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K Islam

Publications and source records attributed to K Islam.

71 records · Page 4Linked to original sources
PubMed1992

Epithelial cells in culture as a model for the intestinal transport of antimicrobial agents.

The bioavailabilities of orally administered drugs depend to a great extent on their capability of being transported across the intestinal mucosa. In an attempt to develop an in vitro model for studying the intestinal transport of drugs, we used an intestinal epithelial cell line (Caco 2) derived from a human colon adenocarcinoma. A renal epithelial cell line (MDCK) was also used to determine the tissue specificity of drug transport. These cell lines, which were grown on filters, form a monolayer of well-polarized cells coupled by tight junctions and can be used for transcellular transport experiments. We studied the transport of nine antimicrobial agents with different physicochemical and pharmacokinetic characteristics using these epithelial cell monolayers to determine whether this model could be predictive of oral bioavailability. The transepithelial passage was assayed from the apical (AP) to the basolateral (BL) side and in the opposite direction (BL to AP) in both cell lines. Radioactively labeled mannitol was used to monitor the intactness of the cell monolayer during drug passage. The results indicated that all antimicrobial agents tested tended to behave in vitro generally according to their known in vivo absorptive characteristics. In addition, the use of epithelia from different tissues enabled us to divide the drugs into four groups according to their behaviors and suggested the existence of different transport mechanisms. In particular, two antibiotics, gentamicin and teicoplanin, showed no passage in either direction or cell line, in accordance with their very poor in vivo absorbances after oral administration. In contrast, rifapentine, rifampin, and nalidixic acid passed very efficiently at similar rates in both directions and cell lines in a concentration-dependent, nonsaturable manner, which is suggestive of passive diffusion down a concentration gradient. Of the remaining drugs, isoniazid and novobiocin sodium showed some differences in passage between the two cell lines and, given their ionized state at the pH that was used, may use the paracellular route. Finally, trimethoprim and D-cycloserine exhibited differences in passage both with respect to polarity and cell line; in particular, trimethoprim had a faster rate of passage only in Caco 2 cells and in the BL to AP direction, while D-cycloserine was exclusively transported by Caco 2 cells in the AP to BL direction. In both cases it is possible that active transport mechanisms are involved.

Adenocarcinoma↗
PubMed1989

A gene for the suppression of anchorage independence is located in rat chromosome 5 bands q22-23, and the rat alpha-interferon locus maps at the same region.

Cell hybrids between malignant mouse hepatoma cells and normal rat fibroblasts with approximately one set of chromosomes from each parent exhibited remarkable karyotypic stability. Most chromosomes of both parents were retained even after prolonged culture in vitro. Normally, such hybrids showed suppression of the transformed phenotype and formed no colonies in soft agar. However, two hybrids, BS140 and BS181, formed a few colonies in soft agar when many cells were seeded, and also occasional foci of cells were detected piling up in monolayer cell cultures. We isolated soft agar colonies (a-subclones) and sub-clones from foci (h-subclones) of both hybrids, and, as a control, subclones of cells from random areas without foci of one hybrid (BS181 p-subclones). When tested for soft agar growth, cells from the a- and h-subclones of both BS140 and BS181 formed colonies at frequencies comparable to the malignant mouse hepatoma parent, whereas the control cells of the BS181 p-subclones (like the normal rat parental cells) yielded no soft agar colonies. All the cell lines were subjected to detailed karyotype analysis in G-banding, which resulted in the finding that cells from the original BS140 hybrid contained at least one copy of each rat chromosome, whereas BS140 a- and h-subclones had lost both copies of rat chromosome 5. Similarly, the original BS181 hybrid contained at least one copy of each rat chromosome, whereas BS181 a- and h-subclones displayed a deletion of the segment q22-23 of rat chromosome 5. In contrast, the control BS181 p-subclones contained one or two copies of non-deleted rat chromosome 5. The conclusion is that a gene for the suppression of anchorage independence is located in the segment 5q22-23. We propose to call this gene SAI1 (for suppression of anchorage independence). Using Southern blotting, we tested whether any of several gene probes, known to correspond to DNA sequences in rat chromosome 5, were homologous to sequences in the deletion. Only one probe, corresponding to the active alpha1-interferon gene, was shown to be located within the deletion. Hence, the SAI1 gene is closely linked to the alpha 1-interferon gene, and might be identical to this locus.

Animals↗
PubMed1985

Microtubules and nucleoside diphosphate kinase. Nucleoside diphosphate kinase binds to co-purifying contaminants rather than to microtubule proteins.

Nucleoside diphosphate (NDP) kinase has been postulated to generate GTP from the GDP bound to tubulin. The purified chick brain enzyme was studied with respect to its kinetic parameters, and the protein-protein interactions between the NDP kinase and tubulin were examined. No specific interaction is observed between the enzyme and assembled microtubules, tubulin dimers, or tubulin-microtubule-associated protein (MAP) oligomers under a variety of nucleotide conditions. The apparent association is demonstrated to result from NDP kinase binding to a co-purifying contaminant. The absence of detectable NDP kinase-tubulin interactions indicates that NDP kinase does not directly charge up tubulin-GDP.

Animals↗
PubMed1985

Microtubules and nucleoside diphosphate kinase. Comparison of kinetics of GTP- and CTP-induced assembly.

The kinetics of assembly of MAP2-tubulin microtubule protein were examined as a function of the GTP concentration in order to test the hypothesis that CTP-induced assembly results from the generation of GTP by nucleoside diphosphate kinase. These studies show that (a) there is no assembly below a minimum GTP concentration and that this represents a nucleation requirement, (b) the rate of elongation is inconsistent with a single assembly-species, and (c) the elongation rate increases markedly as the GTP concentration is raised, although GTP is not absolutely required for elongation. These assembly kinetics have been compared with those with increasing CTP concentrations, by using microtubule protein containing a very low nucleoside diphosphate kinase activity of known substrate specificity. Neither nucleation nor the observed rates of elongation can be attributed to the formation of GTP, either (a) in terms of the generation of free GTP and subsequent binding to tubulin or (b) by the direct charging of GDP bound to the tubulin exchangeable site. The results show that nucleoside diphosphate kinase is not required for CTP-induced microtubule assembly, and suggest that CTP directly effects microtubule assembly.

Animals↗
PubMed1984

Assembly of microtubules with ATP: evidence that only a fraction of the protein is assembly-competent.

Chick brain microtubule protein can be assembled in vitro with ATP, although the extent of assembly is less than that with GTP. The ATP-induced assembly is not the result of generation of GTP by the co-purifying nucleoside diphosphate kinase. Neither an observed increase in the critical concentration nor the phosphorylation of MAP2 can account for the decreased extent of assembly. However, whereas microtubules are formed with both ATP and GTP, incubation with ATP yields additional filaments and polymorphic aggregates. The results demonstrate that of the total protein which can be assembled into microtubules by GTP, about 25-35% is assembled into other structural forms in the presence of ATP.

Adenosine Triphosphate↗
PubMed1984

Phosphorylation of the microtubule-associated protein MAP2 by GTP.

The chick brain microtubule-associated protein MAP2 can be phosphorylated in vitro to the extent of 12 mol/mol with GTP at the same sites as can be labelled by the cyclic AMP-independent protein kinase utilizing [gamma-32P]ATP as the phosphoryl donor. Consequently, the microtubule protein is chemically modified by the conditions usually employed for studies of microtubule assembly, so that the derived kinetic parameters may not relate to steady-state conditions.

Adenosine Triphosphate↗
PubMed1984

Direct incorporation of microtubule oligomers at high GTP concentrations.

Chick brain microtubule protein consists primarily of a mixture of MAP2:tubulin oligomers and dimeric tubulin. The assembly of this protein is described by a single pseudofirst-order reaction at 20 microM GTP, but by the summation of two pseudofirst-order reactions at 1 mM GTP. The protein contains two GTP-binding species, corresponding to the tubulin dimers and the oligomers, and conditions which alter the dimer: oligomer equilibrium, affect the kinetics of microtubule assembly. The results indicate that the oligomers are only direct assembly intermediates at high GTP concentrations.

Animals↗
PubMed1984

Stoichiometry of microtubule-associated protein (MAP2):tubulin and the localisation of the phosphorylation and cysteine residues along the MAP2 primary sequence.

The stoichiometry of the dimer between microtubule-associated protein 2 (MAP2) and tubulin has been determined by quantitative dodecylsulphate/polyacrylamide gel electrophoresis to be 1:12 mol X mol-1, a value equal to the number of phosphorylation sites that can be labelled in vitro. The distribution of these sites along the MAP2 primary sequence has been determined by cleaving pre-labelled MAP2 with either alpha-chymotrypsin or at the five cysteine residues with nitrothiocyanobenzoic acid. The phosphorylation sites lie in two clusters: ten within the known tubulin-binding domain at one end of the primary sequence, and a pair midway along the sequence. It is postulated that the tertiary structure of MAP2 is folded to bring all twelve sites into association with the twelve tubulin dimers.

Amino Acid Sequence↗
PubMed1984

The multiple phosphorylation of the microtubule-associated protein MAP2 controls the MAP2:tubulin interaction.

Pre-phosphorylation of the microtubule-associated protein MAP2 with the co-purifying cAMP-independent protein kinase (a) decrease the affinity of MAP2 for taxol-stabilised microtubules, (b) increases the dissociation rate constant for microtubule polymerisation, each of which is dependent upon the level of phosphorylation, but (c) has no effect on the association rate constant. Microtubule assembly has no effect on the kinetics of phosphorylation, whereas phosphorylation of pre-assembled microtubules causes their immediate depolymerisation at a rate which is proportional to the initial rate of phosphorylation. The results suggest that the modulated phosphorylation of MAP2 may regulate microtubule length in vivo.

Adenosine Triphosphate↗
PubMed1984

Purification of a soluble monoisozyme of nucleoside diphosphate kinase from chick brain: exploitation of ionic characteristics.

A procedure for the rapid purification of nucleoside diphosphate kinase, 24 h with a single operator, from the chick brain soluble fraction is described. The influence of the ionic conditions on the association-disassociation properties of the enzyme are exploited to obtain yields of 30% from the crude homogenate. The enzyme has been purified 500-fold with a maximal specific activity of 1500 mumol/min/mg at 25 degrees C (using thymidine diphosphate as the phosphate acceptor and ATP as the donor) and is demonstrated to be monoisozymic.

Ammonium Sulfate↗
PubMed1983

PHC in Bangladesh--too much to ask?

In Bangladesh where the number of those without land is increasing; 30% of the budget (national) goes for "maintaining law and order'; there is massive illiteracy; the common people and more specifically, women, are not involved in decision-making. It is unrealistic to think that PHC has a chance to survive or succeed. Should Government (with all good intentions) try to run a PHC while socio-economic and political factors remain unchanged, it (PHC) will not succeed. Its fate is either a collision, in which PHC will be the victim, or a compromise of some type. In most Third World countries, it is a compromised PHC that one sees. In spite of this, we still believe that community-oriented health care can help to bring about social, economic and political changes, provided such changes are accompanied by structural reform in the political economy of the country.

Bangladesh↗
PubMed1982

Characterisation of the chick brain high molecular weight multiply phosphorylated microtubule associated protein.

A high-molecular-weight microtubule-associated protein from chick brain, which has previously been shown to be phosphorylated in vitro to the extent of 10-12 mol X mol-1, is shown to have the same properties as reported for the microtubule-associated protein MAP2 from calf and porcine brain in terms of its molecular weight, sensitivity to trypsin and alpha-chymotrypsin, the molecular weights of the primary proteolytic degradation products, and the failure of certain of the latter components to bind to assembled microtubules. The primary 260,000-Mr tryptic product can be phosphorylated to at least 4 mol X mol-1, in contrast to an earlier report with calf brain MAP2 which noted a single phosphorylation.

Animals↗
PubMed1981

Nucleosidediphosphate kinase associates with rings but not with assembled microtubules.

Microtubules reassembled in vitro from chick brain contain significant nucleosidediphosphate (NDP) kinase activity (EC 2.7.4.6) although the specific activity decreases with successive cycles of reassembly. However, while the recovery of microtubule protein, as a function of initial protein concentration, exhibits a critical concentration below which there is no polymerisation, the recovery of NDP kinase activity is, at low initial protein concentrations, directly proportional to the initial protein content indicating that microtubule protein and the kinase activity are independently recovered. This was confirmed by pelleting the reassembled microtubules through a sucrose cushion: the specific activity of the pelleted microtubules was reduced by approximately 90%. By contrast, when cold-dissociated microtubule protein, which is predominantly in the form of rings, is fractionated on a Biogel A 15 m column the microtubule protein and NDP kinase activity coeluted in the void volume and the specific activity remained constant throughout the ring fraction. Similarly, when microtubules were dissociated in the presence of NDP kinase the enzyme bound to the generated rings. These results suggest that NDP kinase binds preferentially to the rings compared with the microtubules, and a model is proposed to account for the presence of this enzyme in pellets of microtubule protein.

Animals↗
PubMedDate not supplied

Solubilization and identification of essential functional groups of Candida albicans oxidosqualene cyclase.

The enzyme properties and location of essential functional groups of solubilized oxidosqualene cyclase of Candida albicans have been studied. We show that the C. albicans enzyme is much more heat-labile compared with Saccharomyces cerevisiae and rat liver cyclases, requires a histidyl residue for enzyme activity, contains an essential thiol residue either close to or in the active site and exhibits a carbocationic mechanism for catalysis, as the enzyme-bound substrate protects the enzyme from inactivation by a site-directed inactivator.

Animals↗