Potent slow-binding inhibition of cathepsin B by its propeptide.

A peptide (PCB1) corresponding to the proregion of the rat cysteine protease cathepsin B was synthesized and its ability to inhibit cathepsin B activity investigated. PCB1 was found to be a potent inhibitor of mature cathepsin B at pH 6.0, yielding a Ki = 0.4 nM. This inhibition obeyed slow-binding kinetics and occurred as a one-step process with a k1 = 5.2 x 10(5) M-1 s-1 and a k2 = 2.2 x 10(-4) s-1. On dropping from pH 6.0 to 4.7, Ki increased markedly, and whereas k1 remained essentially unchanged, k2 increased to 4.5 x 10(-3) s-1. Thus, the increase in Ki at lower pH is due primarily to an increased dissociation rate for the cathepsin B/PCB1 complex. At pH 4.0, the inhibition was 160-fold weaker (Ki = 64 nM) than at pH 6.0, and the propeptide appeared to behave as a classical competitive inhibitor rather than a slow-binding inhibitor. Incubation of cathepsin B with a 10-fold excess of PCB1 overnight at pH 4.0 resulted in extensive cleavage of the propetide whereas no cleavage occurred at pH 6.0, consistent with the formation of a tight complex between cathepsin B and PCB1 at the higher pH. The synthetic propeptide of cathepsin B was found to be a much weaker inhibitor of papain, a structurally similar cysteine protease, and no pH dependence was observed. Inhibition constants of 2.8 and 5.6 microM were obtained for papain inhibition by PCB1 at pH 4.0 and 6.0, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Polymerization of τ into Filaments in the Presence of Heparin: The Minimal Sequence Required for τ - τ Interaction

Abstract: Paired helical filaments isolated from the brains of patients with Alzheimer's disease are composed of a major protein component, the microtubule-associated protein termed τ, together with other nonprotein components, including heparan, a glycosaminoglycan, the more extensively sulfated form of which is heparin. As some of these nonprotein components may modulate the assembly of τ into filamentous structures, we have analyzed the ability of the whole τ protein or some of its fragments to self-assemble in the presence of heparin. Different τ fragments, all of them containing some sequences of the tubulin-binding motif, can assemble in vitro into filaments. We have also found formation of polymers with the 18-residue-long peptide corresponding to the third tubulin-binding motif of τ. This suggests that the ability of τ for self-assembly could be localized in a short sequence of amino acids present in the tubulin-binding repeats of the τ molecule.     

Physiological studies and determination of chromosome number in Maca,Lepidium Meyenii (Brassicaceae)

Experiments based on four accessions of maca (Lepdium meyenii) disclosed higher developmental rates in plants grown in neutral pH (6.6) soil when compared with those grown in acidic soil (5.3). Photoperiod response studies revealed similar growth rate for plants grown under either long day or short day condition. Plants in the field and growth chambers completed their life cycle in 11 months or less, therefore maca can be considered an annual crop. These results suggest that the range of adaptation of maca is not as narrow as previously believed, and therefore it can be successfully produced outside its natural habitat. Chromosome counts and predominance of bivalents in diakinesis and metaphase I disclosed that maca is a disomic octoploid of 2n=8x=64 chromosomes. Field and growth chamber observations and morphological uniformity of the plants within accessions indicate that maca relies mainly on self-fertilization for its reproduction.     

Trinuclear iridium and rhodium complexes: the solution to a puzzle involving the multiple coordination possibilities of 1,8-naphthyridine-2-one ligands

The multiple coordination possibilities of 1,8-naphthyridine-2-one (HOnapy) and 5,7-dimethyl-1,8-napthyridine-2-one (HOMe₂napy) ligands allow the synthesis of a variety of tri- di- and mononuclear complexes, showing fluxional behaviour and frequent exchange of the coordinated ML₂ fragments. Thus, reactions of [M₂(μ-OMe)₂(cod)₂] (cod = 1,5-cyclooctadiene) with HOnapy and HOMe₂napy yield the compounds of the general formula [M(μ-OR₂napy) (cod)]_n (M = Ir, R = Me (1a, 1b, H (2); M = Rh, R = Me (3a, 3b). They crystallise as inconvertible yellow (a) and purple/orange (b) forms and also show a puzzling behaviour in solution. X-ray diffraction studies on both forms (3a, 3b) and spectroscopic data reveal that the yellow forms are mononuclear complexes whilst the dark-coloured crystals contain dinuclear complexes. In solution, the nuclearity of the complexes depends on the solvent. In addition both types of complexes are fluxional. The mixed-ligand complexes [M₂(μ-OMe₂napy)₂(CO)₂(cod)] M = Ir (5), Rh (6) have been isolated and characterised; they are found to be intermediates in the synthesis of the trinuclear complexes [M₃(μ₃-OMe₂napy)₂(CO)₂(cod)₂]⁺ M = Rh (8), Ir (9). Reactions of [IrCl(CO)₂(NH₂-p-tolyl] with the complexes [Rh(μ-OR₂napy)(diolefin)]_n followed by addition of a poor donor anion is a general one-pot synthesis for the hetertrinuclear complexes [Rh₂Ir(μ₃-OR₂napy)₂(CO)₂(diolefin)₂]⁺ (R=Me, DIOLEFIN = cod (10), tetrafluorobenzo-barrelene (tfbb) (11), 2,5-norbornadiene (nbd) (12); R=H, DIOLEFIN=cod (13)). This synthesis follows a stepwise mechanism from the mononuclear to the trinuclear complexes in which mixed-ligand heterodinuclear complexes are involved as intermediates of the type [(diolefin)Rh(μ-OMe₂napy)₂Ir(CO)₂]. Heteronuclear complexes which possess the core [RhIr₂]³⁺, such as [RhIr₂(μ₃-OR₂napy)₂(CO)₂(cod)₂]BF₄ (R=Me (14), H (15)), result from the reaction of 1 or 2 with [Rh(CO)₂S_x]⁺ (S = solvent). The trinuclear complexes undergo two chemically reversible one-electron oxidation processes. The chemical oxidation of 10, 14 and 9 with silver salts gives the mixed-valence trinuclear radicals [Rh₂Ir(μ₃-OMe₂napy)₂(CO)₂(cod)₂]²⁺ (16), [RhIr₂(μ₃-OMe₂napy)₂(CO)₂(cod)₂]²⁺ (17) and [Ir₃(μ₃-OMe₂napy)₂(CO)₂(cod)₂]²⁺ (18), which have been isolated as the perchlorate and tetrafluoroborate salts. The EPR spectrum of 16 indicates that the unpaired electron is essentially in an orbital delocalised on the metals. The molecular structures of the complexes 3a, 3b, 6, 10b and 16a are described. Crystals of 3a are triclinic, P-1, with a = 9.7393(2), b = 14.0148(4), c = 16.0607(4) Å, α = 88.122(3), β = 83.924(3), γ = 87.038(3)°, Z = 4; 3b crystallises in the Pna2_i orthorhhombic space group, with a = 16.7541(3), B = 11.7500(8), c = 17.7508(7) Å, Z = 4; complex 6 is packed in the monoclinic space group P2_i/c, a = 9.6371(1), b = 11.8054(4), c = 27.2010(9) Å, β = 90.556(4)°, Z = 4; crystals of 10b are monoclinic, P2₁/n, with a = 17.546(7), b = 13.232(6), c = 17.437(8) Å, β = 106.18(1)°, Z = 4; crystals of 16a are triclinic, P-1, with a = 10.318(4), b = 12.562(6), C = 19.308(8) Å, α = 92.12(8), β = 97.65(9), γ = 90.68(5)°, Z = 2. The five different structures show the coordination versatility of the OMe₂napy molecule as ligand, which behaves as a N,N′-chelating (3a), bidentate N,O-donor (3b, 6), or as a tridentate N,N′,O-donor bridging ligand (10b, 16a).     

The Jurassic succession in west-central Argentina: stratal patterns, sequences and paleogeographic evolution

Jurassic rocks in west-central Argentina are predominantly marine and marginal-marine siliciclastics, associated with prominent but volumetrically subordinate carbonates and evaporites. Facies developments were ruled by paleogeographic persistence within the southern-infratropical latitude belt (within 40–50°S) and by siliciclastic derivation from the Patagonian hinterland to the southwest. Minor volcanic and volcaniclastic supply arrived from the west, out of a magmatic belt related to Circum-Pacific convergence. Timing of the main marine-flooding events and general correspondence with the high-rank stratal packaging recorded in western North America, suggest that global eustasy was also a factor in controlling the local stratigraphic record. Early Jurassic sedimentation occurred within a series of semi-isolated depocentres linked to fault-bounded Triassic troughs. The Sinemurian-Toarcian deposits record depocentre expansion and coalescence. These trends were coeval with progressively more widespread marine invasions from the northwest and west, leading to an elongate marine seaway which connected central Patagonia with the Pacific domain. During the Aalenian-Bajocian the region was subject to a more subdued tectonism and the foreland side of the basin became fringed by an extensive clastic embankment. Bathonian and Early Callovian were times when coarse clastics prograded into the basin, while the marine embayment shrank as a result of stepwise forced regressions. During Late Callovian to Oxfordian globally rising sea-level, the depocentre witnessed the appearance of cosmopolitan invertebrates and a stratal pattern of basin widening and depositional underbalance, that promoted cratonward onlap and inception of widespread carbonate deposition. Ooidal-coralline carbonate development was terminated after a relatively sudden (Messinian-style) event that desiccated a large tract of the Andean basin, and favored massive precipitation of anhydrite. In the course of the Kimmeridgian the evaporite basin was largely flooded by siliciclastics and turned into a broad and featureless mudflat-salina complex, linked to a widespread erg and to an ephemeral drainage system. By the Tithonian, at a period of peak oceanic stand, marine connection was reinstated and recorded as the most widespread Jurassic transgression across the Neuquén-Aconcagua embayment. Shelfal deposition consisted of molluscs and ooid-dominated carbonate terraces that grew in pulses tuned to eustatic fluctuations. Like in other prolific petroleum provinces around the world the Tithonian basinal strata involve widespread euxinic deposits featuring unusually high organic content.