PubMed Health⌕ Search

Biomedical subjects

P Dancker

Publications and source records attributed to P Dancker.

32 records · Page 2Linked to original sources
PubMedDate not supplied

Regulation of actin-myosin interaction.

In the presence of the regulatory protein complex tropomyosin-troponin ATPase activity of vertebrate skeletal muscle actomyosin is either higher or lower than in the absence of tropomyosin-troponin. The actual behavior depends on ionic strength, Ca2+ concentration, ATP concentration (determining the amount of rigor complexes present), and the ratio between actin and myosin. This effect of the myosin-actin ratio implies that under certain conditions cooperativity in actin-myosin interaction can be seen.

Actins↗
PubMedDate not supplied

ATPase activity and light scattering of acto-heavy meromyosin: dependence on ATP concentration and on ionic strength.

1. The dependence on ATP concentration of ATPase activity and light scattering decrease of acto-HMM could be described at very low ionic strength by one hyperbolic adsorption isotherm with a dissociation constant of 3 X 10(-6)M. Hence the increase of ATP ase activity was paralleled by a decrease in light scattering. At higher values of ionic strength ATPase activity stopped rising before HMM was completely saturated with ATP. Higher ionic strength prevented ATPase activity from further increasing when the rigor links (links between actin and nucleotide-free myosin), which have formerly protected the ATPase against the suppressing action of higher ionic strength have fallen below a certain amount. This protecting influence of rigor links did not require tropomyosin-troponin. 2. For complete activation of ATPase activity by actin less actin was needed when HMM was incompletely saturated with ATP than when it was completely saturated with ATP. 3. The apparent affinity of ATP to regulated acto-HMM (which contained tropomyosin-troponin) was lower than to unregulated acto-HMM (which was devoid of tropomyosin-troponin). In the presence of rigor complexes (indicated by an incomplete decrease of light scattering) the ATPase activity of regulated acto-HMM was higher than that of unregulated acto-HMM. At increasing ATP concentrations the ATPase activity of regulated acto-HMM stopped rising at a similar degree of saturation with ATP as the ATPase activity of unregulated acto-HMM at the same ionic strength.

Actins↗
PubMedDate not supplied

Calcium sensitivity of actomyosin ATPase: its modification by substitution of myosin sulfhydryl groups.

SH group substitution by DTNB enabled natural actomyosin to split ATP (in the prescence of Mg2+) also in the absence of Ca2+, when assayed at low ionic strength. At higher KCl concentrations the ATPase activity of SH group substituted actomyosin was still Ca-dependent. Addition of unsubstituted myosin to natural actomyosin whose SH groups had been substituted increased the ATPase activity. This increase was Ca-insensitive indicating that SH group substitution of myosin in actomyosin can make the interaction of additional myosin molecules Ca-independent. In natural actomyosin Ca-insensitivity of ATPase activity was attained at a lower degree of SH group substitution when substitution was performed in the presence of EDTA. The part of ATPase activity which still remained Ca-sensitive after DTNB treatment could be activated by lower concentrations of free Ca2+ than the Ca-sensitive ATPase of untreated actomyosin. In reconstituted actomyosin the Ca-sensitivity of ATPase activity could more easily be reduced when the myosin-actin ratio was high. For demonstrating remaining Ca-sensitivity in SH group substituted reconstituted actomyosin more tropomyosin-troponin was needed than for sensitizing unsubstituted actomyosin to Ca2+. The similarities between the ATPase acitivity of SH group substituted actomyosin on the one hand and that of actomyosin at low concentrations of ATP on the other hand suggest that SH group substitution modifies actin-myosin interaction in a similar way as does nucleotide-free myosin (rigor myosin).

Actomyosin↗
PubMedDate not supplied

Tropomyosin-troponin-induced changes in the partitioning of free energy release of actomyosin-catalyzed ATP hydrolysis as measured by ATP-phosphate exchange.

ATPase activity and ATP-Pi exchange of unregulated (without tropomyosin-troponin) and regulated (with tropomyosin-troponin) acto-HMM were measured in media containing 0.2 mg/ml actin, HMM, and (when present) tropomyosin-troponin, 2 mM MgCl2, 10 mM KCl, 2 mM NaN3, 10 mM Pi (pH 7.0), 3 MM ATP. The following mean values for ATPase activity and for the rate of incorporation of Pi into ATP (each per mg HMM and per min) were obtained: unregulated acto-HMM 0.33 mumol Pi and 0.33 nmol Pi, regulated acto-HMM 0.54 mumol Pi and 1.06 nmol Pi. The ratio of Pi incorporation rate to ATPase activity was 1.01 x 10(-3) for unregulated and 2.02 x 10(-3) for regulated acto-HMM. From these ratios and from the overall free energy change of ATP hydrolysis it was calculated that under the prevailing experimental conditions in unregulated acto-HMM 62% and in regulated acto-HMM 66% of the free energy change of ATP hydrolysis occurs after the release of phosphate from actomyosin. It is probably this part of the free energy change that is used by the muscle for the performance of work.

Actins↗
PubMedDate not supplied

Cytochalasin B-induced ATPase activity of actin: dependence on monomer concentration.

We simultaneously measured polymerization and ATPase activity of actin induced by cytochalasin B. It was found that under all conditions tested, ATPase activity was proportional to the concentration of actin which was unpolymerized. In addition, it was found that conditions increasing ATPase activity also increase the velocity of polymerization. From this we conclude that CB-induced ATPase activity is a property of the actin monomers and that the "readiness" of the monomers to hydrolyze ATP is correlated with an increased capacity of the monomers to polymerize.

Actins↗
PubMedDate not supplied

Polymerization of actin in the absence and presence of cytochalasin B: problems of determining "critical concentration".

Various concentrations of actin (0.3 or 1 mM MgCl2, 1 mM ATP, 1 mM EGTA) reached their final degree of polymerization (measured by a pyrene dye attached to actin) earlier in the presence of cytochalasin B than in its absence. The curves relating concentrations of polymeric F-actin to total actin concentration were under these conditions highly nonlinear making an unambiguous extrapolation to zero F-actin concentration (to deduce the "critical concentration" of actin polymerization) impossible. The concentration of actin, above which polymerization occurred, was unaltered by cytochalasin B (although for reasons not yet understood the specific fluorescence intensity of polymerized actin was lower in the presence of cytochalasin B than in its absence). The results show that a distinct "critical concentration" of actin polymerization must not always be well defined.

Actins↗
PubMedDate not supplied

Stabilization of actin filaments by ATP and inorganic phosphate.

Both inorganic orthophosphate and ATP stabilize actin filaments. This is reflected by a reduced nucleotide exchange and by a protection against filament breakdown by SDS or KI. When the filament-stabilizing effect of ATP was maximal, only about 15% of the actin subunits of the filament had bound one molecule of the nucleotide offered in the stabilizing solution.

Actin Cytoskeleton↗
PubMedDate not supplied

Product release is not the rate-limiting step during cytochalasin B-induced ATPase activity of monomeric actin.

Under conditions where cytochalasin B induces ATPase activity of monomeric actin (0.3 mM MgCl2, 1 mM EGTA, 30 microns cytochalasin B, 1 mM ATP) the rate constant of the exchange of actin-bound epsilon-ATP for free ATP is about 4-6 times faster than steady state ATPase activity. When a stoichiometric ATP-actin complex is extracted with PCA (single turnover experiment) the apparent rate constant of Pi generation is not faster than steady state ATPase activity. - The experiments suggest that the hydrolysis of actin-bound ATP and not the subsequent release of hydrolysis products is rate-limiting during cytochalasin-induced ATPase activity of actin.

Actins↗