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At least 73 records · Page 4Linked to original sources
PubMed1991

An expanded function for superoxide dismutase.

alpha-Hydroxyalkylperoxyl radicals were generated from the primary and secondary alcohols methanol, ethanol and 2-propanol in N2O/O2-saturated aqueous solutions by pulse radiolysis. These radicals reduced a ferric iron porphyrin complex, tetrakis-(4-N-methylpyridyl)porphine, with diffusion-controlled rate constants. The extreme sensitivity of the shift of the Soret absorption band in this reaction was used to determine, by competition kinetics, the reactivity of the peroxyl radicals with different proteins. Only native Cu,Zn-superoxide dismutase and metallothionein showed competitive behavior, with SOD exhibiting rate constants close to the dismutation rate for O2-. Metallothionein was slower by a factor of 30 with hydroxymethylperoxyl radicals. We propose, that SOD has unique properties of the protein surface in addition to the prosthetic copper site, having possibly evolved as a 'general-purpose radical-scavenging protein'.

Alcohols↗
PubMed1992

[An analysis of the joint activities of primates in a situation of delayed spatial choice].

A leader-dominant baboon of a couple took practically all the food in the situation of spatial delayed choice. The subdominant baboon managed to get some food, too, using fast changes of behavioral tactics during the experimental session. The obvious negative state of the subdominant baboon resulted in a severe neurotic breakdown. Long delays in the experimental scheme enabled the subdominant to get up to 50% of the food reinforcement due to the dominant baboons mistakes. The role of changes in the tactics of the monkeys is discussed.

Animals↗
PubMed1992

Modulation of the mitochondrial megachannel by divalent cations and protons.

In patch-clamp experiments on rat liver mitoplasts, the 1.3 nanosiemens (in 150 mM KCl) mitochondrial megachannel was activated by Ca2+ and competitively inhibited by Mg2+, Mn2+, Ba2+, and Sr2+. Cyclosporin A, which inhibits the megachannel, also showed a competitive behavior versus Ca2+. The pore is regulated by pH in the physiological range; lower pH values cause its closure in a Ca(2+)-reversible manner. The modulating sites involved in these effects are located on the matrix side of the membrane. As illustrated in the companion paper (Bernardi, P., Vassanelli, S., Veronese, P., Colonna, R., Szabó, I., and Zoratti, M. (1992) J. Biol. Chem. 267, 2934-2939), the calcium-induced permeability transition of mitochondria is affected by these various agents in a similar manner. The results support the identification of the megachannel with the pore believed to be involved in the permeabilization process. The kinetic characteristics of the single channel events support the idea that the megachannel is composed of cooperating subunits.

Animals↗
PubMed1987

Characterization of Na+/H+ exchange in platelets.

Acidification of the cytoplasm of human blood platelets leads to activation of Na+/H+ exchange. As a result, alkalinization occurs that is detectable by an intracellular fluorescent pH indicator. The activity of the exchanger can also be measured by the swelling of platelets suspended in Na-propionate medium using a Coulter Counter: the rapid entry of propionic acid leads to acidification and activation of Na+/H+ exchange with the parallel entry of Na+ and propionic acid leading to osmotic swelling. The Na+/H+ exchanger is sensitive to amiloride and to derivatives that are reported to be more specific inhibitors; it is specific for Na+ and Li+ with no measurable transfer of K+, Rb+ or Cs+; its Km for Na+ is 75 to 90 mM; it displays competitive behavior between Na+ and amiloride; its activity is decreased in cells loaded with Na+ by prolonged ouabain treatment; and it has a high temperature coefficient. These properties are in general similar to those of the exchanger in other cells. It is suggested that the Na+/H+ exchanger plays a role in platelet pH regulation.

Adult↗
PubMed1983

Fluorescence stopped flow analysis of the interaction of virginiamycin components and erythromycin with bacterial ribosomes.

The kinetics of the interaction between the 50 S subunits (R) of bacterial ribosomes and the antibiotics virginiamycin S (VS), virginiamycin M (VM), and erythromycin have been studied by stopped flow fluorimetric analysis, based on the enhancement of VS fluorescence upon its binding to the 50 S ribosomal subunit. Virginiamycin components M and S exhibit a synergistic effect in vivo, which is characterized in vitro by a 5- to 10-fold increase of the affinity of ribosomes for VS, and by the loss of the ability of erythromycin to displace VS subsequent to the conformational change (from R to R*) produced by transient contact of ribosomes with VM. Our kinetic studies show that the VM-induced increase of the ribosomal affinity for VS (K*VS = 25 X 10(6) M-1 instead of KVS = 5.5 X 10(6) M-1) is due to a decrease of the dissociation rate constant (k*-VS = 0.008 s-1 instead of 0.04 s-1). The association rate constant remains practically the same (k+VS approximately k*+VS = 2.8 X 10(5) M-1 s-1), irrespective of the presence of VM. VS and erythromycin bind competitively to ribosomes. This effect has been exploited to determine the dissociation rate constant of VS directly by displacement experiments from VS . 50 S complexes, and the association rate constant of erythromycin (k+Ery = 3.2 X 10(5) M-1 S-1) on the basis of competition experiments for binding of free erythromycin and VS to ribosomes. By making use of the change in competition behavior of erythromycin and VS, after interaction of ribosomes with VM, the conformational change induced by VM has been explored. Within the experimentally available concentration region, the catalytic effect of VM has been shown to be coupled to its binding kinetics, and the association rate constant of VM has been determined (k+VM = 1.4 X 10(4) M-1 S-1). Evidence is presented for a low affinity binding of erythromycin (K*Ery approximately 3.3 X 10(4) M-1) to ribosomes altered by contact with VM. A model involving a sequence of 5 reactions has been proposed to explain the replacement of ribosome-bound erythromycin by VS upon contact of 50 S subunits with VM.

Erythromycin↗
PubMed1976

Reduced nicotinamide adenine dinucleotide phosphate, a structural and conformational probe of chicken liver fatty acid synthetase.

Structural and conformational organization of chicken liver fatty acid synthetase has been probed using its fluorescent coenzyme, NADPH. Three NADPH binding sites per mole of the enzyme complex, of apparently identical dissociation constant (KD = 0.6 muM) can be titrated at temperatures above 12 degrees. These results are in disagreement with the earlier studies of Hsu and Wagner (Hsu, R. Y., and Wagner, B. J. (1970) Biochemistry, 9, 245-251) in which four such sites could be titrated. At 12 degrees, the composite sites split into two subsets: a pair of sites with a KD of 0.3 muM and a third site with a Kd of 1.1 muM. At lower temperatures (5 degrees or 2 degrees), the site with weak affinity disappears, leaving a pair of sites with a Kd of 0.5 muM. Similar observations were made when the enzyme was modified with phenylmethylsulfonyl fluoride, a specific and selective inhibitor of fatty acyl-CoA deacylase (s) of the pigeon liver enzyme complex (Kumar, S. (1975) J. Biol. Chem. 250, 5150-5158). Partial modification with phenylmethylsulfonyl fluoride elicits a NADPH binding response similar to the binding observed at 12 degrees, i.e. two sets of binding sites with nonidentical dissociation constants. Further modification corresponding to the complete loss of deacylase function results in a set of two apparently identical binding sites, and the third site is not available for titration. The modified enzyme retains the two reductase functions as measured by the model substrates, acetoacetyl-N-acetylcysteamine and crotonyl-CoA. Furthermore, the addition of acetyl- and malonyl-CoA (100 muM each) to the modified enzyme lowers the NADPH binding affinity by a factor of 3. Other observations show that the quantum yield, as measured by the ratio of fluorescence intensity of bound and free NADPH, changes with temperature and ionic strength. Lowering the temperature from 30 degrees to 2 degrees increases the enhancement ratio by 50%, whereas increase in ionic strength from 0.05 to 0.2 M potassium phosphate lowers it to 50% of the original level. Measurement of NADPH binding in the presence of NADP+, NADH, NAD+ and adenosine-2'-monophospho-5'-diphosphoribose demonstrates that NADP+ shows competitive behavior for NADPH sites (KD = 10.6 muM), whereas NADH and NAD+ show noncompetitive (KD (apparent) = nearly 600 muM) and rather complicated interactions implicating nonspecific conformational alteration of the enzyme complex. The behavior of adenosine 2'-monophospho-5'-diphosphoribose is intermediate between NADP+ and NADH. These data are discussed in terms of substrate-mediated conformational changes and the moles of each of the reductase enzymes per mole of the enzyme complex, the polarity of the NADPH binding region, and the probable structure of the nicotinamide moiety when bound to the enzyme.

Animals↗