Biomedical subjects

K F Fok

Publications and source records attributed to K F Fok.

At least 37 records · Page 2Linked to original sources

PubMed1990

Endothelin and sarafotoxin S6b have similar vasoconstrictor effects and postsynaptically mediated mechanisms.

In the isolated perfused mesenteric vascular bed, porcine endothelin (ET) and sarafotoxin S6b produced direct vasoconstriction and potentiated nerve stimulation-induced vasoconstriction. ET also enhanced the vasoconstrictor response to exogenous norepinephrine (NE). Basal or stimulated endogenous NE release was not affected either by ET or sarafotoxin S6b. Qualitatively similar responses to ET and sarafotoxin S6b were always observed, although, in many cases, the response to ET was greater and longer lasting than to sarafotoxin S6b. These results indicate that vasoconstrictor responses to ET or sarafotoxin S6b are mediated primarily by postsynaptic mechanisms. No initial vasodilator response to ET or sarafotoxin S6b was observed in this mesenteric vascular preparation.

Animals↗

PubMed1990

Cardiac and vascular actions of sarafotoxin S6b and endothelin-1.

Snake venom-derived sarafotoxin S6B (SRT) and porcine endothelium-derived endothelin-1 (ET) have striking structural similarities. In conscious, freely-moving rats, ET (0.67 nmol/kg) produced a transient tachycardia and fall in arterial blood pressure which was followed by a long-lasting increase in arterial pressure, bradycardia, decrease in cardiac output (CO) and marked increase in total peripheral resistance. In contrast, SRT (0.67 nmol/kg) produced only the sustained cardiovascular responses. The sustained cardiovascular effects of SRT or ET were similarly attenuated by nifedipine. SRT and ET (30 nM) produced vasoconstriction in the isolated perfused mesenteric vascular bed without initial vasodilation. SRT and ET had potent positive inotropic and negative chronotropic effects on isolated perfused hearts and induced toxic reactions including coronary vasospasm, arrhythmias, A-V block and ventricular fibrillation. In addition to SRT lacking the initial depressor response in vivo, several differences in the activities of the peptides were also observed. ET produced greater and longer-lasting actions than SRT in producing pressor and vasoconstrictor responses in all 3 preparations, and in its ability to induce toxic effects on the heart.

Animals↗

PubMed1990

Multiple peptide synthesis using a single support (MPS3).

An automated multiple peptide synthesis method to synthesize, cleave, and purify several peptides simultaneously in a single batch has been developed. The technique is based on the synthesis of multiple peptides on a single solid phase support and is easily adapted to manual or to automated methods. The approach relies on coupling of amino acid mixtures to the resin and it has been found that DCC/HOBt gives the best coupling performance. Fast Atom Bombardment Mass Spectrometry (FAB-MS) was used to rapidly and efficiently identify the peptides in each synthetic mixture which significantly assisted the purification process by HPLC. The method has been successfully applied to the synthesis of magainin 2 and angiotensinogen peptides.

Amino Acid Sequence↗

PubMed1989

Identification of an allatostatin from adult Diploptera punctata.

A peptide (allatostatin) causing strong and rapid inhibition of juvenile hormone synthesis in vitro by corpora allata from reproductively active females has been isolated from brain/retrocerebral complexes of the cockroach Diploptera punctata. The primary structure of this 13-residue peptide has been determined: Ala-Pro-Ser-Gly-Ala-Gln-Arg-Leu-Tyr-Gly-Phe-Gly-Leu-NH2. Removal of the terminal amide group caused at least a ten thousandfold loss of activity. This neurohormone has no sequence similarity with any other known neuropeptide. Its target in the biosynthetic pathway is located prior to the conversion of farnesol to juvenile hormone.

Animals↗

PubMed1989

In vitro and in vivo activity of chymotrypsin-activated big endothelin (porcine 1-40).

We investigated whether big endothelin (porcine 1-40) had contractile activity in isolated rat aorta or pressor activity when injected intravenously into the anesthetized rat. When isolated rat aorta was exposed to a 100 nM concentration of big endothelin, 4.8% of a maximal KCl contraction was observed, compared to 131% of KClmax when paired aortic rings were exposed to an equivalent concentration of synthetic endothelin. Likewise, big endothelin had very weak pressor activity when injected intravenously into anesthetized, ganglion-blocked rats at 10 nmol/kg. When big endothelin was incubated with chymotrypsin, native endothelin and other peptide fragments were formed. Chymotrypsin-treated big endothelin produced an endothelin-like contraction when applied to isolated rat aortic rings, and a characteristic endothelin-like effect on blood pressure in vivo. Our results indicate that the biological activity of endothelin could be effectively blocked by inhibiting the enzyme which converts big endothelin to endothelin.

Amino Acid Sequence↗

PubMed1989

Plasmin's peptide-binding specificity: characterization of ligand sites in alpha 2-antiplasmin.

The basis for specific binding of plasmin to alpha 2-antiplasmin (AP) was analyzed by preparing overlapping synthetic peptides of 11, 17, 18, 19, 26, 33, and 40 amino acid residues corresponding to the carboxy-terminal sequence of AP. Affinities of the peptides for plasmin were estimated by competitive inhibition of the association of AP with plasmin. Dissociation constants with increasing peptide length were: 200, 54, 19, 18, 9.8, 4.7, and 2.8 microM, respectively. Peptides blocked binding sites on plasmin, not the catalytic site, as evidenced by lack of effect on the hydrolysis of chromogenic substrates. Substituting arginine for lysine at the carboxy-terminus or the 17th residue from the carboxy-terminus decreased the affinity of peptides for plasmin 9-fold and 5-fold, respectively, implicating these lysine residues of AP as major ligand sites for plasmin. Several stepwise increases in affinity of peptides for plasmin as peptide length increased up to 40 residues suggest contributions by additional sites, possibly other lysine residues. A potential plasmin binding site in fibrin, analogous to that in AP, is identified by affinity for plasmin of synthetic peptides corresponding to part of the alpha-chain ending with residue 207. To explain these data, we propose that plasmin recognizes physiological ligands by binding two or more lysine residues which are optimally presented to favor simultaneous interaction with separate lysine-binding site in plasmin.

Arginine↗

PubMed1989

Effects of endothelin on regional hemodynamics in conscious rats.

Endothelin is a potent vasoactive peptide in anesthetized rats and isolated vascular smooth muscle. This study was performed to describe the hemodynamic effects of endothelin in conscious, freely moving rats. Endothelin (0.067-2 nmol/kg i.v.) produced long-lasting, dose-dependent increases in arterial pressure, mesenteric and, to a lesser degree, hindquarters vascular resistances and decreases in heart rate. We suggest that endothelin may play an important role in regulation of arterial pressure by modulating peripheral vasomotor tone.

Animals↗

PubMed1989

Regional hemodynamic and baroreflex effects of endothelin in rats.

Endothelin is a peptide with potent, long-lasting pressor effects characterized by increases in mesenteric and hindquarters vascular resistance and bradycardia following an initial, transient depressor response. This study examined the mechanisms of action of endothelin on regional hemodynamics in conscious, freely moving rats and on baroreflex sensitivity both in conscious and chloralose-anesthetized rats. The pressor response to endothelin (0.67 nmol/kg) was attenuated by nifedipine (25 micrograms/kg) and augmented by chloralose anesthesia. The bradycardia was attenuated by pentolinium (10 mg/kg), atropine methyl sulfate (0.5 mg/kg), or chloralose anesthesia. Hindquarter vaso-constriction was attenuated by nifedipine, pentolinium, and atropine, whereas mesenteric vasoconstriction was less sensitive to blockade. The vasopressin V1 antagonist, [d(CH2)5Tyr(Me)]-AVP (20 micrograms/kg), indomethacin (5 mg/kg), or verapamil (150 micrograms/kg) did not affect any of these cardiovascular responses. Renal sympathetic nerve activity was reduced similarly in chloralose-anesthetized rats to pressor responses elicited by either phenylephrine or endothelin, and the slope of the baro-reflex function curve after endothelin was similar to that of phenylephrine. These results suggest that endothelin is a potent vasoconstrictor in which its action on visceral and skeletal muscle vasculature is mediated by somewhat different mechanisms. Endothelin does not alter baroreceptor reflex control of sympathetic nerve activity or heart rate.

Animals↗

PubMed1988

Alpha 2-antiplasmin's carboxy-terminal lysine residue is a major site of interaction with plasmin.

alpha 2-Antiplasmin (AP) inhibits plasmin in a two-step reaction in which AP reversibly binds to lysine-binding sites of plasmin and, then, more slowly complexes covalently with the enzyme's active site. Here, we show that the C-terminal lysine residue of AP has a key role in binding of the inhibitor to plasmin. A synthetic peptide corresponding to the C-terminal 26 amino acid residues of AP blocked association of AP with plasmin, but this activity of the peptide was lost when its C-terminal lysine residue was removed with carboxypeptidase B. The essential role of this lysine residue was shown more directly by treating AP with carboxypeptidase B and observing that AP lost its ability to inhibit plasmin rapidly.

Binding Sites↗

PubMed1988

The design of school furniture for Hong Kong schoolchildren. An anthropometric case study.

Anthropometric data of Hong Kong schoolchildren have been collected and analysed in order to develop recommendations for the design of chairs and tables for use in Hong Kong Government coeducational schools. The anthropometric data for Hong Kong have been compared with data from a Western population (United Kingdom) and another Asian population (Japan). Five sizes of chair and table combinations have been proposed to accommodate six primary and seven secondary forms (pupils aged from 6 to 18 years). The recommended design dimensions, based on the anthropometric characteristics of the Hong Kong target populations, are discussed in relation to recommendations from previous research in this area.

Journal Article↗

PubMed1987

Sulfation of a tyrosine residue in the plasmin-binding domain of alpha 2-antiplasmin.

Sulfation of human alpha 2-antiplasmin, the major plasma inhibitor of fibrinolysis, was examined using both protein isolated from human plasma and protein synthesized and biosynthetically labeled with [35S]sulfate by a human hepatoma-derived cell line. Linkage of sulfate to tyrosine was demonstrated by recovery of labeled tyrosine sulfate after base hydrolysis of sulfate-labeled alpha 2-antiplasmin. Analysis by reverse-phase high performance liquid chromatography of peptides released from alpha 2-antiplasmin by cleavage with trypsin or cyanogen bromide indicated that sulfate is linked to a single segment of the protein. A cyanogen bromide peptide corresponding to the sulfate-labeled peptide was prepared from alpha 2-antiplasmin isolated from human plasma. Consistent with the presence of tyrosine sulfate in this peptide, its chromatographic elution was altered by treatment with acid under conditions which release sulfate from a tyrosine residue. No peptide in the total digest of alpha 2-antiplasmin by cyanogen bromide eluted at the position of the peptide following desulfation, suggesting that all of the protein is in a sulfated form. The sequence of the sulfate-containing cyanogen bromide peptide as determined by sequential Edman degradation, amino acid composition, and fast atom-bombardment-mass spectrometry was: Glu-Glu-Asp-Tyr(SO4)-Pro-Gln-Phe-Gly-Ser-Pro-Lys-COOH. This peptide is a segment of the previously identified plasmin-binding domain of alpha 2-antiplasmin.

Amino Acid Sequence↗

PubMed1986

Sequence analysis of the COOH terminus of the alpha-chain of the fourth component of human complement. Identification of the site of its extracellular cleavage.

The predominant form of the fourth component of complement in human and murine plasma (C4p) has an Mr approximately 5,000 less than C4 secreted by hepatocytes or macrophages (C4s). Here we demonstrate that the difference in Mr results from excision of a peptide from the COOH terminus of the alpha-chain of C4s. The site of truncation of the alpha-chain of C4 in plasma was established by sequence analysis of the COOH-terminal cyanogen bromide fragment isolated by high performance liquid chromatography. Sequential Edman degradation, digestion with carboxypeptidase Y, analysis of amino acid composition, and partial sequence analysis of an overlapping tryptic peptide established the sequence of this peptide to be Glu-Ala-Asn-Glu-Asp-Tyr-Glu-Asp-Tyr-Glu-Tyr-Asp-Glu-Leu-Pro-Ala. Comparison of our results with reported sequences establishes the COOH-terminal alanine to be residue 748 in the alpha-chain. This identifies the site at which C4s is cleaved by an extracellular protease and suggests that the protease has an elastase-like activity.

Amino Acid Sequence↗

PubMed1986

Proteolytic processing of atriopeptin prohormone.

The metabolism of atriopeptin prohormone ANF1-126 was examined with the aid of two separate radioimmunoassays, one detecting the C-terminal atriopeptins and the other detecting a fragment of the prohormone N-terminus. Intact prohormone standards are recognized in both assays, whereas the C-terminal atriopeptins are only detected by the atriopeptin assay. Both atriopeptin and N-terminal fragment immunoreactivities were detected in rat plasma and were simultaneously elevated following intravenous administration of desamino-arginine-vasopressin. Atriopeptin immunoreactivity returned to basal levels within 60 min after desamino-arginine vasopressin administration, whereas the N-terminal fragment immunoreactivity remained elevated for more than 2 hr. Analysis of both acid-boiled and sodium dodecyl sulfate-boiled rat atrial extracts by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting revealed the presence of a single high molecular weight species which reacted to both antisera and which comigrated with atriopeptin prohormone standards. Western blots of plasma from desamino-arginine vasopressin-stimulated rats yielded both the low molecular weight C-terminal atriopeptin and a high molecular weight N-terminal fragment-reactive peak which was smaller than the prohormone standards and which did not possess atriopeptin immunoreactivity. A recombinant 128-amino acid atriopeptin prohormone construct, ANF1-126-Arg-Arg, was used as a model substrate for prohormone metabolism. ANF1-126-Arg-Arg was specifically cleaved followed incubation with thrombin to yield the 98-amino acid N-terminal fragment and the C-terminal atriopeptin, AP28-Arg-Arg. Processing of ANF1-126-Arg-Arg by reperfusion through an isolated heart or by incubation in serum yielded identical metabolites to those generated by incubation with thrombin. No significant metabolism was observed following incubation of the prohormone with rat plasma. We conclude that the rat heart contains the necessary enzyme to cleave both endogenous and exogenous prohormone to atriopeptin and that processing by blood enzymes is not required.

Amino Acid Sequence↗

PubMed1985

Ser-Leu-Arg-Arg-atriopeptin III: the major circulating form of atrial peptide.

Vasopressin induces a concentration-dependent increase in atriopeptin immunoreactivity in plasma. Rat plasma, rat atrial extract, and synthetic atriopeptin III (APIII) produced parallel displacement curves of iodine-125-labeled APIII binding to specific antiserum. Fractionation of plasma atriopeptin immunoreactivity by reverse-phase high-performance liquid chromatography showed that the major portion consists of two species of low molecular weight peptides in a ratio of 10 to 1. Both peaks exhibited potent vasorelaxant activity, suggesting the presence of the carboxyl terminal Phe-Arg sequence of atriopeptin in each species. Sequence determination of the purified peptides indicated that the major peptide is Ser-Leu-Arg-Arg-APIII and the minor peptide APIII. It appears that the former is the major species of atrial peptide in the rat circulation and that it is the product of selective cleavage of the high molecular weight precursor.

Animals↗

PubMed1985

Differential structure-activity relationships of atrial peptides as natriuretics and renal vasodilators in the dog.

Natriuretic-diuretic and vasodilator activities of synthetic atriopeptin (AP)-related peptides were examined in the anesthetized dog. We have selected, the naturally occurring, APIII as the reference compound for comparison with various related peptides. APIII is a 24 amino acid peptide with the sequence ser-ser-cys-phe-gly-gly-arg-ile-asp-arg-ile-gly-ala-gln-ser-gly-leu-gly- cys-asn-ser-phe-arg-tyr-OH. APII, another peptide isolated from atrial extracts, lacks the C-terminal arg- of APIII. N-terminal amino acid extensions on APIII or APII, exhibited enhanced natriuretic-diuretic effectiveness. Furthermore, the maximum response obtained by ser-leu-arg-arg-APIII and arg-arg-APIII were significantly higher and the dose-response curve was not parallel to that obtained with APIII. In contrast, there were no significant qualitative or quantitative differences between the renal blood flow responses produced by the N-terminal extended peptides and APII or APIII. These results suggest a heterogeneity of AP receptors in vascular and renal tubular tissues.

Amino Acid Sequence↗

PubMed1985

Atriopeptins: correlation between renal vasodilation and natriuresis.

The effect of atrial peptides on renal function was studied in intact anesthetized dogs. A quantitative comparison of bolus intra-arterial injections demonstrated a rank order potency as renal vasodilators and natriuretic/diuretic agents as follows: ser-leu-arg-arg-atriopeptiin III (SLRR-APIII) greater than high molecular weight artrial peptide greater than or equal to atriopeptin (AP)III = APII much greater than API (essentially inactive). A sustained infusion of APIII was employed in order to study the temporal and quantitative correlation of the renal functional changes induced by the atrial peptide. Both intra-arterial and intravenous administration of the peptide produced concentration-dependent increases in renal blood flow, urine volume, sodium excretion, and osmotic clearance. Infusion of APIII into the renal artery did not alter systemic blood pressure or heart rate. Intravenous infusions of APIII required 10 times higher doses to induce the changes in renal vascular resistance and electrolyte excretion, and a fall in blood pressure and tachycardia resulted. The natriuretic-diuretic effect of the atriopeptins appears to be closely associated with renal vasodilation, exhibiting a positive linear correlation between the peptide-induced changes in sodium excretion and changes in renal blood flow.

Animals↗

PubMed1985

Comparative vascular pharmacology of the atriopeptins.

The atriopeptins are potent relaxants of norepinephrine-constricted aortic strips or are dilators of renal blood vessels in isolated perfused rat kidneys that are constricted by norepinephrine. This vasorelaxant property of the atriopeptins requires the presence of phenylalanine arginine (i.e., atriopeptin II, III, or ser-leu-arg-arg atriopeptin III) residues in the carboxy terminus which are considerably more effective than atriopeptin I (the 21 amino acid peptide which lacks the phe-arg C-terminus) or the core peptide (residues 3-19). However, these artificially in vitro precontracted preparations do not accurately predict the vascular effectiveness of the atriopeptins in intact rats. Intravenous administration of the atriopeptins (including atriopeptin I) to anesthetized rats produces concentration-dependent hypotension, a selective decrease in renal resistance in low doses (determined with microspheres), and pronounced diuresis. At higher doses, atriopeptins increase blood flow in other vascular beds. On the other hand, in the anesthetized dog, injection (intraarterially) of the phe-arg-containing peptides produces a concentration-dependent increase in both renal blood flow and sodium excretion, whereas atriopeptin I is inactive. Although there is a species difference in responsiveness to atriopeptin I, these data demonstrate a direct correlation between the renal vasodilation and diuresis produced by this novel family of atrial peptides.

Animals↗

PubMed1984

The sequence of an atriopeptigen: a precursor of the bioactive atrial peptides.

The high molecular weight fraction ( atriopeptigen -APG) obtained by gel filtration chromatography of rat atrial extracts was fractionated by isoelectric focusing and reverse phase HPLC to obtain a pure APG. Purification of cyanogen bromide digests of the crude high molecular weight fraction resulted in the isolation of a single biologically active cyanogen bromide cleavage peptide. Sequence analyses of these peptides coupled with recent reports of sequence analyses of intermediate molecular weight atrial peptides ( Thibault , et al. (1984) FEBS Letters 167, 352-356, and Kangwa , et al., Biochem. Biophys. Res. Commun 119, 933-940) provide the complete primary structure of an 111 residue APG.

Amino Acid Sequence↗