Stochastic free energy transduction

Theoretical free-energy coupling systems in which the free energy coupling intermediate (e.g., the proton) occurs only in small numbers of molecules per coupling unit are shown to exhibit a number of peculiar properties: (i) the reactions involving the intermediates do not follow conventional kinetic (or nonequilibrium thermodynamic) rate laws in terms of the average concentration or chemical potential of the intermediate, (ii) the variation of the output reaction rate with the average intermediate concentration (or apparent chemical potential) is not unequivocal but depends on whether the input reaction or the leak is varied to alter that concentration, and (iii) when the apparent free energy contained in the average concentration of the intermediate is compared with the average free energy recovered in the output reaction, apparent violations of the second law of thermodynamics can occur. These phenomena are reminiscent of experimental observations in proton-linked free-energy transducing systems that suggest a more direct coupling between electron transfer chains and H+-ATPases than only through a bulk proton gradient, delta muH. Consequently, the chemiosmotic coupling theory can account for those observations if it limits the number of free energy coupling protons per chemiosmotic coupling unit to small values.

Iron-montmorillonite: a spectral analog of Martian soil

Light absorption and reflectance by smectite clays containing various adsorbed ions were measured in the UV, VIS, and NIR ranges and compared to Martian dust and surface soil spectra. Structural iron in the octahedral sheet of smectites is responsible for a characteristic absorption feature in the UV at 240-260 nm, resulting from an O2 --> Fe3+ charge transfer that is similar to one observed in the Martian spectrum. Adsorbed iron affects, via crystal field absorptions, the reflectance of montmorillonite in the VIS and NIR (to 1.3 micrometers), causing stronger absorption and higher opacity in the wavelength range 0.4-0.6 micrometer, without developing any specific pronounced absorption feature. In both general appearance and presence of, or lack of, spectral features, the iron-montmorillonite reflectance spectra in the VIS and NIR are similar to the Martian spectra. At present, however, spectral similarity cannot be used as the sole criterion for constraining Martian mineralogy since several other minerals, other than Fe-smectites, show sufficient similitude to the Martian spectra; other properties have to be explored and combined to obtain a definitive identification.

Phase changes at the end of a microtubule with a GTP cap

Examination of Monte Carlo kinetic simulations, based on a realistic set of microscopic rate constants that apply to the end of a microtubule with a GTP cap, suggests that the end of a microtubule alternates between two quasimacroscopic phases. In one phase, the microtubule end has a GTP cap that fluctuates in size; in the other phase, the GTP cap has been lost. These repeated phase changes take place at any given tubulin concentration in a wide range of concentrations. While in the first phase, the microtubule grows slowly; while in the second phase, it shortens rapidly and may disappear completely. These results are closely related to the recent experimental work of Mitchison and Kirschner [Mitchison, T. & Kirschner, M.W. (1984) Nature (London), in press].

Interference of GTP hydrolysis in the mechanism of microtubule assembly: an experimental study

This paper reports an experimental study of the interference of GTP hydrolysis in the mechanism of microtubule assembly, following the model and theory previously published [Hill, T. L. & Carlier, M.-F. (1983) Proc. Natl. Acad. Sci. USA 80, 7234-7238]. Results from dilution experiments show that microtubules depolymerize faster below the critical concentration than expected with a reversible polymerization model. The experimental plot of flux versus tubulin concentration exhibits a slope discontinuity at the critical concentration, in agreement with the theory. Theoretical points calculated by the Monte Carlo method can be fitted qualitatively to the data. A consequence of this peculiar dynamic behavior of microtubules is that the ratio of tubulin dissociation and association rate constants measured, respectively, below and above the critical concentration does not yield the true value of the critical concentration. It is emphasized that the presence of GTP at microtubule ends is necessary to maintain the stability of the polymer.

Use of Monte Carlo calculations in the study of microtubule subunit kinetics

GTP-tubulin forms a cap on microtubule ends during aggregation. The bulk of the microtubule is GDP-tubulin. This complicates the usual simple kinetic theory of subunit exchange at microtubule ends to such an extent that Monte Carlo calculations are needed to handle the complications, except in special cases. The Monte Carlo method is introduced here, for this problem, and illustrated with steady-state and transient examples. Monte Carlo transients are needed to simulate dilution experiments. Preliminary results (with M. F. Carlier) have been obtained applying these theoretical procedures to experimental data.

Acute infectious torticollis

Acute torticollis has occurred in epidemics in eastern and northern China since 1975, but has not yet been reported in other countries. A general account of this disease, including its terminology, epidemiology, clinical features, laboratory findings, diagnosis, differential diagnosis, and treatment, is presented. The etiology, though highly suggestive of a virus infection, remains to be determined.

Immunosuppression in plasmacytoma: induction of suppressor cells

Immunosuppression in BALB/c mice with plasmacytomas (PC) is, at least in part, due to increased suppressor activity of splenic adherent cells. A PC subcellular fraction rich in intracisternal A particles, previously shown to have an immunosuppressive effect in normal BALB/c mice, is now shown to exert this effect through the development of splenic suppressor cells. The splenic suppressor cells induced with this subcellular fraction were found to be radioresistant adherent macrophages that suppressed the immune response of normal splenic cells to sheep erythrocytes through a diffusible factor, but did not influence their polyclonal IgM secretion ater LPS stimulation in vitro. These characteristics are identical to those of the naturally occurring suppressor macrophages in mice with PC. Whether the induction of suppressor cells is due to the intracisternal A particles themselves or a factor co-purified with them, remains to be established.

The mode of action of splenic suppressor cells in murine plasmacytoma

Polyclonal IgM secretion after LPS stimulation and antigen-dependent antibody production by splenic cells of BALB/c mice with plasmacytoma were found to be markedly reduced. The former occurred without mediation of suppressor cells, probably as the result of a quantitative reduction of mitogen-responsive B lymphocytes. On the other hand, the reduction in the generation of antibodies to sheep erythrocytes in vitro, as determined by the number of PFC, was found to be dependent on the activity of suppressor cells which had the functional characteristics of macrophages. The targets of these suppressor cells were found to be normal macrophages, B and T lymphocytes which were all independently suppressed. A diffusible factor appeared to be responsible for the suppressive effect of the suppressor macrophages interfering mainly with those early stages of the immune response involving antigen-triggered B cell activation.