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The degree of racemization in the coupling reaction, Pro-Val + Pro, with the several other methods than the activated ester methods was measured and the results were compared with that in the coupling reaction, Leu-Phe + Val, as well as in the previous paper.1) In the azide and the mixed anhydride methods, no or almost no racemization was detected, whereas in the other tested methods of peptide synthesis (Pachornik-, DCCD- and phosphazo-methods) the significantly large racemization was observed. It can be attributed to the strong nucleo- philic N-atom in the penultimate amino acids (Pro) and the steric hindrance of C-terminal amino acid (Val), which are favourable to the formation of the oxazolone ring.

This assumption was further systematically confirmed in the synthesis of the other several tripeptides with the DCCD method. The separation of the diastereoisomers (LLL and LDL) of the resulting tripeptides by gas chromatography with a packed column was also here reported.  相似文献   
3.
A large fraction of the Ca-2plus- and Mg-2plus-dependent ATPase (EC 3.6.1.3) in sarcoplasmic reticulum membranes solubilized with Triton X-100 was phosphorylated with Pi. The phosphorylation required Mg-2plus but was strongly inhibited by low concentrations of Ca-2plus. A Ca-2plus ion concentration of 30 muM caused half-maximum inhibition in the presence of 50 mM MgCl2. The phosphorylated enzyme showed a rapid turnover and was in dynamic equilibrium with Pi in the medium. At equilibrium the amount of the phosphorylated enzyme increased markedly with increased in the reaction temperature. The apparent standard free energy change, the apparent standard enthalpy change, and the apparent standard entropy change in the formation of the phosphorylated enzyme from the enzyme-phosphate complex in the presence of excess Mg-2plus at 37 degrees and pH 7.0 were, respectively, 0.35 Cal per mol, 15.9 Cal per mol, and 50.2 e.u. per mol. The susceptibility of the acid-denatured phosphorylated enzyme to hydroxylamine showed that the phosphorylated enzyme is of an acyl phosphate type. The present results are consistent with the probability that the phosphorylation results from reversal of late steps in the Ca-2plus transport process. The results clearly show that the phosphorylated enzyme is stabilized by a great increase in entropy upon its formation from the enzyme-phosphate complex.  相似文献   
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Sarcoplasmic reticulum vesicles were modified with a fluorescent thiol reagent, N-iodoacetyl-N'-(5-sulfo-1-naphthyl)ethylenediamine. One mol of readily reactive thiols per mol of the Ca2+-ATPase was labeled without a loss of the catalytic activity. The fluorescence of the label increased by 8% upon binding of Ca2+ to the high affinity sites of the enzyme. This fluorescence enhancement probably reflects a conformational change responsible for Ca2+-induced enzyme activation. Upon addition of ATP to the Ca2+-activated enzyme, the fluorescence decreased by 15%. This fluorescence drop and formation of the phosphoenzyme intermediate were determined under the same conditions with a stopped-flow apparatus and a rapid quenching system. The amplitude of the fluorescence drop thus determined was saturated with 3 microM ATP. This shows that the fluorescence drop was caused by ATP binding to the catalytic site. In contrast, the rate of the fluorescence drop was not saturated even with 50 microM ATP. The fluorescence drop coincided with phosphoenzyme formation at 0.5 or 3 microM ATP, but it became much faster than phosphoenzyme formation when the ATP concentration was raised to 100 microM. These results indicate that the ATP-induced fluorescence drop reflects a conformational change in the enzyme.ATP complex. The fluorescence drop was accompanied by a red spectrum shift, which suggests that the label was exposed to a more hydrophilic environment. The electrophoretic analysis of the tryptic digest of the labeled enzyme (10.9 kDa) showed that almost all of the label was located on the 5.2-kDa fragment which includes the carboxyl terminus and the putative ATP-binding domain. The sequencing of the two major labeled peptides, which were isolated from the thermolytic digest of the labeled enzyme, revealed that the labeled site in either of these peptides was Cys674. It seems likely that the label bound to this Cys674 could be involved in the observed fluorescence changes.  相似文献   
6.
Incubation of insulin-treated rat adipocytes with chloroquine, in a time- and concentration-dependent manner, was observed to inhibit the insulin-stimulated increase in insulin-like growth factor II (IGF-II) binding activity, whereas no significant change in IGF-II binding was observed in the absence of insulin. The incremental increase of insulin-stimulated IGF-II binding was inhibited 50% by 0.2 mM chloroquine within 15 min and was nearly completely abolished by 60 min. Interestingly, IGF-II binding was never observed to decrease below the binding value in cells without insulin treatment even when incubation was extended to 180 min. Scatchard analysis of IGF-II binding as well as the specific binding of an anti-IGF-II receptor antibody demonstrated that the loss of IGF-II binding in the insulin-stimulated chloroquine-treated adipocytes was due to a decrease in the number of cell-surface IGF-II receptors, whereas the total number of cellular IGF-II receptors was unaltered. The effect of chloroquine was observed to be reversible, temperature-dependent, and sensitive to the metabolic poison KCN. Furthermore, NH4Cl was also observed to inhibit insulin-stimulated increase in IGF-II binding. In contrast, chloroquine or NH4Cl did not inhibit the basal or insulin-stimulated glucose transport activity. Photoaffinity labeling of the glucose transporter with [3H]cytochalasin B also demonstrated that the basal and insulin-stimulated subcellular distribution of the glucose transporters was unaltered by chloroquine treatment. These results suggest that 1) insulin induces a constitutive, acidotropic agent-sensitive recycling of IGF-II receptor and 2) the glucose transporter and IGF-II receptor do not share the same insulin-regulated intracellular trafficking pathways.  相似文献   
7.
Hepatic granulomas of euthymic (nu/+) mice infected with Schistosoma mansoni were freeze-dried or freeze-thawed 3 times and transplanted subcutaneously into naive nu/+ and athymic (nu/nu) mice. The grafted sites, studied histologically, showed formation of organized granulomas in nu/+ mice similar to donor granulomas as observed after grafting of freshly isolated granulomas. On the other hand, in nu/nu mice, the nonviable transplants elicited small and disorganized granulomas, like hepatic granulomas in nu/nu mice with schistosomiasis, but different from fresh nu/+ transplants in nu/nu skin. The findings indicate viable cells are not required for transfer of granulomatous reactions, but T cells are needed for full expression.  相似文献   
8.
Summary Liposomes composed of chemically synthesized glyceroglycolipids, such as 1,2-dipalmityl-[-cellobiosyl-(1 3)]-glycerol (Cel-DAG), 1,2-dipalmityl-[-lactosyl-(1 3)]-glycerol, or 1,2-dipalmityl-[-maltosyl-(1 3)]-glycerol, were found to enhance protective immunity against transplantable tumor cells (sarcoma 180) in ICR mice. Peritoneal exudate cells prepared from mice treated in vivo with Cel-DAG showed cytostatic activity in vitro against the mouse leukemia cell line, EL-4. Adherent cells separated from this preparation showed similar activity. Peritoneal cells from polypeptone-injected mice acquired appreciable cytostatic activity when incubated in vitro in the presence of glyceroglycolipid liposomes. The adherent cell fraction alone showed rather weak cytostatic activity when pretreated with the glyceroglycolipids, and full activity was restored by supplementing with the nonadherent cell fraction. The ability of glycolipids to induce tumoricidal effects was affected by cholesterol content: with increasing cholesterol content, the activities decreased. Cholesterol-free glycolipid liposomes were taken more efficiently by macrophages than cholesterol-containing liposomes. Cholersterol modifies the surface property of glyceroglycolipid liposomes. Activation of macrophages is responsible for enhancement of protective immunity against tumor cells by injection of these glycolipids in vivo.This work was supported in part by Grants-in-Aid (Nos. 58010010, and 59870076) for Scientific Research from the Ministry of Education, Science and Culture of Japan  相似文献   
9.
Electrophoretic mobilities of plant protoplasts from varioussources were measured, as a function of the pH of the medium,by a micro-electrophoresis technique to characterize the protoplastsin terms of curves of zeta potential vs. protoplast surfacepH (pHs). The shape of the curves of zeta potential vs. pHscurves differed among preparations of protoplasts isolated fromvarious species and strains. The isoelectric points (pI) ofthe protoplasts measured in this study were between 3.0 and4.0. These differences among the protoplasts suggest that itmay be possible to develop an electrophoretic method for theselection of protoplasts. The shape of the curves of zeta potentialvs. pHs also indicated that carboxyl groups, as well as phosphategroups, may contribute to the negative charges on the surfaceof protoplasts. (Received October 14, 1988; Accepted February 22, 1989)  相似文献   
10.
The phase transitions in fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases have been studied by lowangle time-resolved x-ray diffraction under conditions similar to those employed in calorimetry (scan rates 0.05-0.5°C/min and uniform temperature throughout the samples). This approach provides more adequate characterization of the equilibrium transition pathways and allows for close correlations between structural and thermodynamic data. No coexistence of the rippled gel (Pβ') and liquid-crystalline (Lα) phases was found in the main transition of DPPC; rather, a loss of correlation in the lamellar structure, observed as broadening of the lamellar reflections, takes place in a narrow temperature range of ~100 mK at the transition midpoint. Formation of a long-living metastable phase, denoted by Pβ'(mst), differing from the initial Pβ' was observed in cooling direction by both x-ray diffraction and calorimetry. No direct conversion of Pβ'(mst) into Pβ' occurs for over 24 h but only by way of the phase sequence Pβ'(mst) → Lβ' → Pβ'. According to differential scanning calorimetry (DSC), the enthalpy of the Pβ'(mst)-Lα transition is by ~5% lower than that of the Pβ'-Lα transition. The effects of ethanol (Rowe, E. S. 1983. Biochemistry. 22:3299-3305; Simon, S. A., and T. J. McIntosh. 1984. Biochim. Biophys. Acta 773:169-172) on the mechanism and reversibility of the DPPC main transition were clearly visualized. At ethanol concentrations inducing formation of interdigitated gel phase, the main transition proceeds through a coexistence of the initial and final phases over a finite temperature range. During the subtransition in DPPC recorded at scan rate 0.3°C/min, a smooth monotonic increase of the lamellar spacing from its subgel (Lc) to its gel (Lβ') phase value takes place. The width of the lamellar reflections remains unchanged during this transformation. This provides grounds to propose a “sequential” relaxation mechanism for the subgel-gel transition which is not accompanied by growth of domains of the final phase within the initial one.  相似文献   
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