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Biomedical subjects

M K Cheung

Publications and source records attributed to M K Cheung.

9 recordsLinked to original sources
PubMed1991

Affinity labeling of the membrane protein-binding component of human polymorphonuclear leukocyte receptors for leukotriene B4.

A radiolabeled N-(3-aminopropyl)-leukotriene B4 amide ([3H]LTB4-APA) analog of the potent leukocyte chemotactic factor leukotriene B4 (LTB4) binds to receptors for LTB4 in plasma membrane-enriched preparations from human blood polymorphonuclear leukocytes (PMNL) and intact PMNL with respective mean dissociation constants of 2.3 nM and 69 nM at 4 degrees C. The [3H]LTB4-APA bound to plasma membrane-enriched preparations from PMNL was covalently cross-linked to membrane proteins with disuccinimidyl suberate. Solubilization and resolution by SDS-PAGE of proteins from [3H]LTB4-APA-labeled PMNL membranes revealed predominant labeling of a 60-kDa protein. Labeling of the PMNL membrane protein was inhibited by LTB4 and its analogs at concentrations similar to those inhibiting the binding of [3H]LTB4 to its receptor, with an identical rank order of potency of LTB4 greater than 20-hydroxy-LTB4 greater than LTB4-APA = 5(S),12(R)-dihydroxy-eicosa-14-cis-6,8,10-trans-tetraenoic acid much greater than LTD4 = LTC4. GTP suppressed the labeling of the 60-kDa PMNL membrane protein to an extent consistent with the decrease in receptor affinity for LTB4 induced by GTP. The stereospecificity of the affinity cross-linking reaction and the regulation by GTP support the identification of an approximately 60-kDa protein as the binding component of the PMNL receptor for LTB4.

Affinity Labels↗
PubMed1988

Decrease in light diffraction intensity of contracting muscle fibres.

Single fibres from the semitendinosus muscle of frog were illuminated normally with a He-Ne laser. The intensity transient and fine structure pattern of light diffracted from the fibre undergoing isometric twitches were measured. During fibre shortening, the intensity decreased rapidly and the fine structure pattern preserved its shape and moved swiftly away from the undiffracted laser beam. The fine structure patterns of the contracting and resting fibre were nearly identical. The ratio of intensities of the contracting and resting fibre of the same sarcomere length was determined as a function of the time elapsed after fibre stimulation. The time-resolved intensity ratio increased with sarcomere length and became unity when sarcomere length was between 3.5 micron and 3.7 micron. A diffraction theory based on the sarcomere unit was developed. It contained a parameter describing the strength of filament interaction. The comparison between the theory and data shows that the initial intensity drop during contraction is primarily due to filament interactions. At a later stage of contraction, sarcomere disorder becomes the major component causing the intensity to decrease. Diffraction models which use the Debye-Waller formalism to explain the intensity decrease are discussed. The sarcomere-unit diffraction model is applied to previously reported intensity measurements from active fibres.

Animals↗
PubMed1987

Polarization changes in light diffracted from contracting muscle fibers.

Single fibers were isolated from the semitendinosus muscle of frog and illuminated with an He-Ne laser. The polarization of the laser beam was varied by a photoelastic modulator. The time course of the degree of polarization of light diffracted from the muscle fiber during an isometric contraction was measured directly with a time resolution of 1 ms. Tension, sarcomere length, and diffraction intensity were also measured. During the contraction cycle, the degree of polarization of the active fiber exhibited a biphasic variation relative to that of the resting fiber. Analysis identifies the movement of heavy meromyosin toward actin and the rise in myoplasmic calcium ion concentration as the main contributors to the polarization transient of active fibers. A quantitative theory describing the polarized diffraction from muscle fibers is formulated. There is good agreement between the theory and measurements.

Actins↗
PubMed1985

Experimental methyl mercury neurotoxicity: locus of mercurial inhibition of brain protein synthesis in vivo and in vitro.

Brain cell-free protein synthesis is inhibited by methyl mercury chloride (MeHg) following in vivo or in vitro administration. In this report, we have identified the locus of mercurial inhibition of translation. Intraperitoneal injection of MeHg (40 nmol/g body wt) induced variable inhibition of amino acid incorporation into the post-mitochondrial supernatant (PMS) harvested from the brain of young (10-20-day-old) rats. No mercurial-induced disaggregation of brain polyribosomes nor change in the proportion of 80S monoribosomes was detected on sucrose density gradients. No difference in total RNA was found in the PMS. Initiation complex formation was stimulated by MeHg, as detected by radiolabelled methionine binding to 80S monoribosomes following continuous sucrose density gradient centrifugation. After micrococcal nuclease digestion of endogenous mRNA, both in vivo and in vitro MeHg inhibited polyuridylic acid-directed incorporation of [3H]phenylalanine. However, the in vivo inhibition was no longer observed when [3H]phenylalanyl-tRNAPhe replaced free [3H]phenylalanine in the incorporation assay. The formation of peptidyl[3H]puromycin revealed no difference from controls. There was significant mercurial inhibition of phenylalanyl-tRNA Phe synthetase activity in pH 5 enzyme fractions derived from brain PMS of MeHg-poisoned rats. These experiments revealed that the apparent MeHg inhibition of brain translation in vivo and in vitro is due primarily to perturbation in the aminoacylation of tRNA and is not associated with defective initiation, elongation, or ribosomal function.

Animals↗
PubMed1985

Cross-reactive idiotypes in immunoglobulin A-deficient sera.

Mice of inbred strains immunized with simple antigens can produce antibodies that share similar V regions, which result in serologic similarities called cross-reactive idiotypes (CRI). In this study, we considered the possibility that IgA-deficient humans, who are continuously immunized via the intestinal tract by dietary protein, might also produce antibodies sharing CRI. For this, anti-casein antibodies were isolated from the blood of 16 adult IgA-deficient donors (4 Finns and 12 North Americans) and an autologous anti-anti-casein from the blood of one of the Finnish donors. In addition, a heterologous anti-anti-casein was raised to the casein-anti-casein immune complexes of this donor. Comparing the activities of the two anti-idiotypes, it was found that both bind anti-casein in the region of the antigen binding site, but that each binds additional determinants not located within this region, with the heterologous reagent having more affinity for these latter determinants than the autologous anti-idiotype. Using both reagents in enzyme-linked immunosorbent assay inhibition assays, extensive cross-reactivities between anti-caseins were demonstrated. Using the autologous anti-idiotype, 5 of 16 anti-caseins were found to share CRI, and with the heterologous reagent 12 of 16 shared CRI. In both assays, the anti-caseins of Finnish donors displayed more cross-reactivity than those derived from Northern American donors. These studies show that specific, commonly shared CRI can be identified in this human system in which antibodies are raised as a result of natural immunization across the gastrointestinal mucosa.

Adult↗