[Chemical constituents of Cassia occidentalis].

Compounds(1-6) were isolated and identified from 90% ethanol extract of the stems and leaves of Cassia occidentalis through column chromatography with silica gel, ODS, and Sephadex LH-20. These compounds were identified as 7-hydroxy-5-(3-hydroxy-2-oxopropyl)-2-methyl-4H-chromen-4-one(1), saccharonol A(2), S-6-hydroxymullein(3), 2-methyl-5-acetonyl-7-hydroxy-chromone(4), 2-(2'-hydroxypropyl)-5-methyl-7-hydroxychromone(5) and 7,4'-dihydroxyflavone(6) based on their physicochemical and spectroscopic data. Among them, compound 1 was a new compound, and all the compounds were isolated from this plant for the first time. DPPH method was employed to determine the antioxidant activities of these compounds in vitro. Six compounds exhibited weak antioxidant activities.

Consequences of a six residual deletion from the N-terminal of rabbit muscle creatine kinase.

A mutant of dimeric rabbit muscle creatine kinase (CK), in which six residues (residues 2-7) at the N-terminal were removed by the PCR method, was studied to assess the role of these residues in dimer cohesion and to determine the structural stability of the protein. The specific activity of the mutant was 70.39% of that of the wild-type CK, and the affinity for Mg-ATP and CK substrates was slightly reduced compared with the wild-type protein. The structural stability of the mutant was investigated by a comparative equilibrium urea denaturation study and a thermal denaturation study. The data acquired by intrinsic fluorescence and far-UV circular dichroism (CD) during urea unfolding indicated that, the secondary and tertiary structures of the mutant were more stable than those of wild-type CK. Furthermore, results of 8-anilino-1-naphthalene-sulfonic acid (ANS) fluorescence demonstrated that the hydrophobic surface of the mutant CKND(6) was more stable during urea titration. Data from size exclusion chromatography (SEC) experiments indicated that deletion of the six N-terminal residues resulted in a relatively loose molecular structure, but the dissociation of the mutant CKND(6) occurred later during the unfolding process than for wild-type CK. Consistent with this result, the differential scanning calorimetry (DSC) profiles demonstrated that the thermal stability of the enzyme was increased by removal of the six N-terminal residues. We conclude that a more stable quaternary structure was obtained by deletion of the six residues from the N-terminal of wild-type CK.

Urea induced inactivation and unfolding of arginine kinase from the sea cucumber Stichopus japonicus.

Urea titration was used to study the inactivation and unfolding equilibrium of arginine kinase (AK) from the sea cucumber Stichopus japonicus. Both fluorescence spectral and circular dichroism spectral data indicated that an unfolding intermediate of AK existed in the presence of 1.0 to 2.0 M urea. This was further supported by the results of size exclusion chromatography. The spectral data suggested that this unfolding intermediate shared many structural characteristics with the native form of AK including its secondary structure, tertiary structure, as well as its quaternary structure. Furthermore, according to the residual activity curve, this unfolding intermediate form still retained its catalytic function although its activity was lower than that of native AK. Taken together, the results of our study give direct evidence that an intermediate with partial activity exists in unfolding equilibrium states of AK during titration with urea.