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Biomedical subjects

H C Cheung

Publications and source records attributed to H C Cheung.

99 records · Page 6Linked to original sources
PubMed1972

Computer simulation of fluorescence depolarization due to brownian motion.

A computer program has been written to simulate the Brownian motion of rigid fluorescent molecules. The time dependence of the fluorescence polarization anisotropy as generated by this simulation is in agreement with that predicted by the recent theoretical treatment of Belford, Belford, and Weber (Proc. Nat. Acad. Sci. USA (1972) 69, 1392-1393). The program thus serves as a verification of their equation. It is being generalized to cover the case of nonrigid molecules.

Journal Article↗
PubMedDate not supplied

A rotational offset model for two-stranded F-actin.

We propose the following "rotational offset" model for two independent strands of F-actin to account for the observation that it is possible, at times, for the crossover repeat to either alternate between long and short periods or remain constant (Bremer et al., 1991. J. Cell Biol. 115, 689-703). Rotational offset is the manifestation of the angular component of "lateral slipping" between the two long-pitch, right-handed strands comprising the actin filament. The present model is based on the premise that the longitudinal bond connecting the subunits along a single long-pitch strand is substantially stronger than the diagonal bond that connects interstrand subunits. We pose that, over fairly long stretches, the backbones of the two right-handed strands are individually close to being ideal helices, and that it is possible for the backbone of one strand to "roll across" the other. The rotational offset angle (epsilon 0) is the amount that one helical strand is angularly displaced relative to the position that otherwise would allow the two strands to be described as an ideal single genetic helix. Such an independent movement of the two strands is shown to shift the monomers that are involved in crossover points and produce the different patterns in crossover periods which have been observed from electron micrographs analysis. We specifically demonstrate that for a constant nonzero rotational offset the length of the crossover periods alternates, whereas for a constant offset of zero the crossover period remains constant. We also show that changes in the rotational offset angle along the filament can account for variable (random) crossover periods.

Actins↗
PubMedDate not supplied

Translocation of Thy-1 antigen and a fluorescent lipid probe during lymphoblastoid cell interaction with Mycoplasma hyorhinis.

The translocation of membrane-associated components during in vitro surface interactions between BW5147 murine T-lymphoblastoid cells and Mycoplasma hyorhinis was investigated. Thy-1 antigen (shown to be selectively associated with mycoplasmas following their detachment from infected BW5147 cells) was found by immunoferritin techniques to be localized at the surface of organisms colonizing lymphoblastoid cells. These results are consistent with a transfer of Thy-1 to membranes of mycoplasmas on the lymphoid cell surface. The exchange of membrane lipid-associated components was further investigated with the fluorescent lipid probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Fluorescence polarization of DPH was much less pronounced in uninfected BW5147 cells than in mycoplasmas--a result indicating markedly different probe environments. The increase of fluorescence intensity and the characteristic polarization in mycoplasmal supernatant fractions obtained after incubation of organisms with DPH-labeled BW5147 cells indicated translocation of DPH from lymphoid cells to mycoplasmas. Incorporation of the probe into mycoplasmas using supernatants from DPH-labeled BW5147 cells (incubated alone) was not demonstrated; this observation is consistent with, but does not prove, a contact-dependent process of exchange. Direct monitoring of the exchange of surface antigens and lipophilic probes may be useful in studying the interaction of hydrophobic surface glycoproteins with membranes and the role of membrane components in mycoplasma-host cell interactions.

Animals↗