Alkaloids from Corydalis decumbens modulate neuronal excitability.

Eight new alkaloids, including five isoquinoline alkaloids, a benzoazepine alkaloid, two isoindole alkaloids, and three synthetic alkaloids firstly obtained from the natural sources, together with three known ones were isolated from the bulbs of Corydalis decumbens. The structures were determined by analysis of their spectroscopic data and single-crystal X-ray diffraction. This is the first report of isoindole alkaloid and benzoazepine alkaloid from the genus Corydalis. Full NMR data for 9-11 are reported here for the first time. Moreover, the ability to modulate neuronal Ca2+ mobilization of the isolated alkaloids was tested in primary cultured neocortical neurons. Compound 7 inhibited spontaneous Ca2+ oscillations in primary neocortical neuron cultures at low micromolar concentrations.

Phthalideisoquinoline Hemiacetal Alkaloids from Corydalis decumbens That Inhibit Spontaneous Calcium Oscillations, Including Alkyl Derivatives of (+)-Egenine That Are Strikingly Levorotatory.

The new phthalideisoquinoline hemiacetal alkaloids (2-7) and the known analogues (1 and 8) were isolated from the bulbs of Corydalis decumbens. The new compounds were characterized by analysis of their NMR spectroscopic data, chemical degradation syntheses, X-ray crystallography, and comparison of experimental and calculated ECD data. All the isolates were screened in vitro for inhibitory activity of spontaneous calcium oscillations in primary cultured neocortical neurons. Compounds 1-3 and 5-7 were found to be active in the suppression of spontaneous calcium oscillations with IC50 values of 6.8, 5.6, 11.6, 10.2, 8.3, and 3.1 µM, respectively. It was also observed that the presence of hydroxy, methoxy, and ethoxy groups at the remote stereogenic center C-7' of some isolated phthalideisoquinoline hemiacetal alkaloids could alter the preferred conformation and invert the sign of optical rotation, rather than this resulting from configurational isomerism at C-1 or C-9, and that the 3J1,9 coupling constants of these analogues varied accordingly. For example, compounds 1 and 6 are levorotatory, despite these molecules having the same carbon skeleton and absolute configuration as (+)-egenine. This emphasizes the potential risk of incorrectly assigning absolute configuration based only on observed coupling constants or optical rotation when comparing the data of new compounds with literature values for known analogues, especially within this class of molecules.

New phthalideisoquinoline hemiacetal alkaloid derivatives from Corydalis decumbens.

Two new phthalideisoquinoline hemiacetal alkaloid derivatives, named corybensines A and B (1 and 2), and four known alkaloids (3-6) were isolated from the bulbs of Corydalis decumbens. Their structures were characterized by analysis of 1D/2D NMR and ECD data, quantum chemical ECD calculations, and X-ray diffraction analysis. Among them, compound 2 represents the first naturally occurring phthalideisoquinoline hemiacetal alkaloid derivative with a 2-pyrrolidinone moiety. The activity of the isolated compounds towards neuronal excitability was examined.

Equilibrium folding of porcine insulin precursor in the presence of redox buffer: implications for the common intermediates shared by its unfolding/ refolding processes.

We use the procedure established for 'disulfide stability analysis in redox system' to investigate the unfolding process of porcine insulin precursor (PIP). Six major unfolding intermediates have been captured, in which four contain two disulfides, two contain one disulfide. Based on the characterization and analysis of the intermediates an unfolding pathway has been proposed, by which the native PIP unfolded through in turn 2SS and 1SS intermediates into fully reduced form. Besides, the comparison of the intermediates captured in PIP unfolding process with those intermediates captured in its refolding process revealed that some intermediates captured during both unfolding/refolding processes of PIP have identical disulfide pairing pattern, from which we suggest that the unfolding/refolding processes of PIP share some common intermediates but flow in the opposite direction.

In vitro unfolding of insulin: characterization of intermediates and putative unfolding pathway.

The in vitro insulin unfolding had been studied using the "equilibrium unfolding" method where protein is unfolded by reducing reagents in the presence of trace amounts of oxidants such as oxidized glutathione. Nine intermediates were captured in the unfolding process, named as P1A, P2A, P3A, P4A, P3B, P4B, P5B, P6B, and P7B, which were all either A chain derivatives or B chain derivatives. No intermediate with inter-A-B chain disulfide was captured. Based on the character of the intermediates, their distribution during the unfolding process and the hypothetic "transient" intermediates, an in vitro putative unfolding pathway of insulin had been proposed. Besides, the comparison of the intermediates captured in unfolding with the intermediates captured in the refolding process of insulin revealed that both unfolding/refolding processes of insulin shared common intermediates. Based on these observations we suggested that the unfolding pathway of insulin was similar to the refolding pathway but flowed in the opposite direction.

The sequence determinant causing different folding behaviors of insulin and insulin-like growth factor-1.

Although insulin and insulin-like growth factor-1 (IGF-1) belong to the insulin superfamily and share highly homologous sequences, similar tertiary structure, and a common ancestor molecule, amphioxus insulin-like peptide, they have different folding behaviors: IGF-1 folds into two thermodynamically stable tertiary structures (native and swap forms), while insulin folds into one unique stable structure. To further understand which part of the sequence determines their different folding behavior, based on previous reports from the laboratory, two peptide models, [B9A][1-4]porcine insulin precursor (PIP) and [B10E][1-4]PIP, were constructed. The plasmids encoding the peptides were transformed into yeast cells for expression of the peptides; the results showed that the former peptide was expressed as single component, while the latter was expressed as a mixture of two components (isomer 1 and isomer 2). The expression results together with studies of circular dichoism, disulfide rearrangement, and refolding lead us to deduce that isomer 1 corresponds to the swap form and the isomer 2 corresponds to the native form. We further demonstrate that the sequence 1-4 plus B9 of IGF-1 B-domain can make PIP fold into two structures, while sequence 1-5 of insulin B-chain can make IGF-1 fold into one unique structure. In other words, it is the IGF-1 B-domain sequence that 1-4 allows IGF-1 folding into two thermodynamically stable tertiary structures; this sequence plus its residue B9E can change PIP folding behavior from folding into one unique structure to two thermodynamically stable structures, like that of IGF-1.