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Biomedical subjects

A K Tan

Publications and source records attributed to A K Tan.

50 records · Page 3Linked to original sources
PubMed1993

The fixed dilated pupil after cataract surgery--is it related to intraocular use of hypromellose?

Hypromellose can be used as a viscoelastic substance during cataract surgery. Two groups of patients, one group operated on using hypromellose, the other using sodium hyaluronate as a viscoelastic substance were followed up 2-6 months postoperatively. A total of 16.7% of the eyes operated on using hypromellose developed a non-reactive semidilated pupil whereas none of the eyes from the control group developed this phenomenon. It was concluded that there is a probable link between the intraocular use of hypromellose and abnormal pupils after cataract surgery.

Aged↗
PubMed1993

Ambulatory tonsillectomy and adenoidectomy: complications and associated factors.

This is a case-control study of over 500 cases of tonsillectomy and adenotonsillectomy performed at the Montreal Children's Hospital over a 10-month period. Demographics of patient population and selection criteria for out-patient procedure are presented. The hot (cautery) and cold (knife) dissection techniques are compared. Complications including intraoperative and postoperative hemorrhage are presented. The recognition of aerodigestive tract complications necessitating re-admission or emergency room management cannot be over emphasized. Recent history of upper airway infection (UAI) was noted to be associated with postoperative aerodigestive tract complications. The triad of UAI, knife dissection technique and increased intraoperative bleeding was found to be associated clinically and statistically with primary postoperative hemorrhage.

Adenoidectomy↗
PubMed1993

Bilateral proptosis after post-traumatic hydrocephalus: a case report.

Hydrocephalus can cause several neuro-ophthalmological complications but only rarely has it been reported to cause proptosis. A 23-year-old woman developed post-traumatic hydrocephalus after a road traffic accident. She was found to have deteriorating visual acuity bilaterally and bilateral proptosis. Both these complications resolved after a ventriculo-peritoneal shunt was inserted.

Adult↗
PubMed1992

Identification of the site of covalent attachment of nafcillin, a reversible suicide inhibitor of beta-lactamase.

Nafcillin was shown to reversibly inhibit beta-lactamase from Staphylococcus aureus PC1 with characteristics indicative of a type A inhibitor [Citri, Samuni & Zyk (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 1048-1052]. At nafcillin concentrations above 80 mM, complete inactivation occurred within 200 s. Upon removal of the excess nafcillin the inhibited enzyme was re-activated completely, with a rate constant of 2.0 x 10(-3) s-1 (25 degrees C). The inhibited enzyme was shown to be in the form of a covalent acyl-enzyme intermediate. Digestion by pepsin and trypsin yielded a single nafcillin-labelled peptide fragment which was isolated, sequenced and shown to be: Ala-Tyr-Ala-Ser-Thr-Ser-Lys. This sequence corresponds to the region surrounding the active-site serine residue, Ser-70, indicating that the inhibitor is covalently attached to the same residue as productive substrates.

Binding Sites↗
PubMed1992

Hospitalized croup (bacterial and viral): the role of rigid endoscopy.

This is a retrospective study of 500 cases of hospitalized patients with the diagnosis of croup (laryngotrachitis), admitted between January 1986 and August 1988, at the Montreal Children's Hospital. The patient's age, sex, clinical history, physical examination, number of admissions, season of admission, method of diagnosis, treatment and management were reviewed. Two per cent of these patients were diagnosed as bacterial croup. All of them required intubation and endoscopic evaluation in their management. Of all the viral croup patients, less than one-third were severe enough to require an intensive care setting for their management. In these patients, 6% required endotracheal intubation, and on endoscopy, a significant number of these patients had an endoscopic airway abnormality in addition to croup (subglottic edema). According to our findings, we suggest diagnostic micro-laryngoscopy and bronchoscopy be performed on certain groups of croup patients because of their higher yield of airway abnormality on endoscopy.

Bacterial Infections↗
PubMed1991

Differences in substrate specificities of monoamine oxidase A from human liver and placenta.

The substrate specificities of monoamine oxidase (MAO) A isolated from human placenta and of human liver expressed in yeast have been compared in homogeneous preparations with respect to Vmax and Km values for natural and synthetic substrates and Ki values for competitive inhibitors. MAO A from these two sources is known to differ in at least 5 amino acid residues. While the Km and Ki values were found to be nearly identical in the enzymes from these two sources, the Vmax differed significantly on bulky synthetic substrates.

1-Methyl-4-phenylpyridinium↗
PubMed1991

Site-directed mutagenesis of beta-lactamase leading to accumulation of a catalytic intermediate.

Site-specific mutation of Glu-166 to Ala in beta-lactamase causes a millionfold reduction in catalytic activity toward both penicillin and cephalosporin substrates and results in the stoichiometric accumulation of a normally transient acyl-enzyme intermediate. Kinetic analysis indicated that substitution of Glu-166 by Ala leads to negligible effect on the acylation half of the reaction but effectively eliminates the deacylation reaction. Such differential effects on the rates of formation and breakdown of an enzyme-substrate intermediate have not been previously reported. Thus, unlike the situation for most transfer enzymes, e.g., the serine proteases, acylation and deacylation in beta-lactamase catalysis are not "mirror" images and must involve different mechanisms. The results suggest an explanation for the different catalytic activities between the beta-lactamases and the penicillin-binding proteins involved in bacterial cell-wall synthesis.

Acylation↗
PubMed1990

Cryoenzymology of staphylococcal beta-lactamase: trapping a serine-70-linked acyl-enzyme.

Various cryosolvents were investigated for their suitability in cryoenzymological experiments with beta-lactamase from Staphylococcus aureus PC1. On the basis of the minimal effects on the catalytic and structural properties of the enzyme, ternary solvents containing ethylene glycol, methanol, and water were found most suitable. The interaction of beta-lactamase with a number of substrates was studied at subzero temperatures. In general, the reaction profiles were similar to those in aqueous solution at above-zero temperatures, with the exception of the slower rates. For cephalosporin substrates, such as PADAC, in which the 3'-substituent may leave to form a more stable form of the acyl-enzyme [Faraci, W., & Pratt, R. (1985) Biochemistry 24, 903-910], this intermediate could be readily stabilized at subzero temperatures. At -40 degrees C the slow rate of deacylation in the reaction with the chromophoric substrate 6 beta-[(furylacryloyl)amino]penicillanic acid permitted the acyl-enzyme to be stoichiometrically accumulated. This intermediate was then stabilized at low pH with trifluoroacetic acid. Isolation by centrifugal gel filtration, followed by pepsin digestion, gave a penicilloyl-labeled peptide which was isolated by HPLC. Subsequent trypsinolysis of this peptide gave a single labeled peptide, corresponding to the octapeptide surrounding the active-site serine, Ser-70.

Acylation↗
PubMed1989

Trapping the acyl-enzyme intermediate in beta-lactamase I catalysis.

Cryoenzymology techniques were used to facilitate trapping an acyl-enzyme intermediate in beta-lactamase I catalysis. The enzyme (from Bacillus cereus) was investigated in aqueous methanol cryosolvents over the 25 to -75 degrees C range, and was stable and functional in 70% (v/v) methanol at and below 0 degree C. The value of kcat. decreased linearly with increasing methanol concentration, suggesting that water is a reactant in the rate-determining step. In view of this, the lack of incorporation of methanol into the product means that the water molecule involved in the deacylation is shielded from bulk solvent in the enzyme-substrate complex. From the lack of adverse effects of methanol on the catalytic and structural properties of the enzyme we conclude that 70% methanol is a satisfactory cryosolvent system for beta-lactamase I. The acyl-enzyme intermediate from the reaction with 6-beta-(furylacryloyl)amidopenicillanic acid was accumulated in steady-state experiments at -40 degrees C and the reaction was quenched by lowering the pH to 2. H.p.l.c. experiments showed covalent attachment of the penicillin to the enzyme. Digestion by pepsin and trypsin yielded a single labelled peptide fragment; analysis of this peptide was consistent with Ser-70 as the site of attachment.

Acylation↗
PubMed1989

Clavulanate inactivation of Staphylococcus aureus beta-lactamase.

The interaction of clavulanic acid with beta-lactamase from Staphylococcus aureus was investigated, particularly with a view to determining whether conformational effects are involved. The inactivation at neutral pH is essentially stoichiometric, leading to an inactive species with an enamine chromophore. Two forms of the enamine were observed, the first-formed having a positive ellipticity with a maximum near 290 nm. This species slowly converted into the stable form of the inactivated enzyme that had a negative ellipticity with a minimum at 275 nm. This change in sign of the ellipticity of the enamine is consistent with the previously proposed cis-trans isomerization of the enamine [Cartwright & Coulson (1979) Nature (London) 278, 360-361). Both the far-u.v.c.d. and the intrinsic viscosity of the inactivated enzyme indicated that negligible change in conformation of the enzyme accompanied inactivation. The rates of inactivation and enamine formation were compared at low temperatures, where the initial rates were slow enough to be monitored. The rate of loss of 95% of the catalytic activity was almost 100-fold faster than the rate of formation of the first-formed enamine species. The remaining 5% activity was lost with a rate comparable with that for formation of the initial enamine. The simplest explanation of these results is that a relatively stable acyl-enzyme intermediate builds up initially and more slowly partitions between turnover (hydrolysis) and enamine formation. The initially formed enamine is in the cis conformation but slowly isomerizes to the more stable trans form.

Amines↗
PubMed1987

Kinetic and structural characterization of reversibly inactivated beta-lactamase.

The reversible inhibition of beta-lactamase I from Bacillus cereus by cloxacillin, methicillin, and nafcillin has been systematically investigated. For these substrates the enzymatic reaction involves partitioning of the substrate between turnover and inhibition. Typically, concentrations of several hundred millimolar are necessary for complete inactivation. The completely inactivated enzyme could be formed by incubation at temperatures above 20 degrees C, where inhibition competes more effectively with turnover, and then stabilized by dropping the temperature to 0 degrees C or lower. The inactivated enzyme was rapidly separated from unreacted substrate and product at low temperature by centrifugal gel filtration or ion exchange and examined by far-UV circular dichroism for evidence of a conformational change. At pH 7 the inactivated enzyme had a secondary structure essentially identical with that of the native enzyme. The fluorescence emission spectrum of the inactivated enzyme (at pH 7) was also identical with that of the native enzyme. However, the inactivated enzyme was found to be considerably more sensitive to thermal denaturation, to acid-induced conformational isomerization, and to trypsinolysis than the native enzyme. We conclude from the circular dichroism results that the structure of the reversibly inactivated enzyme cannot be significantly different from that of the native enzyme. Therefore, previous findings that have been interpreted as indicating a major conformational change must be reevaluated. From examination of the low-resolution crystallographic structure of the enzyme we propose that the most likely cause of the inactivation is an alternate conformational state of the acyl-enzyme intermediate involving movement of one or more of the alpha-helices forming part of the active site. Such a structural effect could leave the secondary structure unchanged but have significant effects on the tertiary structure, catalysis, mobility, and susceptibility to trypsin and denaturation. We propose that the underlying physical reason why certain beta-lactam substrates bring about this "substrate-induced deactivation", or suicide inactivation, of the enzyme is due to the presence of the alternative acyl-enzyme conformation of similar free energy to the productive one, in which one (or more) essential catalytic group is no longer optimally oriented for catalyzing deacylation. Thus for substrates with a slow rate of deacylation (less than or equal to 100 s-1) the conformational transition can compete effectively on the time scale of the turnover reaction.

Bacillus cereus↗
PubMed1980

Laparoscopy as a diagnostic aid in women with localized ureteral obstruction due to endometriosis.

Endometriosis is a common disease which can produce localized ureteral obstruction. Many women will have significant disease without any clinical signs or symptoms until they present with distal ureteral obstruction. In the reported literature, the diagnosis of ureteral involvement by endometriosis usually has been made at surgery. It is therefore suggested that women with distal ureteral obstruction not attributable to calculous disease undergo laparoscopy in an attempt to rule out endometriosis as the cause of ureteral obstruction.

Adult↗