Characterisation of lattice damage formation in tantalum irradiated at variable temperatures.

The formation of radiation-induced dislocation loops and voids in tantalum at 180(2), 345(3) and 590(5)°C was assessed by 3MeV proton irradiation experiments and subsequent damage characterisation using transmission electron microscopy. Voids formed at 345(3)°C and were arranged into a body centred cubic lattice at a damage level of 0.55 dpa. The low vacancy mobility at 180(2)°C impedes enough vacancy clustering and therefore the formation of voids visible by TEM. At 590(5)°C the Burgers vector of the interstitial-type dislocation loops is a<100>, instead of the a/2 <111> Burgers vector characteristic of the loops at 180(2) and 345(3)°C. The lower mobility of a<100> loops hinders the formation of voids at 590(5)°C up to a damage level of 0.55 dpa.

Inhibition studies of S-adenosylhomocysteine hydrolase purified from Acinetobacter calcoaceticus ULA-501.

Adenosine analogs previously reported as reversible inhibitors of mammalian S-adenosylhomocysteine hydrolase (SAHase) have been found to exert similar effects on Acinetobacter calcoaceticus ULA-501 enzyme. In addition, two metal coordination compounds, cis-platinum diammine dichloride (cis-DDP) and its trans isomer (trans-DDP), the former well known for its employment in anticancer chemotherapy, were assayed on both bacterial and mammalian SAHases. In our conditions, trans-DDP appeared to be the strongest inhibitor toward both SAHases. Finally, treatment of the bacterial enzyme with a mixture of ATP-Mg acetate and KC1 caused only a slight reversible inhibition, in contrast to the complete inactivation exerted on the mammalian SAHase.

The binding of human serum transferrin to its specific receptor reconstituted into liposomes.

Human placental transferrin receptor (HPTR), purified following a procedure based on affinity chromatography step, was reconstituted by the detergent dialysis method into various kinds of phosphatidylcholine vesicles and the receptor ability to bind 125I-labelled human serum transferrin (HST) was then evaluated. In our experimental conditions, the binding of the labelled protein to its specific receptor showed several features, in particular: (1) in cholesterol/1-alpha-dipalmitoylphosphatidyl choline (CHO/DPPC) liposomes, a positive cooperatively of the transferrin binding resulted at the lowest cholesterol/phospholipids (C/P) ratio; 1-alpha-dioleylphosphatidyl choline (DOPC) and phosphatidic acid (PA) containing liposomes showed an opposite binding curve trend; (2) the apparent dissociation constant (K'd) did not change significantly as a function of the lipid composition, being always around 1.00 x 10(-6) M; (3) the encapsulation capacity of liposomes decreased from 27% to about 13% with increasing amounts of cholesterol and was around 20% in the presence of DOPC or PA; about 8-13% of this receptor was found to be functional; (4) receptor-loaded liposomes treated with polyclonal anti-HPTR rabbit antibodies showed a remarkable binding decrease for transferin. All these results seem to point out the crucial role played by the environment in the binding behaviour of the transferrin receptor.

S-adenosylhomocysteine hydrolase from Acinetobacter calcoaceticus: purification and partial characterization.

S-adenosylhomocysteine hydrolase (SAHase) was purified to homogeneity from the Gram negative strain Acinetobacter calcoaceticus 501. The molecular weight of the native enzyme, estimated by gel permeation, was about 288 KDa, while sodium dodecyl sulfate polyacrylamide gel electrophoresis yielded a relative molecular mass of 48 KDa. The determination of the coenzyme content gave 4 mol of NAD+ and 2 mol of NADH per mol of enzyme. The isoelectric point of native SAHase was at pH 5.1. When assayed in the hydrolytic direction, the Km for S-adenosylhomocysteine and the Vmax of the enzyme for this substrate were 84 microM and 357 mumol/min/mg, respectively; in the synthetic direction, instead, the Km for adenosine and the corresponding Vmax value were 1.6 microM and 37 mumol/min/mg. Substrate analogs were tested for their ability to act as inhibitors and inactivators of the enzyme. Among these compounds, 9-beta-arabinofuranosyl adenine (Ara A) appeared as the most powerful competitive inhibitor (Ki = 18 microM) as well as the strongest time-dependent inactivator. The common feature of all the assayed analogs was the presence of the adenine ring in their molecular structure. It can thus concluded that the presence of the adenine moiety is an essential element in substrate and/or inhibitor interaction with this bacterial enzyme.

Structural analysis of seminal and serum human transferrin by second derivative spectrometry and fluorescence measurements.

Denaturation of human seminal transferrin (HSmT) compared with human serum transferrin (HSrT) was followed to check structural differences between these two proteins. Second derivative UV spectroscopy indicated that treatment with 6 M guanidine hydrochloride (Gnd.HCl) induced greater structural changes in HSrT than in HSmT and, in particular; (i) the exposure value of tyrosinyl residues was almost 2.5-fold higher in native HSmT than in native HSrT; and (ii) a much more pronounced movement of tryptophanyl residues toward a higher polar environment could be noticed in HSrT after incubation with denaturing agent. Fluorescence measurements showed that: (i) a shift of the maximum emission wavelength of HSmT occurred (maximum emission was centered at 333 nm instead of 323 nm as for HSrT; excitation = 280 nm); (ii) the intrinsic tryptophan fluorescence intensity of HSmT increased after 36 hr in the range of 1.5-4.0 M of denaturant, whereas an opposite behavior was found for HSrT in the range 0.0-2.0 M; and (iii) the wavelength maximum of fluorescence emission changed in a biphasic manner for HSrT and, conversely, under the same experimental conditions, HSmT gave a linear and parallel increase of fluorescence emission after 1 and 36 hr. We can conclude that this different behavior of HSmT with respect to HSrT might be due mainly to the fact that both the number and the exposure of tyrosinyl and tryptophanyl residues are different. Lately, these effects are discussed in relationship with the fact that HSmT contains less than half disulphide bridges than HSrT.