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D D Thomas

Publications and source records attributed to D D Thomas.

At least 181 records · Page 10Linked to original sources
PubMed1982

Orientation of spin labels attached to cross-bridges in contracting muscle fibres.

Electron micrographs showing different cross-bridge orientations in different states of muscle fibres, and X-ray diffraction patterns indicating axial cross-bridge disorder in contracting muscle first suggested that force generation in the contracting muscle involved a change in orientation of the myosin heads that form cross-bridges between thick and thin filaments. This has been supported by subsequent work; the myosin molecule has the required flexibility for changes in orientation. The orientation of muscle tryptophans and of probes attached to the myosin heads of permeable muscle fibres depends on the state of the muscle. Recently, fluorescence polarization fluctuations and time-resolved X-ray diffraction patterns have suggested that cross-bridges of a contracting muscle can rotate. We have used electron paramagnetic resonance (EPR) spectroscopy to monitor the orientation of spin labels attached specifically to a reactive sulphydryl on the myosin heads in glycerinated rabbit psoas skeletal muscle. Previously, it has been shown that the paramagnetic probes are highly ordered in rigor muscle, with a nearly random angular distribution in relaxed muscle. We show here that during the generation of isometric tension, approximately 80% of the probes display a random angular distribution as in relaxed muscle while the remaining 20% are highly oriented at the same angle as found in rigor muscle. These findings indicate that a domain of the myosin head does not change orientation during the power stroke of the contractile interaction.

Actins↗
PubMed1982

Rotational dynamics of protein and boundary lipid in sarcoplasmic reticulum membrane.

We have used spin labels and electron paramagnetic resonance (EPR) to study the correlation between the rotational dynamics of protein and lipid in sarcoplasmic reticulum (SR) membranes. A short-chain maleimide spin label was used to monitor the submillisecond rotational mobility of the Ca-ATPase enzyme (using saturation transfer EPR); a free fatty acid spin label was used to monitor the submicrosecond rotational mobility of the bulk lipid hydrocarbon chains (using conventional EPR); and a fatty acid spin label derivative (long-chain maleimide) attached to the enzyme was used to monitor the mobility of hydrocarbon chains adjacent to the protein (i.e., boundary lipid). In the native SR membranes, the protein was highly mobile (effective correlation time 50 microseconds). The spectra of the hydrocarbon probes both contained at least two components. For the unattached probe, the major component indicated nearly as much mobility as in the absence of protein (effective rotational correlation time 3 ns), while a minor component, corresponding to 25-30% of the total signal, indicated strong immobilization (effective correlation time greater than or equal to 10 ns). For the attached hydrocarbon probe, the major component (approximately 70% of the total) was strongly immobilized, and the mobile component was less mobile than that of the unattached probe. When the lipid-to-protein ratio was reduced 55% by treatment with deoxycholate, protein mobility decreased considerably, suggesting protein aggregation. A concomitant increase was observed in the fraction of immobilized spin labels for both the free and attached hydrocarbon probes. The observed hydrocarbon immobilization probably arises in part from immobilization at the protein-lipid boundary, but protein-protein interactions that trap hydrocarbon chains may also contribute. When protein aggregation was induced by glutaraldehyde crosslinking, submillisecond protein mobility was eliminated, but there was no effect on either hydrocarbon probe. Thus protein aggregation does not necessarily cause hydrocarbon chain immobilization.

Animals↗
PubMed1982

The effect of calcium and prostaglandin inhibitors on the intestinal fluid response to heat-stable enterotoxin of Escherichia coli.

The role of calcium and prostaglandins in the intestinal fluid response to Escherichia coli heat-stable enterotoxin (ST) was investigated in infant mice. Drugs that inhibit calcium uptake-cromolyn sodium, diltiazem, and nifedipine-caused a significant (P less than 0.02, P less than 0.02, and P less than 0.01, respectively) decrease in the fluid response when administered with ST. The effect of cromolyn sodium and nifedipine was dose-dependent; both agents were effective when administered 30 min before toxin challenge. Inhibitors of prostaglandin synthesis-quinacrine hydrochloride and zomepirac sodium-caused a significant (P less than 0.05 and P less than 0.02, respectively) reduction in the fluid response when administered with or 30 min before ST. The fluid response to cyclic 8-bromoguanosine 3',5'-monophosphate or to nitroprusside, which directly activated guanylate cyclase, was not altered by inhibitors of calcium uptake or prostaglandin synthesis. These observations indicate that calcium and prostaglandins are involved in the initial events that result in an intestinal fluid response to E. coli ST.

Animals↗
PubMed1980

Oculoplethysmography/carotid phonoangiography: its value as a screening test in patients with suspected carotid artery stenosis.

A review of 291 carotid arteries that were subjected to both oculoplethysmography/carotid phonoangiography (OPG/CPA) and arteriography showed that although 84% of the vessels that were narrowed less than 40% were properly categorized by the noninvasive test, there was poor grading ability for lesions 40% or greater, and an unacceptably high (40%) false-negative rate for marked carotid stenoses. This has been confirmed by others. Analysis of a 15-item questionnaire that was returned by 12 clinicians showed an unwarranted reliance on the OPG/CPA as a screening test in those patients without transient ischemic attacks. A large majority of responding physicians favored endarterectomy in asymptomatic patients with lesions greater than 70%, and the use of OPG/CPA prevented a substantial number of affected individuals from undergoing angiography and subsequent corrective surgery.

Angiography↗
PubMed1980

Submillisecond rotational dynamics of spin-labeled myosin heads in myofibrils.

The rotational motion of crossbridges, formed when myosin heads bind to actin, is an essential element of most molecular models of muscle contraction. To obtain direct information about this molecular motion, we have performed saturation transfer EPR experiments in which spin labels were selectively and rigidly attached to myosin heads in purified myosin and in glycerinated myofibrils. In synthetic myosin filaments, in the absence of actin, the spectra indicated rapid rotational motion of heads characterized by an effective correlation time of 10 microseconds. By contrast, little or no submillisecond rotational motion was observed when isolated myosin heads (subfragment-1) were attached to glass beads or to F-actin, indicating that the bond between the myosin head and actin is quite rigid on this time scale. A similar immobilization of heads was observed in spin-labeled myofibrils in rigor. Therefore, we conclude that virtually all of the myosin heads in a rigor myofibril are immobilized, apparently owing to attachment of heads to actin. Addition of ATP to myofibrils, either in the presence or absence of 0.1 mM Ca2+, produced spectra similar to those observed for myosin filaments in the absence of actin, indicating rapid submillisecond rotational motion. These results indicate that either (a) most of the myosin heads are detached at any instant in relaxed or activated myofibrils or (b) attached heads bearing the products of ATP hydrolysis rotate as rapidly as detached heads.

Actins↗
PubMed1980

Orientation of spin-labeled myosin heads in glycerinated muscle fibers.

We have used electron paramagnetic resonance (EPR) spectra to study spin labels selectively and rigidly attached to myosin heads in glycerinated rabbit psoas muscle fibers. Because the angle between the magnetic field and the principal axis of the probe determines the position of the EPR absorption line, spectra from labeled fibers oriented parallel to the magnetic field yielded directly the distribution of spin label orientations relative to the fiber axis. Two spin labels, having reactivities resembling iodoacetamide (IASL) and maleimide (MSL), were used. In rigor fibers with complete filament overlap, both labels displayed a narrow angular distribution, full width at half maximum approximately 15 degrees, centered at angles of 68 degrees (IASL) and 82 degrees (MSL). Myosin subfragments (heavy meromyosin and subfragment-1) were labeled and allowed to diffuse into fibers. The resulting spectra showed the same sharp angular distribution that was found for the labeled fibers. Thus is appears that virtually all myosin heads in a rigor fiber have the same orientation relative to the fiber axis, and this orientation is determined by the actomyosin bond. Experiments with stretched fibers indicated that the spin labels on the fraction of heads not interacting with actin filaments had a broad angular distribution. Addition of ATP to unstretched fibers under relaxing conditions produced orientational disorder, resulting in a spectrum almost indistinguishable from that of an isotropic distribution of probes. Addition of either an ATP analog (AMPPNP) or pyrophosphate produced partial disorder. That is a fraction of the probes remained sharply oriented as in rigor while a second fraction was in a disordered distribution similar to that of relaxed fibers.

Actins↗
PubMed1978

Effect of the lipid environment on protein motion and enzymatic activity of sarcoplasmic reticulum calcium ATPase.

In order to investigate the roles of the physical states of phospholipid and protein in the enzymatic behavior of the Ca2+ -ATPase from sarcoplasmic reticulum, we have modified the lipid phase of the enzyme, observed the effects on the enzymatic activity at low temperatures, and correlated these effects with spectroscopic measurements of the rotational motions of both the lipid and protein components. Replacement of the native lipids with dipalmitoyl phosphatidylcholine inhibits ATPase activity and decreases both lipid fluidity, as monitored by EPR spectroscopy on a stearic acid spin label, and protein rotational mobility, as monitored by saturation transfer EPR spectroscopy on the covalently spin-labeled enzyme. Solubilization of the lipid-replaced enzyme with Triton X-100 reverses all three of these effects. Ten millimolar CaCl2 added either to the enzyme associated with the endogenous lipids or to the Triton X-100 soulbilized enzyme inhibits both ATPase activity and protein rotational mobility but has no detectable effect on the lipid mobility. These results are consistent with the proposal that both lipid fluidity and protein rotational mobility are essential for enzymatic activity.

Animals↗
PubMed1978

Large-scale rotational motions of proteins detected by electron paramagnetic resonance and fluorescence.

Direct spectroscopic measurements of rotational motions of proteins and large protein segments are crucial to understanding the molecular dynamics of protein function. Fluorescent probes and spin labels attached to proteins have proved to be powerful tools in the study of large-scale protein motions. Fluorescence depolarization and conventional electron paramagnetic resonance (EPR) are applicable to the study of rotational motions in the nanosecond-to-microsecond time range, and have been used to demonstrate segmental flexibility in an antibody and in myosin. Very slow rotational motions, occurring in the microsecond-to-millisecond time range, are particularly important in supramolecular assemblies, where protein motions are restricted by association with other molecules. Saturation transfer spectroscopy (ST-EPR), a recently developed electron paramagnetic resonance (EPR) technique that permits the detection of rotational correlation times as long as 1 ms, has been used to detect large-scale rotational motions of spin-labeled proteins in muscle filaments and in membranes, providing valuable insights into energy transduction mechanisms in these assemblies.

Actins↗
PubMed1978

Rotational motion of the sarcoplasmic reticulum Ca2+-ATPase.

Using saturation transfer electron paramagnetic resonance, we have detected the rotational motion of a spin label rigidly attached to the sarcoplasmic reticulum Ca2+-ATPase (ATP phosphohydrolase, EC 3.6.1.3). At 4 degrees C, the spectrum indicates an effective rotational correlation time of 60 microsec, determined by comparison with reference spectra obtained from theoretical calculations and from experiments on model systems. This motion appears to correspond to rotation of the enzyme with respect to the membrane, because the motion persists when the membrane fragments are immobilized by sedimentation and the motion stops when the polypeptide chains, but not the membrane vesicles, are crosslinked by glutaraldehyde. The rotational mobility of the enzyme increases with increasing temperature, and this increase becomes more gradual when the temperature exceeds 20 degrees C; the same kind of temperature dependence has been observed previously for lipid fluidity and enzymatic activity.

Animals↗
PubMed1978

Fluorescence energy transfer in the rapid-diffusion limit.

Energy transfer is enhanced by translational diffusion of the donor and acceptor [Steinberg, I. Z. & Katchalski, E. (1968) J. Chem. Phys. 48, 2404-2410]. The effect of diffusion on energy transfer depends on Dtau(0)/s(2), in which D is the sum of the diffusion coefficients of the donor and acceptor, tau(0) is the lifetime of the donor in the absence of transfer, and s is the mean distance between donors and acceptors. In most previous studies, Dtau(0)/s(2) << 1, corresponding to the static limit. We report here steady-state and kinetic fluorescence experiments showing that Dtau(0)/s(2) >> 1, the rapid-diffusion limit, can be attained by using Tb(3+) chelated to dipicolinate as a long-lived energy donor (tau(0) = 2.2 msec). The concentration of rhodamine B, the energy acceptor, resulting in 50% transfer was 0.67 muM, which is three orders of magnitude less than the concentration giving 50% transfer in the static limit. The dependence of the transfer efficiency on diffusion coefficients varying from 5 x 10(-11) to 1.5 x 10(-4) cm(2)/sec, spanning the range from the static limit to the rapid-diffusion limit, is in excellent agreement with theory. It is evident that energy donors with millisecond or longer excited state lifetimes can be used to probe translational motions in membranes and other assemblies. Energy transfer in the rapid diffusion limit is sensitive to the distance of closest approach (a) of the donor and acceptor. For a Tb.(DPA)(3) chelate trapped inside the aqueous space of a membrane vesicle containing eosin phosphatidylethanolamine, a = 10 A. The transverse location of chromophores in model membranes and biological membranes can be determined by this technique.

Journal Article↗