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Biomedical subjects

M H Koch

Publications and source records attributed to M H Koch.

At least 109 records · Page 6Linked to original sources
PubMed1986

Intensity increases of actin layer-lines on activation of the Limulus muscle.

Small angle x-ray diffraction patterns were recorded from isometrically contracting Limulus (horseshoe crab) telson levator muscle using a multiwire proportional-area detector on the storage ring DORIS. In the pattern a substantial increase in intensity is observed on the thin-filament-associated layer-line at 1/38 nm-1 (the first actin layer-line) with a maximum increase at a radial spacing of R = 0.07 nm-1 but there is a much smaller change in the intensity of the 5.9-nm layer-line, which also arises from the thin filament structure. The results suggest that during contraction the myosin heads, presumably being attached to the thin filaments, are arranged along the long-stranded helical tracks of the thin filaments but that the spatial relationship between the heads and the actin monomers varies. Intensity increases have also been observed (Maéda et al., manuscript in preparation) in the part of the patterns from frog muscle and barnacle muscle, which are attributable to the first actin layer-line. It is thus likely that the intensity increase of the first actin layer-line on the Limulus pattern is associated not with structural features which are special to Limulus muscle, but with the tension generating processes that are shared by muscles in general.

Actin Cytoskeleton↗
PubMed1986

Differences in the fibril structure of corneal and tendon collagen. An electron microscopy and X-ray diffraction investigation.

A detailed analysis of the D-period and axial electron density distribution of cornea and tendon collagen was carried out by means of X-ray diffraction and electron microscopy. Ultrastructural observations were made on replicas of freeze fractured and deep-etched specimens. Synchrotron radiation was used to obtain high resolution small angle X-ray diffraction patterns. The data provide evidence that D-period and intraperiod distances in cornea are shorter than in tendon collagen fibrils. The observed different banding observed is interpreted on the basis of the different morphological arrangement of the microfibrils in the two tissues: "helicoidal" in cornea and "straight" in tendon microfibrils.

Animals↗
PubMed1985

Stress-induced molecular rearrangement in tendon collagen.

Tension-induced molecular rearrangements in wet native fibres of rat-tail tendons and human finger flexor tendons are registered with the help of time-resolved diffraction spectra using synchrotron radiation. The tension-induced increase of the 67 nm D period is combined with changes in the intensities of some orders of the meridional small angle reflection. Both effects are reversible when unloading the fibre, but are preserved when the load is held constant until the fibre tears. The increase in the D period is partly due to a sliding of the triple helices relative to each other and partly due to a stretching of the triple helices themselves. The sliding of the triple helices results in an alteration of the D stagger, leading to a change in the length of the gap and overlap regions, and to a stretching of the cross-linked telopeptides. This interpretation is supported by comparison with the relative intensities derived from a model with varying length of gap and overlap regions, as well as by comparison with model calculations that include the telopeptides.

Adult↗
PubMed1985

Time-resolved X-ray scattering study of actin polymerization from profilactin.

The polymerization of actin in solutions of purified calf spleen actin or profilactin (1-10 mg . ml-1) was followed by synchrotron radiation X-ray solution scattering. At the concentration used, polymerization of actin from profilactin or actin occurs without any lag phase. It is shown by a combination of solution scattering, model calculations and electron microscopy that contrary to the conclusions from previous viscometry studies, filaments form without any lag phase in profilactin solution but aggregate in bundles or networks. This phenomenon is independent of the method used to induce polymerization: slow temperature increase, temperature jump in the presence of polymerizing salts or fast mixing with salt. This aggregation explains the lower final viscosity levels, as compared to actin solutions, observed during the polymerization of actin from profilactin.

Actins↗
PubMed1985

A low-angle X-ray diffraction analysis of osteonic inorganic phase using synchrotron radiation.

Using synchrotron radiation the low-angle X-ray diffraction method has been applied to single osteon samples to yield new data on the texture of the inorganic bone fraction. Two sample types--cylindrically shaped osteonic samples and osteonic radial hemisections--were prepared from longitudinal and alternate osteons at both the initial and final stages of calcification. The results indicate that the diffraction pattern is due to the inorganic phase, which reveals the same axial periodicity as native collagen fibrils and fits into the main band. No change is appreciable as osteons pass from the initial to the final stage of calcification. This means that when crystallites covering much more than a collagen axial period are observed under the electron microscope, they do not appreciably affect the calcified banding of collagen fibrils. The osteonic axis corresponds to the main direction of collagen orientation both in longitudinal and alternate osteons. The degree of orientation, however, is lower in alternate osteons than in longitudinal ones, where only few thin, incomplete transversal lamellae are found.

Adult↗
PubMed1985

Synchrotron radiation x-ray scattering in the early stages of in vitro collagen fibril formation.

The time course of in vitro collagen fibril formation was monitored by synchrotron radiation x-ray scattering. Collagen polymerization was induced by a temperature jump from 4 degrees C to 32 degrees C and the solution scattering pattern was recorded continuously with a time resolution of a few seconds. The scattered intensity increased as soon as the final temperature was attained, without discernible lag phase, when the collagen concentration was about 0.56 mg/ml, whereas turbidimetric measurements revealed a lag phase of ca. 2.5 min at 1.05 mg/ml. A direct correlation was found between the temperature and both the rate of formation and the total amount of early aggregates. The formation of these aggregates was only partially reversed by lowering the temperature, except when the collagen had been prevented from forming Schiff base-mediated intermolecular crosslinks by sodium borohydride reduction. In this case, formation of aggregates was completely reversible. We conclude that the aggregates that appear to correspond to the subfibrils proposed on the basis of independent methods are formed and simultaneously crosslinked in the early phases of in vitro collagen self-assembly.

Animals↗
PubMed1984

Chromatin superstructure: synchrotron radiation X-ray scattering study on solutions and gels.

X-ray small angle scattering patterns of solutions and gels of native chicken erythrocyte chromatin and chromatin depleted of H5 histones have been measured under several ionic conditions using synchrotron radiation. Features of the patterns are interpreted as resulting from a superstructure with an outer diameter of about 300A which is already present in uncondensed nucleofilaments. This superstructure which is shown to be maintained by the H5 histones also explains the rapid condensation of the nucleofilaments in higher ionic strengths.

Animals↗
PubMed1983

Changes in the X-ray reflections from contracting muscle during rapid mechanical transients and their structural implications.

During normal contractions of vertebrate striated muscle, it is believed that the cross-bridges which produce the sliding force undergo asynchronous cyclical changes in their structure. Thus, an X-ray diffraction diagram from a muscle under these conditions will give structural information averaged over the whole range of cross-bridge states. Such diagrams show characteristic and informative differences from those given by relaxed muscle, but can give little information about changes in the configuration of the cross-bridges at different stages of their working stroke. However, it is possible to effect a partial synchronization of these changes by applying very rapid changes in length, completed in less than one millisecond to an otherwise isometrically contracting muscle. If the amplitude of these length changes is comparable to the length of the cross-bridge stroke (say 100 A per half-sarcomere), then it should bring about a transient but significant redistribution of cross-bridge states, which would show up in the X-ray diagram. We have made use of synchrotron radiation as a high intensity X-ray source in order to record such patterns with the necessary time resolution (1 ms or less) and have found major changes in the intensity of the 143 A meridional reflection accompanying the rapid length changes of the muscle. These changes appear to arise from specific configurational changes in the cross-bridges during the working stroke. A model is suggested in which the 143 A meridional intensity in a contracting muscle arises mainly from attached cross-bridges and is generated by the part of the myosin head near the S1-S2 junction. During normal contraction, cross-bridges go through their structural cycle asynchronously with each other, since they start at different times, but if the S2 changes in length rather little, then the configurational changes in the myosin heads are synchronized with the actin filament movement in such a way that the S1-S2 junction remains relatively fixed in its axial position. In a quick release, it is suggested that bringing many S1 heads simultaneously to the end of their working strokes on actin disrupts the 143 A axial repeat of their distal ends near S2, and brings about the large decrease of the 143 A meridional reflection. This model therefore involves a large change in the position of part of the myosin head structure relative to actin during the working stroke of the cross-bridge.

Animals↗
PubMed1983

Stopped-flow solution scattering using synchrotron radiation: apparatus, data collection and data analysis.

We have constructed an experimental system, under remote control, for stopped-flow X-ray scattering using synchrotron radiation. It has been used, in conjunction with an annular detector and its associated electronics, to obtain good scattering curves, with time-slices as short as 200 ms, in a new study of the dissociation of the enzyme complex aspartate transcarbamylase. The data have been analysed by new statistical methods, and they agree well with the results from parallel chemical quench experiments. For studying dissociation reactions, stopped-flow X-ray scattering is a quite practical method, which need not use very much more material than conventional stopped-flow experiments.

Aspartate Carbamoyltransferase↗
PubMed1983

Time-resolved small-angle X-ray scattering experiment on association of isolated alpha- and beta-chains of human hemoglobin.

The time course of the recombination of hemoglobin from isolated alpha- and beta-chains (human) was studied by following the change in small-angle scattering from a reaction mixture using synchrotron radiation. The scattering patterns were recorded successively within 51 s at intervals of 200 ms. The results suggest that when 2 mM solutions of CO-liganded alpha- and beta-chains are mixed in stoichiometric amounts at pH 7.4, the tetramerization process is a pseudo-first-order reaction with a rate constant of 0.059 s-1, i.e., a half-time of approx. 12 s.

Hemoglobins↗
PubMed1982

Zinc-sulfur bonds of aspartate transcarbamylase studied by x-ray absorption spectroscopy.

X-ray absorption spectra have been recorded for aspartate transcarbamylase [unligated and ligated with the transition-state analogue N-(phosphonoacetyl)-L-aspartate] and for the model compound zinc dimethyldithiocarbamate. The spectra confirm that, in the enzyme, the zinc atom is ligated to four sulfur atoms, with a mean distance of 2.34 +/- 0.03 A. A spread in bond lengths of 0.1 +/- 0.03 A is possible, due to thermal and/or static disorder. No significant difference was found between the spectra of the ligated and unligated enzymes.

Aspartate Carbamoyltransferase↗
PubMed1982

X-ray diffraction evidence of collagen molecular packing and cross-linking in fibrils of rat tendon observed by synchrotron radiation.

X-ray diffraction patterns of fibres from 90 day (mature) rat-tail tendons were investigated using synchrotron radiation. The specimens were kept isometric at their corresponding in vitro rest length, and effects of pH and ionic strength were studied during short X-ray exposures. The results indicate that fibrils, equilibrated in physiological Ringer prior to exposure, have segregated lateral regions of well ordered collagen molecular packing. Lowering the ionic strength or the pH to 4.0 causes an order/disorder transition during which the fibril crystallinity decreases. At pH 3.5 a dramatic increase in the lateral swelling was observed. This effect was absent for fibres pretreated with sodium borohydride. The results are interpreted on the basis of cross-linking phenomena whereby the aldimine cross-link seems to be a controlling component of the lateral packing arrangement of collagen molecules.

Animals↗
PubMed1981

Millisecond time-resolved changes in x-ray reflections from contracting muscle during rapid mechanical transients, recorded using synchrotron radiation.

Low-angle x-ray diffraction diagrams have been recorded from frog sartorius muscles by using synchrotron radiation as a high-intensity x-ray source. This has enabled changes in some of the principal reflections of interest to be followed with a time resolution of 1 ms, during small but very rapid length changes imposed on a contracting muscle. The 143-A meridional reflection, which is believed to arise from a repeating pattern of myosin cross-bridges along the length of the muscle, shows large changes in intensity in these circumstances. During both rapid releases and rapid stretches, by amounts that produce a translation of actin and myosin filaments past each other by about 100 A and that are completed in about a millisecond (i.e., before significant cross-bridge detachment would be expected), an almost synchronous decrease in 143-A intensity occurs, by 50% or more. This is followed, in the case of quick releases, by a rapid partial recovery of intensity lasting 5--6 ms (which may represent cross-bridge release and reattachment) and then by a more gradual return to the normal isometric value. Quick stretches show only the slower return of intensity. Immediately after the length change, the initial drop in 143-A intensity can be reversed if the release (or stretch) is reversed. These changes provide evidence of a more direct kind than has hitherto been available that the active sliding of actin filaments past myosin filaments during contraction is produced by longitudinal movement of attached cross-bridges.

Actins↗