Assembly of the native heterodimer of Rana esculenta tropomyosin by chain exchange.

Rana esculenta tropomyosin assembles in vivo into a coiled-coil alpha helix from two different subunits, alpha and beta, which are present in about equal concentrations. Although the native composition is alpha beta, a mixture of equal amounts of alpha alpha and beta beta is produced by refolding dissociated alpha and beta at low temperature in vitro. Refolding kinetics showed that alpha alpha formed first and was relatively stable with regard to chain exchange below approximately 20 degrees C. Equilibration of the homodimer mixture at 30 degrees and 34 degrees C for long times, however, resulted in the formation of the native alpha beta molecule by chain exchange. Biosynthesis of alpha beta from separate alpha and beta genes is, therefore, favored thermodynamically over the formation of homodimers, and biological factors need not be invoked to explain the preferred native alpha beta composition.

Digoxin-verapamil interaction.

To explore a possible interaction between digoxin and verapamil, a single-dose kinetic study of digoxin was performed and then repeated after 10 days of verapamil treatment in eight healthy subjects. Verapamil diminished the apparent central distribution volume of digoxin from 0.83 +/- 0.25 to 0.64 +2- 0.17 l/kg (P less than 0.05) and reduced total body clearance of digoxin from 3.28 +/- 0.58 to 2.15 +/- 0.66 ml/min/kg (P less than 0.001) by impairing both renal and extrarenal clearance. Biological digoxin half-life rose from 38.6 +/- 8.5 to 50.5 +/- 8.3 hr (P less than 0.005). Reduction of renal clearance of digoxin may be due to inhibition of tubular secretion. The underlying mechanisms of extrarenal interaction are not known, but impaired hepatic degradation of digoxin induced by verapamil should be considered.

Effect of nifedipine on digoxin kinetics in healthy subjects.

Verapamil has been shown to reduce total-body digoxin clearance by 35% due to impairments of both renal and extrarenal clearances. Our study was undertaken to evaluate the influence of the related calcium antagonist nifedipine on single-dose kinetics. Nifedipine increased extrarenal clearance of digoxin from 1.09 +/- 0.30(SD) to 1.45 +/- 0.23 ml/min/kg (P less than 0.05) and reduced the total urinary recovery of the drug from 69.2% +/- 5.9(SD) to 64.3% +/- 5.2 (P less than 0.05). There were no significant changes in renal digoxin clearance, distribution, or biological half-life or in digoxin distribution volumes during nifedipine coadministration.

Effects of physical activity and immobilization on plasma digoxin concentration and renal digoxin clearance.

Plasma digoxin concentration and renal digoxin clearance were determined during 2 hr of normal physical activity and during 2 hr of complete immobilization in eight healthy subjects on steady-state digoxin dosing. Mean plasma digoxin concentration rose from 0.64 +/- 0.13 ng/ml during physical activity to 1.04 +/- 0.19 ng/ml (63%) after 2 hr of rest. Resumption of physical activity resulted in gradual decline of plasma digoxin, and subsequent strenuous exercise reduced the value to preimmobilization level. Mean renal digoxin clearance was reduced from 168.4 +/- 18.7 ml/min during physical activity to 137.2 +/- 32.7 ml/min during rest whereas creatinine clearance was unchanged. The rise in plasma digoxin during rest is presumed to be due to changes in the binding of the drug to tissues such as skeletal muscles. Our findings indicate that attention should be given to the state of physical activity when kinetic studies are performed or when digoxin therapy is monitored by means of plasma digoxin analysis.

Insulin association in neutral solutions studied by light scattering.

Molecular weights and weight distributions of sulfated, Zn-free, and 2Zn insulins have been measured at pH 7.3 as a function of concentration from 0.1 to 2 mg/ml by use of a combination of light scattering, refractometry, and size-exclusion chromatography. Results show that sulfated insulin is monomeric over the studied concentration range. Weight average molecular weights between those of a monomer and a hexamer were found for both zinc-free and 2Zn insulins. Zinc stabilizes the hexamer, and the dimer-hexamer equilibrium constant is approx. 400-times higher in the presence of zinc than in its absence. An average hydrodynamic radius of 5.6 nm, close to the crystallographic size of the insulin hexamer, was determined from dynamic light scattering of 2Zn insulin solutions.

Differences in thermal stability of frog and rabbit alpha alpha- and alpha beta-tropomyosins determined by optical rotatory dispersion.

Frog and rabbit alpha alpha- and alpha beta-tropomyosins were purified, and their thermal stabilities determined by use of optical rotatory dispersion. The tropomyosins were found to be virtually completely helical at 5 degrees C. Regions of different thermal stabilities were seen for all tropomyosins. Rabbit and frog alpha alpha-tropomyosin show very similar thermal properties, with main transitions near 47-49 degrees C. The main transition for frog alpha beta-tropomyosin is at 32 degrees C. The results show that the alpha beta-tropomyosins are less stable than the alpha alpha-forms. Only thermal transitions of the alpha beta-forms appear to be correlated with the body temperatures of the animals.

Thermally induced chain exchange of frog alpha beta-tropomyosin.

The thermally induced unfolding of the alpha-helix of Rana esculenta alpha alpha, alpha beta and beta beta tropomyosin and two tryptic fragments approximately corresponding to the N- and C-terminal halves of alpha beta have been investigated by use of optical rotation, circular dichroism and UV difference spectroscopy. Reversible unfolding transitions of alpha alpha and beta beta occur around 49 degrees C and 32 degrees C, respectively. The helix unfolding of alpha beta shows two major transitions at 36 degrees C and 48 degrees C, with only the latter being reversible. The major unfolding transitions of each of the N- and C-terminal alpha beta peptides roughly correspond to the low and high temperature transitions of intact alpha beta, respectively. This suggests that the unfolding of alpha beta could be due to unfolding of two independent domains in alpha beta. UV difference data, crosslinking and chromatography results show, however, that the unfolding of alpha beta at 36 degrees C is due to chain exchange with the formation of alpha alpha homodimers and largely unfolded beta monomers, and that the transition at 48 degrees C is due to unfolding of alpha alpha dimers.

Solubility of nonylphenol and nonylphenol ethoxylates. On the possible role of micelles.

The water solubility of nonylphenol (NP) has been estimated to be 4.9 +/- 0.4 mg/l corresponding to (2.22 +/- 0.18) x 10(-5) mol/l at 25 degrees C using shake flask and surface tension techniques. The low solubility in combination with an observed rather slow dissolution process will limit the leachability of NP in the terrestrial environment. Based on indirect evidence, it is suggested that NP, in contrast to nonylphenol ethoxylate (NPEO) with, e.g., 12 ethoxylate moieties, is not subject to micelle formation, and as such does not constitute a potential vehicle for the transport of hydrophobic pollutants in the environment. For NPEOs with a very high number of ethoxy moieties, e.g., 100, the compounds appear water soluble without micelle formation.

Solubilities and surface activities of phthalates investigated by surface tension measurements.

Aqueous solutions of DEP (di-ethyl), DnBP (di-n-butyl), DnH(6)P (di-n-hexyl), and DEHP (di-[2-ethyl-hexyl]) phthalates have been investigated by use of surface tension measurements at temperatures between 10 and 35 degrees C. A tensiometric approach allows for the determination of unimeric solubilities and delta G degree, which is the standard Gibbs free energy change, for the dissolution of phthalates in water. The unimeric solubility of the phthalates increase with decreasing temperature. The delta G degree shows a linear increase with increasing phthalate alkyl chain length. The contribution of enthalpy (delta H degree) and entropy (delta S degree) to delta G degree were calculated from the temperature-dependent solubilities. The contributions of both delta H degree and delta S degree are negative and increase in magnitude with increasing alkyl chain length, suggesting hydrophobic interactions between phthalates and water. The ability of different phthalates to lower the surface tension decreases with increasing alkyl chain length, whereas the relative affinity for adsorption in the air-water interface increases drastically for long-chain phthalates. Despite the low surface activity of phthalates compared with that of common surfactants, they show significant affinity for adsorption in air-water interfaces of natural surface waters. This property, combined with their low solubilities, may affect the fate of these compounds within the natural environment, because they form emulsions above unimeric saturation in aqueous media.