.
A beta-amyloid protein fragment AP21-35 (tetradeapeptide with amino acid residues 21-35) was found to be a highly selective agonist of substance P receptors (NK-1) among three tachykinin receptor subtypes. This peptide fragment contracted the smooth muscle preparations of guinea pig ileum in a dose dependent manner (EC50 = 22 microM). This activity was completely reversed by CP-96,345-1, a specific antagonist of NK-1 receptors, whereas atropine for NK-3 had no effect. The peptide was inactive in rat vas deferens which contains predominantly NK-2 receptors. These results indicated that AP21-35 is a highly selective agonist for NK-1 receptors. In the radio-ligand receptor binding assay using [125I]-Bolton-Hunter substance P to membranes from guinea pig ileum, the fragment exhibited a distinct dose-response curve (IC50 = 11 microM). All the present data strongly suggest that beta-amyloid protein function as a peptide ligand of substance P receptors with some pathophysiologic activities.
Thrombin receptor-derived peptide SFLLRNP (one-letter amino acid code) which corresponds to the N-terminal heptapeptide of tethered ligand is able to activate thrombin receptor and to stimulate the phosphoinositide (PI) turnover. The replacement of Phe-2 by Ala eliminated this activity completely, showing the crucial role of the Phe-phenyl group in receptor activation. It was found that substitution of para-fluorophenylalanine ((p-F)Phe) for Phe-2 enhanced several times the PI-turnover activity of SFLLRNP. This is the first example to date of a substitution with one order of magnitude greater increase in receptor activation. The Phe-2/Tyr substitution diminished the activity drastically (almost 2% of SFLLRNP), indicating the importance of hydrophobicity of Phe2-phenyl. The Phe-2/Leu substitution, however, diminished also the activity (less than 2% of SFLLRNP). These results suggested that highly specific hydrophobic interaction exists between Phe-2 of the tethered ligand and its binding site in thrombin receptor.
In an attempt to develop water-soluble, potent inhibitors for chymotrypsin, structurally rigid dipeptides containing arginine were designed and synthesized. The dipeptide H-D-Arg-Phe-NHBzl inhibited chymotrypsin very strongly (Ki = 5.9 microM). The dipeptide with the inverse sequence, H-D-Phe-Arg-NHBzl, was also a moderate inhibitor for chymotrypsin with Ki of 240 microM. In spite of the presence of arginine in these dipeptides, they inhibited trypsin only weakly, indicating that they are highly specific for chymotrypsin. High resolution 1H-NMR (400-MHz) indicated that these dipeptides can make a strong intramolecular hydrophobic interaction between Arg-beta, gamma, delta-methylenes and Phe-phenyl, producing a rigid hydrophobic core which interacts with the chymotrypsin S2 site. Since these dipeptides are easily soluble in water, they are regarded as the sophisticated and effective inhibitors for chymotrypsin.
A complete series of configurational isomers (L-L, L-D, D-L and D-D) of a dipeptide Leu-Phe benzyl ester have been synthesized and assayed for chymotrypsin. In the conformational analysis by 400 MHz 1H NMR, the L-D and D-L isomers, but not the L-L and D-D isomers, showed fairly large upfield shifts (0.2-0.4 ppm) of Leu-beta CH2 and gamma CH proton signals, indicating the presence of shielding effects from the benzene ring. In addition to distinct signal splitting of Phe-beta CH2, the NOE enhancement observed between Leu-delta CH3 and Phe-phenyl groups revealed that these groups are in close proximity. These data indicated that L-D and D-L isomers form a hydrophobic core between side chains of adjacent Leu and Phe residues. When the dipeptides were examined for inhibition of chymotrypsin using Ac-Tyr-OEt as a substrate, the L-L isomer showed no inhibition, itself becoming a substrate. However, the other three isomers inhibited chymotrypsin in a competitive manner, and the D-L isomer was strongest with Ki of 2.2 x 10(-5) M. It was found that the D-L isomer was only slowly hydrolysed but the L(or D)-D isomer was not. H-D-Phe-L-Leu-OBzl with the inverse sequence of H-D-Leu-L-Phe-OBzl inhibited chymotrypsin more strongly (Ki = 6.3 x 10(-6) M).(ABSTRACT TRUNCATED AT 250 WORDS)
Two phospholipases A2 named PLA2-III and IV were newly isolated from Trimeresurus gramineus (green habu snake) venom in addition to PLA2-I and II reported previously [ODA et al. (1991) Toxicon 29, 157; Fukagawa et al. (1992) Toxicon 30, 133]. Their isoelectric points were determined to be about 4.5. PLA2-III and IV exhibited almost unchanged lipolytic activity toward egg-yolk when compared with PLA2-I. The amino acid sequences were determined by sequencing the native proteins and the peptides produced by enzymatic (Achromobacter protease I and clostripain) and chemical (hydroxylamine) cleavages of the S-carboxamidomethylated derivative of the proteins. Both proteins consisted of 122 amino acid residues. When compared with PLA2-I, PLA2-III showed only a single amino acid substitution at the N-terminal position; namely from His to Asn. PLA2-IV also showed a single substitution from Ala to Asp at position 72. It was inferred that these amino acid substitutions between PLA2-I and PLA2-III or IV are due to the single base substitution at the corresponding codons of genes, which might be preserved independently. The unique presence of Phe at position 28, where Tyr is commonly located and assumed to be a part of the Ca(2+)-binding loop, was conserved in both PLA2-III and IV as in PLA2-I. There was no significant difference in the dissociation constants (4.3-5.2 x 10(-4) M) for Ca2+ between these PLA2S and Tyr-28-containing PLA2S. These results suggested that the p-hydroxy group of Try-28 does not play a crucial role in binding of PLA2S to Ca2+.
When papain was reacted with L-1-acetyl-2,3-dihydropyrrolo[2,3-b]indole- 2-carboxamide at pH 8.0, inactivation occurred accompanied by modification of Cys-25 in the active site. Plots of pseudo first-order rate constants against the reagent concentrations yielded an anomalous sigmoidal curve, suggesting that papain responded to this reagent in an allosteric manner. This is supported by the fact that the presence of a moderate concentration (a twenty-fold molar excess) of N alpha-acetyl-L-tryptophanamide over papain accelerated the inactivation.
The thiol groups of leucinthiol, cysteamine and cysteine incorporated into opioid peptides enkephalin and morphiceptin were activated by the 3-nitro-2-pyridinesulphenyl (Npys) group to form mixed disulphides highly reactive to a free thiol. Enkephalin analogues containing Npys-leucinthiol or -cysteine at positions 4, 5 and 6 exhibited high affinities for both mu and delta receptors, while morphiceptin analogues containing Npys-cysteine at positions 4 and 5 showed relatively weak affinity only for mu receptors. When these S-activated opioid peptides were incubated with rat brain membrane preparations, it was found, by binding assay using radiolabelled and non-labelled [D-Ala2,MePhe4,Gly-ol5]enkephalin, that they label mu opioid receptors in a dose-dependent manner. The concentrations required to label half of the receptors were 0.2-2 microM for enkephalins and 10-30 microM for morphiceptins. These results suggested that the thiol group labelled by S-activated enkephalins and morphiceptins is present in the ligand binding site of receptor protein, but not in GTPase-binding protein.
In addition to phospholipase A2-I (PLA2-I) reported previously (ODA et al., 1991, Toxicon 29, 157), a new PLA2 named PLA2-II was isolated from Trimeresurus gramineus (green habu snake) venom, and its amino acid sequence was determined by sequencing the native protein and the peptides produced by enzymatic (Achromobacter protease I and clostripain) cleavages of the carboxamidomethylated derivative of the protein. The protein consisted of 122 amino acid residues and His-47, Asp-48, and Asp-98 which have been assumed to be essential for PLA2 activity were conserved. Its sequence similarity to PLA2-I was 79%, with 26 residual differences. In contrast to the unique presence of Phe-28 in PLA2-I, PLA2-II contains Tyr-28 as seen in most of other PLA2s. There was no significant difference between the dissociation constants of PLA2-I and PLA2-II for Ca2+. Secondary structure compositions of PLA2-II were similar to those of PLA2-I and Crotalus atrox PLA2. A striking difference was found between these isozymes in contractile activity of isolated smooth muscle preparation of guinea-pig ileum. PLA2-II was over ten times more potent than PLA2-I, although its lipolytic activity toward egg-yolk was even slightly weaker (73%) than that of PLA2-I. The difference in contractile activities of PLA2-I and PLA2-II could be assumed to be due to discriminative lipid recognition brought about by different amino acid residues at the 58th position (Asp for PLA2-I and Asn for PLA2-II).
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