Chemical modifications of PhTX-I myotoxin from Porthidium hyoprora snake venom: effects on structural, enzymatic, and pharmacological properties.

We recently described the isolation of a basic PLA2 (PhTX-I) from Porthidium hyoprora snake venom. This toxin exhibits high catalytic activity, induces in vivo myotoxicity, moderates footpad edema, and causes in vitro neuromuscular blockade. Here, we describe the chemical modifications of specific amino acid residues (His, Tyr, Lys, and Trp), performed in PhTX-I, to study their effects on the structural, enzymatic, and pharmacological properties of this myotoxin. After chemical treatment, a single His, 4 Tyr, 7 Lys, and one Trp residues were modified. The secondary structure of the protein remained unchanged as measured by circular dichroism; however other results indicated the critical role played by Lys and Tyr residues in myotoxic, neurotoxic activities and mainly in the cytotoxicity displayed by PhTX-I. His residue and therefore catalytic activity of PhTX-I are relevant for edematogenic, neurotoxic, and myotoxic effects, but not for its cytotoxic activity. This dissociation observed between enzymatic activity and some pharmacological effects suggests that other molecular regions distinct from the catalytic site may also play a role in the toxic activities exerted by this myotoxin. Our observations supported the hypothesis that both the catalytic sites as the hypothetical pharmacological sites are relevant to the pharmacological profile of PhTX-I.

Amyloid fibril formation by circularly permuted and C-terminally deleted mutants.

The natural N- and C-termini, i.e., the given order of secondary structure segments, are critical for protein folding and stability, as shown by several studies using circularly permuted proteins, mutants that have their N- and C-termini linked and are then digested at another site to create new termini. A previous work showed that circularly permuted mutants of sperm whale myoglobin (Mb) are functional, have native-like folding and bind heme, but are less stable than the wild-type protein and aggregate. The ability of wild-type myoglobin to form amyloid fibrils has been established recently, and because circularly permuted mutations are destabilizing, we asked whether these permutations would also affect the rate of amyloid fibril formation. Our investigations revealed that, indeed, the circularly permuted mutants formed cytotoxic fibrils at a rate higher than that of the wild-type. To further investigate the role of the C-terminus in the overall stability of the protein, we investigated two C-terminally deleted mutant, Mb(1-123) and Mb(1-99), and found that Mb(1-123) formed cytotoxic fibrils at a higher rate than that of the wild-type while Mb(1-99) formed cytotoxic fibrils at a similar rate than that of the wild-type. Collectively, our findings show that the native position of both the N-and C-termini is important for the precise structural architecture of myoglobin.

Dehydrocrotonin and its beta-cyclodextrin complex: cytotoxicity in V79 fibroblasts and rat cultured hepatocytes.

Trans-dehydrocrotonin has antiulcerogenic and antitumor activities. A complex of beta-cyclodextrin with dehydrocrotonin was developed to improve the delivery of dehydrocrotonin. Complex in solid state was evaluated using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM). X-ray diffraction and scanning electron microscopy studies showed that dehydrocrotonin exists in a semicrystalline state in the complexed form with beta-cyclodextrin. Differential scanning calorimetry studies showed the existence of a complex of dehydrocrotonin with beta-cyclodextrin. The thermal gravimetric analysis studies confirmed the differential scanning calorimetry results of the complex. Free dehydrocrotonin and the dehydrocrotonin/beta-cyclodextrin inclusion complex were assayed in freshly isolated rat hepatocytes and in V79 cells. Cytotoxicity was determined using nucleic acid content, methylthiazoletetrazolium (MTT) reduction and neutral red uptake assays. In all assays, there was a large reduction (3.5-16.1-fold) in the cytotoxicity of dehydrocrotonin in hepatocytes when complexed with beta-cyclodextrin, whereas for V79 cells the decrease in cytotoxicity was 1.7- and 1.87-fold for MTT reduction and nucleic acid content assays, respectively. The lower cytotoxicity of the dehydrocrotonin/beta-cyclodextrin complex compared to free dehydrocrotonin in rat hepatocytes and V79 cells suggests that such a complex may be useful for the administration of dehydrocrotonin in vivo.