142-98-3Relevant articles and documents
Counterion effect on the thermodynamics of micellization of alkyl sulfates
Ropers,Czichocki,Brezesinski
, p. 5281 - 5288 (2003)
Thermodynamics of micelle formation of anionic surfactants was investigated by using isothermal titration calorimetry (ITC). Highly purified decyl and dodecyl sulfates have been used to analyze the effect of counterions (Li+, Na+, K+, and Cs+) on critical micelle concentration (cmc) and enthalpy of micellization (ΔHmic) determined between 10 and 60 °C. The enthalpy of micellization decreases strongly with increasing temperature and passes trough zero (endothermic to exothermic processes), while the cmc versus temperature exhibits a minimum. At a given temperature and for a fixed chain length, the decrease of cmc and ΔHmic in the order Li+ > Na+ > K+ > Cs+ is related to the increase of the binding of counterions to micelles. The electrostatic repulsions between ionic headgroups, which prevent the aggregation, are progressively screened as the ionic character decreases with the size of the counterion. The heat of dilution of micelles is markedly dependent on temperature and is correlated with the temperature-dependent shape of micelles. The cmc concept has an exact meaning within the so-called phase separation model of micelle formation. Therefore, free energy and entropy were deduced from the cmc and enthalpy of micellation using this model by taking into account the counterion binding. The temperature changes of ΔSmic indicate that the process of micellization is entropically driven. ΔGmic is always negative (thermodynamically favored process) and slightly temperature and counterion dependent.
Influence of substrate structure on the catalytic efficiency of hydroxysteroid sulfotransferase STa in the sulfation of alcohols
Chen, Guangping,Banoglu, Erden,Duffel, Michael W.
, p. 67 - 74 (2007/10/03)
Sulfotransferase a (STa) is an isoform of hydroxysteroid (alcohol) sulfotransferase that catalyzes the formation of sulfuric acid esters from both endogenous and xenobiotic alcohols. Among its various functions in toxicology, STa is the major form of hepatic sulfotransferase in the rat that catalyzes the formation of genotoxic and carcinogenic sulfuric acid esters from hydroxymethyl polycyclic aromatic hydrocarbons. The goal of the present study was to elucidate fundamental quantitative relationships between substrate structure and catalytic activity of STa that would be applicable to these and other xenobiotics. We have modified previous procedures for purification of STa in order to obtain sufficient amounts of homogeneous enzyme for determination of k(cat)/K(m) values, a quantitative measure of catalytic efficiency. We determined the catalytic efficiency of STa with benzyl alcohol and eight benzylic alcohols that were substituted with n- alkyl groups (C(n)H(2n+1), where n = 1-8) in the para position, and the optimum value for k(cat)/K(m) in these reactions was obtained with n- pentylbenzyl alcohol. Correlations between logarithms of k(cat)/K(m) values and logarithms of partition coefficients revealed that hydrophobicity of the substrate was a major factor contributing to the catalytic efficiency of STa. Primary n-alkanols (C(n)H(2n+1)OH, where n = 3-16) exhibited an optimum k(cat)/K(m) for C9-C11 and a linear decrease in v(max) of the reaction for C3-C14; 15- and 16-carbon n-alkanols were not substrates for STa. These results indicated limits to the length of the extended carbon chain in substrates. Such limits may also apply to hydroxysteroids, since cholesterol was inactive as either substrate or inhibitor of STa. Furthermore, the importance of steric effects on the catalytic efficiency of STa was also evident with a series of linear, branched, and cyclic seven-carbon aliphatic alcohols. In conclusion, our results provide fundamental quantitative relationships between substrate structure and catalytic efficiency that yield insight into the specificity of STa for both endogenous and xenobiotic alcohols.
ENZYMIC SYNTHESIS OF STEROID SULFATES XVI. SPECIFICITY AND REGULATION OF HUMAN ADRENAL HYDROXYSTEROID SULFOTRANSFERASE
Adams, J. B.,McDonald, D.
, p. 575 - 586 (2007/10/02)
Pure hydroxysteroid sulfotransferase (EG 2.8.2.2) of human adrenal glands possesses a wide substrate specificity towards steroids.This wide specificity has now been found to extend to simple alcohols; normal aliphatic alcohols from C3 onwards acting as substrates with C9 showing the highest rate.Increased rate was accompanied by a decrease in Km.In marked contrast to the sulfurylation of steroids such as dehydroepiandrosterone, which exhibit wave-like kinetics, the kinetics with simple alcohols were of the normal Michaelis-Menten type.By means of enzyme antibody and enzyme stability studies evidence was provided that one and the same enzyme was responsible for sulfurylation of hydroxyls on the 3- and 17-positions of steroids and simple alcohols.The data lend support to previous evidence that the enzyme controls the secretion of dehydroepiandrosterone sulfate via steroid-specific binding sites, enabling self-regulation in response to ACTH action.
