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Biomedical subjects

A R Khan

Publications and source records attributed to A R Khan.

At least 37 records · Page 2Linked to original sources
PubMed1999

Roles of IL-1 and TNF in the decreased ileal muscle contractility induced by lipopolysaccharide.

Gastrointestinal stasis during sepsis may be associated with gastrointestinal smooth muscle dysfunction. Endotoxin [lipopolysaccharide (LPS)] impairs smooth muscle contraction, in part through inducible nitric oxide synthase (NOS II) and enhanced nitric oxide production. We studied the roles of tumor necrosis factor-alpha (TNF) and interleukin-1 (IL-1) in this process by using TNF binding protein (TNFbp) and IL-1 receptor antagonist (IL-1ra). Rats were treated with TNFbp and IL-1ra, or their vehicles, 1 h before receiving LPS or saline. At 5 h after LPS, contractility was measured in strips of ileal longitudinal smooth muscle, and NOS II activity was measured in full-thickness segments of ileum. LPS decreased maximum stress (mean +/- SE) from 508 +/- 55 (control) to 355 +/- 33 g/cm2 (P < 0.05). Pretreatment with TNFbp plus IL-1ra prevented the LPS-induced decrease. Separate studies of TNFbp alone or IL-1ra alone indicated that, at the doses and timing used, TNFbp was more effective. LPS also increased NOS II activity by >10-fold (P < 0.01) over control. This increase was prevented by TNFbp plus IL-1ra (P = not significant vs. control). We conclude that the LPS-induced increase in NOS II activity and the decrease in ileal muscle contractility are mediated by TNF and IL-1.

Animals↗
PubMed1998

Lowering the entropic barrier for binding conformationally flexible inhibitors to enzymes.

The design of inhibitors with enhanced potency against proteolytic enzymes has many applications for the treatment of human diseases. In addition to the optimization of chemical interactions between the enzyme and inhibitor, the binding affinity can be increased by constraining the inhibitor to the conformation that is recognized by the enzyme, thus lowering the entropic barrier to complex formation. We have structurally characterized the complexes of a macrocyclic pentapeptide inhibitor and its acyclic analogue with penicillopepsin, an aspartic proteinase, to study the effect of conformational constraint on the binding affinity. The phosphonate-based macrocycle PPi4 (Ki = 0.10 nM) is covalently linked at the P2-Asn and P1'-Phe side chains [nomenclature of Schechter and Berger, Biochim. Biophys. Res. Commun. (1967) 27, 157-162] via an amide bond, relative to the acyclic compound PPi3 (Ki = 42 nM). Comparisons of the high-resolution crystal structures of PPi4-penicillopepsin (0.95 A) and PPi3-penicillopepsin (1.45 A) reveal that the conformations of the inhibitors and their interactions with the enzyme are similar. The 420-fold increase in the binding affinity of PPi4 is attributed to a reduction in its conformational flexibility, thus providing the first rigorous measure of the entropic contribution to the binding energy in a protein-ligand complex and stressing the advantages of the design strategy.

Aspartic Acid Endopeptidases↗
PubMed1998

Structural and functional characterization of Streptomyces plicatus beta-N-acetylhexosaminidase by comparative molecular modeling and site-directed mutagenesis.

We have sequenced the Streptomyces plicatus beta-N-acetylhexosaminidase (SpHex) gene and identified the encoded protein as a member of family 20 glycosyl hydrolases. This family includes human beta-N-acetylhexosaminidases whose deficiency results in various forms of GM2 gangliosidosis. Based upon the x-ray structure of Serratia marcescens chitobiase (SmChb), we generated a three-dimensional model of SpHex by comparative molecular modeling. The overall structure of the enzyme is very similar to homology modeling-derived structures of human beta-N-acetylhexosaminidases, with differences being confined mainly to loop regions. From previous studies of the human enzymes, sequence alignments of family 20 enzymes, and analysis of the SmChb x-ray structure, we selected and mutated putative SpHex active site residues. Arg162 --> His mutation increased Km 40-fold and reduced Vmax 5-fold, providing the first biochemical evidence for this conserved Arg residue (Arg178 in human beta-N-acetylhexosaminidase A (HexA) and Arg349 in SmChb) as a substrate-binding residue in a family 20 enzyme, a finding consistent with our three-dimensional model of SpHex. Glu314 --> Gln reduced Vmax 296-fold, reduced Km 7-fold, and altered the pH profile, consistent with it being the catalytic acid residue as suggested by our model and other studies. Asp246 --> Asn reduced Vmax 2-fold and increased Km only 1.2-fold, suggesting that Asp246 may play a lesser role in the catalytic mechanism of this enzyme. Taken together with the x-ray structure of SmChb, these studies suggest a common catalytic mechanism for family 20 glycosyl hydrolases.

Amino Acid Sequence↗
PubMed1998

Molecular mechanisms for the conversion of zymogens to active proteolytic enzymes.

Proteolytic enzymes are synthesized as inactive precursors, or "zymogens," to prevent unwanted protein degradation, and to enable spatial and temporal regulation of proteolytic activity. Upon sorting or appropriate compartmentalization, zymogen conversion to the active enzyme typically involves limited proteolysis and removal of an "activation segment." The sizes of activation segments range from dipeptide units to independently folding domains comprising more than 100 residues. A common form of the activation segment is an N-terminal extension of the mature enzyme, or "prosegment," that sterically blocks the active site, and thereby prevents binding of substrates. In addition to their inhibitory role, prosegments are frequently important for the folding, stability, and/or intracellular sorting of the zymogen. The mechanisms of conversion to active enzymes are diverse in nature, ranging from enzymatic or nonenzymatic cofactors that trigger activation, to a simple change in pH that results in conversion by an autocatalytic mechanism. Recent X-ray crystallographic studies of zymogens and comparisons with their active counterparts have identified the structural changes that accompany conversion. This review will focus upon the structural basis for inhibition by activation segments, as well as the molecular events that lead to the conversion of zymogens to active enzymes.

Amino Acid Sequence↗
PubMed1998

Esophageal replacement with colon in children.

Between 1974 and 1994, 25 colonic interposition procedures were performed for esophageal replacement in 23 cases of esophageal atresia (EA) and 2 corrosive strictures. Nine patients had one-stage and 16 had two-stage reconstructions. The transthoracic route was used in 16 cases (64%) and the retrosternal route in 9 (36%). Average age at the time of operation in EA patients was 17 months (range 12-33), and the children with corrosive injuries were 3 and 6 years old. Mean age at follow-up was 11.8 years (2.3-20.5 years). There was no mortality in the series. One patient developed full-graft necrosis and had a gastric pull-up procedure later. One child had partial graft necrosis (3 cm at the cervical end), however, enough colon was available for reconstruction. Ten patients developed a leak from the cervical anastomosis (40%) and 7 developed a stricture at the cervical esophago-colonic anastomosis (28%). The strictures were treated by repeated esophageal dilatation, and 3 patients required revision of the anastomosis (12%). Other complications included acid reflux in 2 cases (8%), small-intestinal obstruction in 1 (4%), redundancy of the colon in 1 (4%), and chest infections in 2 (8%). Follow-up included assessment of the patient's symptoms, serial growth measurements (height and weight), and where relevant, endoscopy and a contrast swallow or meal. There was a 52% improvement in weight and height percentiles post-operatively. Each of the 19 patients who had barium swallows showed rapid transit and emptying without any significant delay or hold-up. Overall long-term results were excellent in 13 patients (52%), good in 7 (28%), and fair in 5 (20%). The colon conduit thus provides an excellent substitute esophagus in pediatric patients.

Child, Preschool↗
PubMed1998

Copper, zinc, and magnesium levels in non-insulin dependent diabetes mellitus.

A relationship has been reported between trace elements and diabetes mellitus. This study evaluated the role of such a relationship in 83 patients with non-insulin dependent diabetes mellitus (40 men and 43 women), with a mean duration of diabetes of 3.9 +/- 3.6 years. Patients with nephropathy were excluded. Thirty healthy non-diabetic subjects were studied for comparative analysis. Subjects were subdivided into obese and non-obese. Diabetic subjects were also subdivided into controlled and uncontrolled groups; control was based on fasting blood glucose and serum fructosamine levels. Plasma copper, zinc and magnesium levels were analysed using a GBC 902 double beam atomic absorption spectrophotometer. Plasma zinc and magnesium levels were comparable between diabetic and non-diabetic subjects, while copper levels were significantly elevated (p < 0.01) in diabetic patients. Age, sex, duration and control of diabetes did not influence copper, zinc, or magnesium concentrations. We conclude that zinc and magnesium levels are not altered in diabetes mellitus, but the increased copper levels found in diabetics in our study may merit further investigation of the relationship between copper and non-insulin dependent diabetes mellitus.

Adult↗
PubMed1998

Variants of papillary carcinoma of the thyroid: experience at Asir Central Hospital.

Papillary carcinoma of the thyroid is now further subclassifiable into its different variants. We report incidence, clinical behaviour and outcome of rare variants of papillary thyroid cancers at Asir Central Hospital, Saudi Arabia. These variants differ from each other not only on a morphometric basis, but also in clinical behaviour. Accordingly, we report our experience of different variants in a series of 35 papillary carcinomas. These cases were received in the surgical pathology laboratory of Asir Central Hospital from January 1987 to December 1994. We have reviewed clinical charts, microscopic slides and pathological reports of these carcinomas. The following results have been obtained: 20 cases were classical (usual) papillary thyroid carcinomas: seven were follicular variants; three were tall cell/ columnar variants; two were oxyphilic variants; and one was a diffuse sclerosing variant. There were two de-differentiated papillary carcinomas (anaplastic carcinomas with foci of well-differentiated papillary carcinomas). These variants were also correlated with clinical parameters such as age, sex and nationality of the patients, aggressiveness of the tumours, types of surgery required, and follow-up when available.

Adolescent↗
PubMed1997

Comparative modeling of the three-dimensional structure of the calmodulin-related TCH2 protein from Arabidopsis.

Plants adapt to various stresses by developmental alterations that render them less easily damaged. Expression of the TCH2 gene of Arabidopsis is strongly induced by stimuli such as touch and wind. The gene product, TCH2, belongs to the calmodulin (CaM) family of proteins and contains four highly conserved Ca(2+)-binding EF-hands. We describe here the structure of TCH2 in the fully Ca(2+)-saturated form, constructed using comparative molecular modeling, based on the x-ray structure of paramecium CaM. Like known CaMs, the overall structure consists of two globular domains separated by a linker helix. However, the linker region has added flexibility due to the presence of 5 glycines within a span of 6 residues. In addition, TCH2 is enriched in Lys and Arg residues relative to other CaMs, suggesting a preference for targets which are more negatively charged. Finally, a pair of Cys residues in the C-terminal domain, Cys126 and Cys131, are sufficiently close in space to form a disulfide bridge. These predictions serve to direct future biochemical and structural studies with the overall aim of understanding the role of TCH2 in the cellular response of Arabidopsis to environmental stimuli.

Amino Acid Sequence↗
PubMed1997

Structural characterization of activation 'intermediate 2' on the pathway to human gastricsin.

The crystal structure of an activation intermediate of human gastricsin has been determined at 2.4 A resolution. The human digestive enzyme gastricsin (pepsin C) is an aspartic proteinase that is synthesized as the inactive precursor (zymogen) progastricsin (pepsinogen C or hPGC). In the zymogen, a positively-charged N-terminal prosegment of 43 residues (Ala 1p-Leu 43p; the suffix 'p' refers to the prosegment) sterically prevents the approach of a substrate to the active site. Zymogen conversion occurs in an autocatalytic and stepwise fashion at low pH through the formation of intermediates. The structure of the non-covalent complex of a partially-cleaved peptide of the prosegment (Ala 1p-Phe 26p) with mature gastricsin (Ser 1-Ala 329) suggests an activation pathway that may be common to all gastric aspartic proteinases.

Amino Acid Sequence↗
PubMed1997

Tongue involvement in a patient with granulomatous myositis.

A 64-year-old woman had difficulty swallowing and talking, weakness of the tongue, and progressive muscle weakness that was proven to be caused by a granulomatous myopathy. This case resembled a motor neuron disease, indicating that granulomatous myopathy should be considered in patients with similar presentations, since it is a treatable condition with a more benign prognosis.

Anti-Inflammatory Agents↗
PubMed1997

Lead exposure in papier mâché workers.

Lead exposure was assessed in a random cohort of 70 male papier mâché workers and compared with 35 age and sex matched controls. Blood-lead levels in workers were significantly higher than in controls (Mean 68.07 +/- 44.64 ug/dl vs 25.52 +/- 16.56 ug/dl respectively; P < 0.001). Urinary lead was also higher in workers (128.53 +/- 52.61 ug/l) compared to controls (91.18 +/- 27.06, P < 0.001), but excretion of urinary coproporphyrin in the two groups was not significantly different (102.78 +/- 153.42 vs 70.03 +/- 27.06 ug/l, P > 0.05). Blood levels bore a significant correlation with age but did not bear any correlation with the duration of exposure. Various modes of exposure to lead included hand mixing of paints, skin application of the paint to match shades and making point of the brush using lips and the tongue.

Adolescent↗
PubMed1996

Parenteral nutrition-related cholestasis in postsurgical neonates: multivariate analysis of risk factors.

The medical records of 74 neonates dependent on parenteral nutrition for at least 21 days after emergency abdominal surgery (performed between 1988 and 1992) were reviewed respectively. The role of enteral starvation, prematurity, composition and duration of parenteral nutrition, and sepsis in the evolution of parenteral nutrition-related cholestasis was evaluated by multiple regression analysis. The most important factors for cholestasis were low gestational age (median, 34 weeks), early exposure to parenteral nutrition, and sepsis. Episodes of sepsis were associated with a 30% increase in the bilirubin level. Enteral starvation and composition and the duration of parenteral nutrition solutions did not correlate significantly with the development of cholestasis. Prevention of sepsis should be the priority in minimising cholestasis in postsurgical neonates who are dependent on parenteral nutrition.

Abdomen↗
PubMed1996

Value of touch preparations (imprints) for diagnosis of cutaneous leishmaniasis.

BACKGROUND: The clinical diagnosis of cutaneous leishmaniasis is confirmed by demonstrating the organism on a superficial smear or on a biopsy of the lesion. Misdiagnosis in a biopsy specimen may be due to scanty Leishmania organisms that may not be identified in histologic sections. METHODS: Punch biopsies of skin lesions, suspected clinically to be cutaneous leishmaniasis, from 29 patients were taken. Touch smears on slides were air-dried, fixed with methyl alcohol, and stained with Giemsa stain. Results of routine histologic examination were then compared with those of touch preparation. RESULTS: Twenty-one cases were positive for leishmaniasis. In 18 cases, the organisms were seen both in the biopsy and in the touch preparation. In three cases, the organisms were only identified on touch preparation. CONCLUSIONS: A touch preparation improves the sensitivity of the diagnosis of cutaneous leishmaniasis without incurring additional cost to the laboratory.

Adolescent↗
PubMed1995

Accessibility and dynamics of Cys residues in Bacteriophage IKe and M13 major coat protein mutants.

The filamentous bacteriophage major coat protein occurs as a membrane-spanning assembly intermediate prior to incorporation into the lipid-free virion. To gain insight into how this small, multifunctional protein is able to be stably incorporated into both of these distinct environments, the reactive sulfhydryl group of IKe and M13 coat protein Cys mutants was exploited to probe the mobility and environment of this residue at several loci within the hydrophobic domain of these proteins. IKe mutants P30C, G39C, and G39C-V36A and M13 mutant Y24C-V31A, each previously obtained from randomized mutagenesis, were characterized in the intact virion, the intermediate spheroidal S-form, and in membrane-mimetic sodium dodecyl sulfate (SDS) micelles. The accessibility of the Cys sulfhydryl in the virion was examined by reaction with [14C]iodoacetamide (14C-IAN) and other alkylating agents. The IKe mutants G39C and G39C-V36A were found to be the most reactive with 14C-IAN and thus the most accessible, although this accessibility was subject to strict steric constraints since only the smallest sulfhydryl-specific alkylating agents were able to modify the Cys39 locus. The spin probe proxyliodoacetamide (PIAN) was used to characterize Cys side chain mobility by electron paramagnetic resonance (EPR) spectroscopy. The M13 mutant Y24C-V31A Cys side chain in the phage was observed to be the most mobile, with slightly less mobility for IKe mutant P30C and considerably less for G39C mutants. The SDS micelle-bound forms of the Cys mutants all exhibited enhanced side chain mobility compared to the virion form, with the extent of mobility dependent upon the specific location of the Cys residue. EPR and fluorescence quenching results show that the Cys side chain in the Y24C-V31A S-form is largely immobilized and inaccessible in comparison to the virion and micelle-solubilized forms. The overall results are interpreted in terms of the structural changes accompanying disassembly and insertion of the coat protein into the Escherichia coli inner membrane.

Alkylation↗
PubMed1995

Packing of coat protein amphipathic and transmembrane helices in filamentous bacteriophage M13: role of small residues in protein oligomerization.

Filamentous bacteriophage M13, an important cloning and phage display vector, is encapsulated by ca 2700 copies of its 50-residue major coat protein (gene 8). This protein occurs as a membrane protein while stably inserted into its E. coli host inner membrane, and as a coat protein upon assembly and packing onto phage DNA in the lipid-free virion. To examine the specific protein-protein interactions underlying these processes, we used a combination of randomized and saturation mutagenesis of the entire gene 8 to assess the susceptibility of each position to mutation. In the resulting library of ca 100 viable M13 mutants, "small" residues (Ala,Gly,Ser), which constitute the non-polar face of the N-terminal amphipathic helical segment, and a face of the hydrophobic (effective transmembrane) helical segment, were found to be highly conserved. These results support a model in which coat protein packing is stabilized by the presence within each protein subunit of two "oligomerization segments", i.e. specific helical regions with faces rich in small residues which function to promote the close approach of alpha-helices.

Amino Acid Sequence↗
PubMed1995

An engineered disulfide bridge in the transmembrane region of phage M13 coat protein stabilizes the alpha-helical dimer.

A single Cys-residue (Cys24) was introduced into the 50-amino acid major coat protein of M13 bacteriophage as part of a two-site substitution (Y24C-V31A) within the effective transmembrane (TM) segment (Tyr21 to Ile39) of the coat protein. Mutant Y24C-V31A was able to complete the phage life cycle and was shown to contain free sulfhydryls in the intact virus, as evidenced by susceptibility of Y24C-V31A phage to alkylation by Cys-specific 14C-iodoacetamide (14C-IAN). In contrast, the protein solubilized in deoxycholate micelles was resistant to 14C-IAN modification and was virtually inert to a transition from a characteristic alpha-helical oligomeric state to an aggregated beta-sheet structure relative to WT and V31A coat proteins, as shown by circular dichroism spectroscopy and SDS-PAGE. Reduction of mainly dimeric Y24C-V31A protein using beta-mercaptoethanol (beta-ME) generated monomeric species and resulted in a loss of helical thermostability. The overall results indicated that solubilization of Y24C-V31A coat protein into micelles resulted in formation of thermostable disulfide-bridged helical dimers. The disulfide bridge is deduced to be positioned along the stripe of residues involved in hydrophobic packing of TM parallel helical dimers.

Amino Acid Sequence↗
PubMed1995

Prevalence of diabetes and hypertension in a rural population of Bangladesh.

OBJECTIVE--To determine the prevalence of non-insulin-dependent diabetes mellitus (NIDDM), impaired glucose tolerance (IGT), and hypertension in a rural community of Bangladesh. RESEARCH DESIGN AND METHODS--A cluster sampling of 1,005 subjects > 15 years of age in the rural community of Dohar was investigated. Capillary blood glucose of fasting and 2 h after 75 g oral glucose (2hBG) were estimated. World Health Organization criteria were used for diagnosis of NIDDM and IGT. Blood pressure, height, and weight were also measured. RESULTS--The crude prevalence of NIDDM was 2.1% (men 3.1, women 1.3%) and IGT was 13.3% (men 14.4, women 12.4%). Age-adjusted (30-64 years of age) prevalence was 2.23% (95% confidence interval [CI] 1.01-3.45) for NIDDM and 15.67% (95% CI 12.59-18.75) for IGT. Prevalence of hypertension with systolic blood pressure (sBP) > or = 140 mmHg was 10.5% and with diastolic blood pressure (dBP) > 90 mmHg was 9.0%. Increased age was the risk factor for NIDDM, IGT, and hypertension; whereas increased BMI showed inconsistent association with them. Relative risk for sBP with higher BMI ( < 22.0 vs. > or = 22.1) was 1.94 with CI 1.55-2.43 and for dBP it was 2.2 with CI 1.40-3.46. Correlation of sBP was significant with age, BMI, and 2hBG. Similar correlation was also observed with dBP. CONCLUSIONS--High prevalences of NIDDM, IGT, and hypertension were observed among rural subjects. Increased age was shown to be an important risk factor for all these disorders, whereas BMI-associated risk was significant with NIDDM and hypertension but not with IGT.

Adolescent↗