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F Waechter

Publications and source records attributed to F Waechter.

25 records · Page 2Linked to original sources
PubMed1977

The polymerization reaction of muscle actin.

Recent advances in the studies of the aggregation of G-actin monomers, containing one molecule of ATP, to long filaments of F-actin, with a concomitant hydrolysis of the nucleotide to ADP, are reviewed. With the aid of omega-ATP, the association and dissociation rate constant of the nucleotide could be determined. The binding of the nucleotide is enhanced by the binding of one Ca++ ion, probably at a different site. The delta G value of the Mg++ or Ca++ induced polymerization has been determined to --39 to--59 kJ/mole, the critical protein concentration for the ATP-G-actin to ADP-F-actin conversion is very strongly influenced by the concentration of bivalent cations. The rate constants of the protein monomers, and the rate and equilibrium constants for the propagation step show the process to be extremely cooperative. Actin shows the interesting phenomenon of translocational head-to-tail polymerization, which may be regulated by ATP. The contact sites between the monomers in F-actin have been labeled by chemical modification. Two tryosine residues, 53 and 69, are probably close to one of the two sites. The ATP binding sites has been labeled by an ATP analog, and there is evidence that it is close to the contact site.

Actins↗
PubMed1977

Association kinetics and binding constants of nucleoside triphosphates with G-actin.

The dissociation of the complex between 1:N6-ethenoadenosine, 5'-triphosphate (xiATP) and G-actin was initiated by dilution to concentrations between 1 micronM and 5 nM and monitored by the fluorescence change of xiATP. The results were quantitatively explained by a two-step mechanism: a reversible dissociation of the actin-nucleotide complex followed by a fast irreversible inactivation of nucleotide-free G-actin. Under normal conditions (0.8 mM CaCl2, pH 8.2,21 degrees C), the rate-limiting step was the dissociation of the nucleotide-G-actin complex. The half-time of the dissociation of xiATP from G-actin was 290 s as compared to only 13 s for the following denaturation step of nucleotide-free actin. 1 mM EDTA highly accelerated the dissociation step and, regardless of its concentration, the complex dissociated quantitatively within 1 min. Addition of Ca2+ within 20 s after EDTA addition induced a re-association of xiATP with nucleotide-free but still native G-actin. This reversal was kinetically resolved by means of a multimixing stopped-flow apparatus. The association rate constant was 6 X 10(6) M-1s-1. From the association and dissociation rate constant, a value of 2.5 X (10(9) M-1 was calculated for the binding constant of xiATP to G-actin. The binding constant of ATP (1.4 X 10(10) M-1) was derived from the relative binding constant of xiATP and ATP as determined by fluorescence titration of xiATP-G-actin with ATP. These binding constants are 10(3)-10(4) times higher than values reported earlier on the basis of more indirect data.

Actins↗
PubMed1975

The kinetics of the exchange of G-actin-bound 1: N6-ethenoadenosine 5'-triphosphate with ATP as followed by fluorescence.

1: N6-Ethenoadenosine 5'-triphosphate (epsilonATP), a fluorescent analog of ATP, binds to monomeric actin with a binding constant which is only about 5 times smaller than that of ATP. The spectroscopic changes which occur when epsilonATP binds to actin are studied and used to monitor the kinetics of nucleotide exchange. The first-order rate constant which is measured at a large excess of ATP over epsilonATP strongly depends on the ATP and Ca+ concentrations. This finding is explained by a mechanism in which the nucleotide dissociates much more easily from Ca2+-free than from Ca2+-bound actin. Of special interest is the temperature dependence of the dissociation rate constant. The Arrhenius plot shows a sharp bend near 24 degrees C.

Actins↗
PubMedDate not supplied

Assessment of peroxisome proliferation and liver growth-stimulating potential by nondirectly genotoxic compounds in cultured hepatocytes.

Previously, we have established that some peroxisome proliferators, a class of nongenotoxic hepatocarcinogens, are able to induce replicative DNA synthesis (RDS) in cultured hepatocytes. Hepatomegaly observed after short-term in vivo treatment correlated better with the ability to induce RDS than with the potency as peroxisome proliferator assessed in vitro. To clarify the challenging question of the limited sensitivity of primates to peroxisome proliferators, primary cultures of marmoset hepatocytes have been treated with nafenopin for some days. As expected from in vivo observations, no evidence for peroxisome proliferation could be observed. However, nafenopin induced a dose-dependent increase in the amount of RDS, but this induction was measurable only when the serum was absent from the culture medium. These results confirm that peroxisome proliferation and mitogenicity might be independent properties of peroxisome proliferators. Since in vivo the ability of compounds to induce RDS in liver cells is relevant to at least one key parameter of the hepatocarcinogenic response, it is suggested that measurement of RDS inducibility in cultured hepatocytes from different species might be relevant and useful to assess species differences in the liver tumor potency of nondirectly genotoxic compounds.

Animals↗