摘要:

Nonproductive and productive interactions of lysozyme with some β-aryl di-N-acetylchitobiosides were studied. Difference ultraviolet and proton magnetic resonance spectroscopic methods were used to examine the nature and extent of nonproductive interactions. In the nonproductive binding mode, two glycose residues interact with subsites B and C. Information concerning productive interactions was deduced indirectly from rates of phenol liberation and from product analyses. The productive binding modes which place two glycose residues either at subsites C and D, or at subsites D and E, are consistent with experimental results. The contribution of the aryl aglycone to nonproductive and productive interactions suggests that there is flexibility of substrate specificity in lysozyme catalysis.

ISSN:0008-4018    

DOI:10.1139/o73-001

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Mouse L-cells were grown in medium containing bromodeoxyuridine and fluorodeoxyuridine, and the DNA from these cells was centrifuged to equilibrium in alkaline cesium chloride. The bulk of the bromouracil-substituted DNA had a buoyant density lying between 1.82 and 1.85 g/cc. About 4–5% of bromouracil-substituted DNA was observed as a distinct peak with a buoyant density of 1.79 g/cc. After this DNA was purified by recentrifuging, nucleotide analysis revealed an asymmetric composition in which the base-pairing rules for double-stranded DNA were not obeyed. Comparison of the nucleotide composition with published data indicated that the bromouracil-substituted light strand of mouse satellite DNA had been isolated.

ISSN:0008-4018    

DOI:10.1139/o73-002

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

The isolated, nonworking guinea-pig heart was perfused with14C-labelled glucose, pyruvate, and acetate. Labelled and total alanine, glutamate, aspartate, and glutamine were measured. The alanine content of the heart varied directly with the medium pyruvate concentration. The sum of the concentrations of glutamate and aspartate varied inversely with the alanine concentration. The sum of alanine, glutamate, aspartate, and glutamine in the heart was constant under most perfusion conditions, but loss into the medium increased in the presence of ouabain, low Ca2+concentration, or malonate. The sum of the total amino acids from the heart and medium remained constant under all conditions except in the presence of malonate, when it was decreased.Labelling of amino acids from14C-labelled substrates indicated that alanine, glutamate, and aspartate exchanged readily with their α-oxo acids. However, only 30% of the glutamine exchanged with glutamate during 1 h; this proportion varied little with perfusion conditions or the metabolic flux. The results indicate that in a closed system, most of the changes in the concentrations of amino acids are brought about by transamination.

ISSN:0008-4018    

DOI:10.1139/o73-003

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Pyridoxal phosphate phosphorylasebprepared from native phosphorylasebby the specific incorporation of 1–2 mol of pyridoxal phosphate (PLP) differs from the native enzyme in its absorbance, fluorescence, and kinetic properties. The secondary site of attachment of the PLP group responsible for these changes has been identified as a lysine residue from the formation of PLP-lysine on NaBH4reduction. The peptide containing this derivative was isolated from a pepsin digest of the modified protein by Sephadex G-25 and Chromobeads type P resin column chromatography, followed by rechromatography on the resin after removal of the phosphate by alkaline phosphatase. The amino acid sequence of the purified hexapeptide was determined to be Tyr–(Pxy)Lys–Asn–Pro–Arg–Glu (Pxy, pyridoxyl). A second lysine reacting with PLP was found in the sequence Phe–Glu–Gly-(Pxy)Lys–Glu.

ISSN:0008-4018    

DOI:10.1139/o73-004

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

The Pictet–Spengler condensation, involving interaction of properly activated aromatic compounds having an ethylamine side chain (e.g. dopamine) and aldehydes (e.g. acetaldehyde, aldehyde derivatives of biogenic amines, etc.), leads to the formation of tetrahydroisoquinoline derivatives like salsolinol or tetrahydropapaveroline. It may he arrested in the presence of compounds like ascorbate, cysteine, and glutathione. Cysteine is by far the most effective in preventing this condensation. The mechanism of inhibition is different with each agent: ascorbate and glutathione act either as reducing or complexing agents, while cysteine, which is the most effective of compounds studied so far, is a displacing agent and tends to form thiazolidine derivatives with various substituted aldehydes.

ISSN:0008-4018    

DOI:10.1139/o73-005

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Nonionic celluloses are recommended to replace ionic celluloses as substrates in viscometric determinations of low levels of cellulase. A rapid, sensitive assay is described using hydroxyethylcellulose as substrate, and a correction factor is applied to compensate for deviations from linearity in viscosity measurement. The assay system has been used successfully for the measurement of low levels of extracellular and intracellular cellulases of both fungal and plant-cell origin.

ISSN:0008-4018    

DOI:10.1139/o73-006

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Initial rate kinetic studies of succinyl coenzyme A synthetase ofE.coliin the direction of succinyl-CoA cleavage are consistent with the operation of a partially random sequential kinetic mechanism with initial binding of ADP followed by random association of succinyl-CoA and Pi. The mechanism is analogous to that proposed previously for the succinyl-CoA formation reaction, and thus the kinetic mechanism of the overall reversible succinyl-CoA synthetase reaction appears to be symmetrical.Studies of the kinetics ofisotope exchange at equilibrium show that this partially random sequential kinetic mechanism is not an exclusive pathway.isotope exchange rates did not show complete substrate inhibition when CoA or succinate was varied in constant ratio with Pi. However, when CoA or succinate was varied in constant ratio with succinyl-CoA, nearly complete substrate inhibition was observed. These results can be interpreted in terms of a wide variety of minor pathways of substrate binding and product release available to the enzyme under various conditions.

ISSN:0008-4018    

DOI:10.1139/o73-007

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Previous studies have demonstrated that rabbit skeletal tropomyosin consists of two or more chemically non-identical but highly homologous polypeptide chains. Attempts by a variety of techniques to prepare pure tropomyosin chains in amounts adequate for chemical characterization have been unsuccessful to date. To provide more extensive information for the purpose of elucidating the relationship between amino acid sequence and the coiled-coil structure of tropomyosin, a cyanogen bromide treatment of theS-carboxymethylated protein was carried out. The fragments were separated into small and large components by gel filtration on Sephadex G-50. The small fragments were fractionated by ion-exchange chromatography and electrophoresis on paper and their sequences elucidated by conventional methods. Coupled with previous data, these results indicate a minimum of seven unique methionine sequences and are consistent with a high degree of homology in the tropomyosin polypeptide chains. From the mixture of the larger cyanogen bromide polypeptides, a fragment was isolated by ion-exchange chromatography on QAE-Sephadex. In aqueous buffer it had a molecular weight of 35 000 and an α-helical content of about 60% as estimated by circular dichroism. In 8 Murea its molecular weight was reduced to 15 000, a value in reasonable agreement with a minimal molecular weight of 17 000 calculated from its amino acid composition. From its histidine content (two residues) and the known COOH-terminal amino acid sequence of the protein, the fragment was concluded to be derived from the COOH-terminal half of the molecule. These results are consistent with a degree of 'coiled-coil' structure in a fragment representing about one-half of the tropomyosin molecule.

ISSN:0008-4018    

DOI:10.1139/o73-008

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

The thermal denaturation of myosin and actomyosin was studied by active site analysis (enzymatic activity) and measurements were related to overall conformational changes (viscosity) over the temperature range 19–65 °C. The role of sulfhydryl (SH) groups on the temperature-induced denaturation of actomyosin was investigated. The temperature of maximum change in the overall conformation (the melting temperature,TM) was unaffected by the binding of F-actin to myosin. For both myosin and actomyosin theTMwas 43 ± 2 °C. However, the range of temperature over which large conformational changes were observed was affected by the binding of F-actin to myosin. For myosin and dissociated actomyosin, changes were observed between 37 and 50 °C, while for actomyosin large changes were observed between 20 and 50 °C. With actomyosin there was an irreversible increase in titratable sulfhydryl groups from 19 to 60 °C. Temperature effects on the calcium-activated ATPase were studied. The temperature of maximum enzymatic activity for actomyosin was 45 ± 2 °C, which corresponds to theTMand the temperature at which increases in SH content were apparent. However, for myosin the temperature of maximum enzymatic activity was 33 °C, considerably below theTM. Overall conformational changes were reversible below theTM, while changes in enzymatic activity were reversible below the temperature of maximum enzymatic activity. Actin offers considerable protection to the temperature inactivation of the active site of myosin even though the F-actin–myosin complex is very highly dissociated in the presence of ATP. However, there is no significant stabilization of myosin by F-actin in terms of the temperature sensitivity of the overall conformation.

ISSN:0008-4018    

DOI:10.1139/o73-009

出版商:NRC Research Press    

年代:1973

数据来源: NRC

摘要:

Na+, K+, Mg2+, Cu2+, Zn2+, and Mn2+were determined in several regions of rat brain, using atomic absorption spectroscopy. Cu2+was highest in the hypothalamus and lowest in the medulla oblongata. Zn2+was also low in the medulla oblongata and highest in the hippocampal region. Mn2+was found in high concentration in the hypothalamus.

ISSN:0008-4018    

DOI:10.1139/o73-010

出版商:NRC Research Press    

年代:1973

数据来源: NRC