An ESR study of titanium-silicalite in presence of H2O2.

The interaction of H2O2 with TS-1 has been investigated by the ESR technique. A well manufactured TS-1 has been considered, where the presence of extra-structural titanium can be excluded. The two main signals observed are attributed to oxygen radical species co-ordinated on structural titanium sites, and the relevant effect of water presence in the paramagnetic complex is discussed. A comparison is performed with O2-/Ti species in TS-1 obtained by reduction/re-oxidation procedures of Ti(IV). Moreover, some possible structural configurations of the paramagnetic sites are proposed, for two of which ab initio calculation yield g(z)-values quite close to the experimental ones.

Purification and stability of glutaryl-7-ACA acylase from Pseudomonas sp.

The enzyme glutaryl-7-ACA acylase from Pseudomonas sp. NCIMB 40474, produced by a recombinant Escherichia coli host, was purified to homogeneity. The enzyme is a tetramer composed of two couples of asymmetric dimers, each of them constituted of two subunits of mol wt 18 and 52 kDa, respectively. It was found that glutaric acid, one of the products of the substrate hydrolysis, is an effective acylase inhibitor. Between pH 6.0 and pH 10.0, the enzymatic activity is almost constant, but below pH 6.0 it progressively declines. The acylase activity decreased sharply as a function of guanidine HCl concentration. The loss is significant even at concentrations of denaturant lower than those causing unfolding, as suggested by UV spectroscopy and fluorescence emission studies. In these conditions (low denaturant concentration and low pH) the inactivation of the enzyme is caused by the tetramer dissociation into dimers. The lability of the quaternary structure of the enzyme is a key feature that must be taken into account for the improvement of the catalyst stability.

Effect of a new model of hemodialysis potassium removal on the control of ventricular arrhythmias.

The primary aim of this multicenter, prospective, randomized cross-over study was to clarify whether a new model of hemodialysis (HD) potassium (K) removal using a decreasing intra-HD dialysate K concentration and a constant plasma-dialysate K gradient (treatment B) is capable of reducing the arrhythmogenic effect of standard HD, which has a constant dialysate K concentration and decreasing plasma-dialysate K gradient (treatment A). The secondary aim was to verify whether this new model is clinically safe. In treatment B, the initial dialysate K concentration had to be 1.5 mEq/liter less than the plasma K concentration, and exponentially decrease to 2.5 mEq/liter at the end of HD. Forty-two chronic HD patients with an increase in premature ventricular complexes (PVC) during dialysis were enrolled from 18 participating centers, and randomly assigned to either sequence 1 (ABA) or sequence 2 (BAB). A pool of 333 of 378 expected ECG Holter recordings were checked for signal quality; 269 (71%) from 36 patients (86%) had a satisfactory signal quality and 108 were selected for analysis (1 per patient per period). There was a difference in the natural logarithm of the increase in PVC/hr and PVC couplets/hr during HD between treatments A and B (1.70 +/- 1.59 vs. 1.09 +/- 1.76 and 0.94 +/- 0.86 vs. 0.64 +/- 1.01, a reduction of 36% and 32%, P = 0.011 and 0.047, respectively) without any carry over effect (P = 0.61 and 0.24, respectively). The fact that this decrease of one third is due to a lower plasma-dialysate K gradient is supported by the observation that it was more evident during the first than the last two hours of HD (a reduction in the natural logarithm of the increase in PVC/hr and PVC couplets/hr of 60% and 60%, P 0.002 and 0.009, vs. 26% and 17%, P = 0.098 and 0.332, respectively): the initial plasma-dialysate K gradient was 2.3 times lower during treatment B than during treatment A, without adversely affecting pre-HD plasma K levels. These results could have a considerably clinical impact not only because of the possibility of physiologically decreasing the arrhythmogenic effect of HD, but also because this effect can be considered a "marker" of the electrophysiological derangement induced by the administration of standard HD three times a week for years ("electric disequilibrium syndrome").

Potassium removal as a factor limiting the correction of acidosis during dialysis.

The diffusional fluxes of urea, potassium and bicarbonate across the dialysis membrane (external balance) were determined in seven patients during haemodialysis using potassium free dialysate and dialysate containing 2.0mEq/L of potassium. The results show an inverse correlation between extraction of potassium and intake of bicarbonate in both external and internal balances. This is probably due to the increase in cell membrane electrical potential resulting from a fall in blood potassium and emphasises the importance of electrical driving forces in diffusional fluxes across cellular membranes.

[Potassium removal: a factor limiting the correction of acidosis during dialysis]. / La soustraction potassique: facteur limitant la correction de l'acidose pendant la dialyse.

The diffusional fluxes of urea, potassium and bicarbonate across the dialytic membrane (external balance), and across the cellular membrane (internal balance), were determined in 7 patients in haemodialysis using potassium free dialysate and dialysate containing 2.0 mEq/1 of potassium. The results obtained show an inverse correlation between extraction of potassium and intake of bicarbonate in both external and internal balance. This is probably due to the increase in membrane electrical potential resulting from a fall in blood potassium and emphasizes the importance of electrical driving forces in diffusional fluxes across cellular membranes.