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851.
We have isolated two proteolytic fragments of subfragment 1 (S-1) of myosin from rabbit skeletal muscle. These fragments, identified by their molecular weights of 20 and 50 kDa, may be functional domains that, when isolated, retain their specific function. We have studied several structural and functional features of the 20 and 50 kDa fragments. Considerable secondary structure in both fragments has been observed in CD spectrum studies. Previously CD spectra showed 64% ordered structure for the 20 kDa fragment (Muhlrad and Morales, M.F. (1984) Proc. Natl. Acad. Sci. 81, 1003) and here we show 71% ordered structures for the 50 kDa fragment. Fluorescence lifetime studies of tryptophan residues in the 50 kDa fragment and 1,5-IAEDANS-labeled SH-1 in the 20 kDa fragment are used to investigate the tertiary structure of the fragments. We find the tertiary structure relating to this measurement of both fragments to be intact; however, the reaction of 1,5-IAEDANS with SH-1 on the isolated 20 kDa fragment is less specific than with S-1. Furthermore, the fragments showed a tendency to aggregate. The domain concept of S-1 was supported by the characteristic biochemical function of the isolated fragments. Both of the fragments were effective in competing with S-1 for binding to actin in acto-S-1 ATPase measurements. From these studies and in direct binding measurement the 20 kDa fragment proved to bind with higher affinity to actin than did the 50 kDa fragment.  相似文献   
852.
Several reports have suggested that variations of albumin concentration in the incubation medium can modulate the magnitude of transferrin binding to the cells. We have investigated this problem further using K562 cells. In the absence of human serum albumin, transferrin binding demonstrated a non-saturable curve which, upon Scatchard analysis, showed two components with high and low affinities. In the presence of 0.5% human serum albumin, the low-affinity but not the high-affinity component was totally inhibited and, thus, the binding showed a saturation plateau at transferrin concentration of 6 micrograms/ml. Increasing concentrations of human serum albumin in the incubation medium led to progressive inhibition of transferrin binding, reaching a plateau at 0.2% human serum albumin. At this concentration transferrin binding was about 12 ng/10(6) cells, corresponding to the saturation plateau for high-affinity binding. Low-affinity transferrin binding in the absence of human serum albumin could readily be displaced by subsequent addition of albumin. Similar inhibition was obtained by another serum protein, ceruloplasmin, suggesting that this inhibition is not unique to albumin and may be a common property of all proteins. Incubation at 37 degrees C with 59Fe-labeled transferrin indicated that all iron uptake occurs through high-affinity binding. We conclude that the reported variations in magnitude of transferrin binding by the cell due to variations in albumin concentration are the result of inhibition of low-affinity binding of transferrin by albumin.  相似文献   
853.
Influence of 1,5-(3,3'-dimethylphosphate)diphenoxy-3-oxapentane (DDOP) on conductivity (G) of bilayers of common fabbit brain lipids is studied. It has been found that DDOP increases the bilayer conductivity in the presence of Ca++ and Mg++ (G-maximum at pH = 7.0) they do not act in the presence of K+, Na+. pK'DDOP, pK"DDOP values are equal to 1.2, and 7.7 respectively as determined by titration. Formation of "pseudomacrocyclic" DDOP structure is suggested. The role of Ca++, Mg++ ions seems to consist in lipophilisation of ionized forms of DDOP.  相似文献   
854.
X-irradiated oxymyoglobin (MbO2) exhibits electron spin resonance (ESR) spectra at 77 K due to two distinct [FeO2]-centers formed by electron addition to the dioxygen. In single crystals with 17O and 57Fe isotope enrichment in the heme-ligand complex, the full set of spectral parameters is analyzed for one of the centers (gmax = 2.23, gint = 2.13, gmin = 1.97; HOmax = 2.66 mT, HOint = 1.61 mT, HOmin = 0.57 mT; HFe max = 1.62 mT, HFe int = 0.57 mT, HFe min = 0.49 mT) and the iron-dioxygen spin-density distribution and bonding geometry is derived. The g-tensor is evaluated for the second species at 77 K (gmax = 2.25, gint = 2.11, gmin = 1.95). Both centers transform into secondary species at 180 K for which the g-tensor elements are analyzed (gmax = 2.31, gint = 2.18, gmin = 1.93; gmax = 2.35, gint = 2.21, gmin = 1.91).  相似文献   
855.
Uptake and degradation of lysozyme in the rat kidney were studied in vivo. The protein was labeled with 125I by way of a moiety (tyramine-cellobiose or 'TC') which remained trapped inside the cells even after proteolysis of the peptide chain (in contrast, the label from conventionally labeled proteins escapes after degradation). Following the injection of 'trapped-label' lysozyme, the radioactivity in the kidneys represented the total amount of lysozyme that was taken up during the experiment. Proteolysis could be followed by determining the amount of acid-soluble degradation products. By adding the radioactivity in the urine to that in the kidneys, a measure of the total filtered load was obtained. When only a trace dose of 125I-labeled TC lysozyme was injected into rats, the amount of radioactivity in the kidneys increased on average by 0.09% per min, after the concentration in the blood had become nearly stable. After 100 min, 30% of the injected dose was recovered in the kidneys. The labeled protein was degraded to acid-soluble molecules of Mr less than 1000. There was apparently a 'lag period' between the endocytosis in the kidneys and the start of degradation. 40 min after the injection of a trace dose, about 0.6% of the 'trapped-label' lysozyme in the kidneys was degraded per min.; subsequently, there was a decline in the fraction which was degraded per min. The amount of lysozyme in the urine increased after the injection of increasing amounts of lysozyme, showing that the capacity of the uptake mechanism was being exceeded, but truly saturating levels of lysozyme could not be reached in vivo.  相似文献   
856.
This paper proposes a new estimator of the variance of the Mantel-Haenszel estimator of common odds ratio that is easily computed and consistent in both sparse data and large-strata limiting models. Monte Carlo experiments compare its performance to that of previously proposed variance estimators.  相似文献   
857.
The female reproductive system is exposed to a great variety of environmental stresses. These include many noxious chemicals consumed either intentionally in the form of therapeutic and recreational drugs, or unwittingly as residues in the food we eat or pollutants in the air we breathe. These stresses and noxious agents influence ovarian function through actions at a number of sites and by diverse mechanisms. Sites of action include: the hypothalamo-hypophyseal system, resulting in disruption of the normal pattern of gonadotropin secretion; the ovary, resulting in direct destruction of the oocyte (ovotoxicity) or genetic damage (mutagenicity); and other organs, leading indirectly to altered ovarian function, e.g., through metabolic alterations that change the balance of feedback control of the hypothalamo-pituitary-ovarian system. Susceptibility of the ovaries to the different classes of agents depends on the stage of development at which exposure occurs. Consequences may be temporary and reversible when the source of the "stress" is removed, or permanent if exposure occurs at a "critical stage" in ovarian or hypothalamic differentiation.  相似文献   
858.
T Eckhardt  M Koch 《Blut》1986,53(1):39-48
Fibrinopeptides were measured as direct indices of thrombin, plasmin and elastase in plasma samples obtained from patients with AML. Peptide patterns observed were consistent with spontaneous or drug induced plasmin-specific fibrinogenolysis (AML FAB M 1/3), elastase mediated proteolysis (AML FAB M 3/4) or DIC (AML FAB 4/5). DIC was also observed in septic, agranulocytotic patients.  相似文献   
859.
Microsomal preparations from human kidney were found to contain enzymic activity capable to transfer N-acetylgalactosamine from UDP-N-acetylgalactosamine to native bovine fetuin. The acceptor structures on the fetuin molecules were identified as N- as well as O-linked glycans with a markedly higher incorporation into the N-linked carbohydrate chains. Analysis of the alkali-labile transferase products by thin-layer chromatography indicated that the enzyme is able to synthesize structures having mobilities identical with those found on glycophorin from Cad erythrocytes. Mild acid treatment and enzymic hydrolysis with N-acetylhexosaminidase from jack beans of the N-linked transferase products suggested that beta-D-GalpNAc-(1----4)-[alpha-NeuAc-(2----3)]-beta-D-Galp-(1----s tructures were formed by the enzymic reaction on both N- and O-linked acceptors. The enzyme might, therefore, be involved in the biosynthesis of Sda (and Cad) antigenic structures. By use of various oligosaccharides, glycopeptides, and glycolipids having well characterized carbohydrate sequences, the acceptor-substrate specificity of the N-acetylgalactosaminyltransferase was determined. The enzyme generally recognized alpha-NeuAc-(2----3)-beta-D-Gal groups as acceptors, but in a certain conformation. Thus, tri- and tetra-saccharide alditols, native human glycophorin A, and GM3 were not acceptor substrates although they carry the potential disaccharide acceptor unit. When these structures were presented as sialyl-(2----3)-lactose or as a tryptic peptide from glycophorin A, they were shown to be rather good acceptor substrates for the N-acetyl-beta-D-galactosaminyltransferase from human kidney.  相似文献   
860.
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