ECOLOGICAL CORRELATES OF REPRODUCTIVE TRAITS OF MEXICAN RAIN FOREST TREES

Sexual systems of 139 tree species from a tropical rain forest at Los Tuxtlas, Mexico were investigated to: 1) estimate the relative proportions of hermaphroditic, monoecious, and dioecious species; 2) describe flowers, fruits, and seeds in terms of size and weight; 3) describe flowering and fruiting phenology; and 4) correlate sexuality to pollination and dispersal syndromes, and the successional status occupied in the forest. Hermaphroditism occurred in 63% of the species, monoecism in 9%, and dioecy in 27%. Nondioecious species had larger flowers, but dioecious species had more seeds per fruit. The dioecious condition was associated with small flowers pollinated with unspecialized insects and fleshy fruits dispersed by animals at both species and generic levels. Reproductive traits were more correlated among nondioecious species than dioecious species. Pioneer species had more seeds per fruit, and longer flowering and fruiting periods, but persistent species produced heavier seeds and fruits. Flower and fruit morphological traits, sexual systems, and tree guilds are related in a comprehensive way, and a flow model based on data from this study is presented.     

Comparative study of the genetic effect of various tritium compounds in male mice

Comparative study of the genetic effect of some tritiated biogenic++ compounds and tritiated water was carried out. Reciprocal translocations induced in mice stem spermatogonia by 3H-glucose, 3H-glycine and 3H-lysine occur with similar frequency, whereas the difference in values of the absorbed doses of beta-irradiation in some cases was about two-fold. The highest genetic effect per unit of activity of 3H-nucleosides (3H-TdR and 3H-CdR) was revealed at lowest activities. The frequency of reciprocal translocations induced by 3H-TdR was twice as that induced by 3H-CdR.     

Effect of different doses of chromium picolinate on protein metabolism in infant rats.

This 12-day study was conducted to evaluate the effects of three different levels of dietary chromium (100, 200, and 500 microg/day) in the form of chromium picolinate (CrPic) on growth and protein use in weaned rats. No significant effect of CrPic on body weight gain, food intake, or food conversion rate was observed. Elevated doses of CrPic seemed to increase muscle mass, either by stimulating protein anabolism by activation of insulin by chromium or by lowering protein degradation. However, these effects had no repercussions on overall growth, suggesting that any anabolic effect of chromium due to the action of insulin was probably marginal.     

Litter quality and nutrient cycling affected by grazing-induced species replacements along a precipitation gradient

One of the potential mechanisms for the impact of herbivores on nutrient cycling is the effect of selective grazing on litter quality through changes in species composition. However, the scarce evidence collected on this mechanism is controversial and seemingly influenced by site-specific variables. In this paper, we explored the consequences of grazing-induced changes in species composition on litter quality and nitrogen cycling with a regional perspective. Along a 900-mm of mean annual rainfall gradient, we selected species promoted and diminished by grazing from three natural rangelands of Argentina, analyzed their litter quality, and determined their decomposition and nutrient release kinetics under common greenhouse conditions. Litter quality and decomposition rates were strongly associated with plant response to grazing. However, the magnitude and direction of these differences depended on the ecosystem considered. In the wettest site, the species promoted by grazing (forbs) had higher nitrogen and phosphorus contents, faster decomposition rates, and higher release of nitrogen to the soil than species diminished by grazing (C₃ and C₄ grasses). In the intermediate and dry sites, species promoted by grazing had lower nitrogen and phosphorus contents, and slower decomposition rates than those diminished by grazing (C₃ grasses in both cases). Decomposition of the entire group of species was not correlated with mean annual rainfall, but when litter of the species diminished by grazing was analyzed, it was negatively correlated with precipitation. Nitrogen was immobilized more often than mineralized, even after one year of incubation. Immobilization was negatively correlated with precipitation. All these results indicate that grazing may significantly alter nutrient cycling by affecting litter quality through changes in species composition. These effects seem to be larger when species replacements induced by grazing either involve functional groups, as it was the case in our wettest site, or change root to shoot ratios. Therefore, the functional groups involved in the replacement of species as well as shifts between belowground and aboveground allocation should play a key role in grazing-induced changes on nitrogen cycling.     

Conformational changes induced by cloxacillin in class a beta-lactamase from Bacillus cereus.

Class A beta-lactamases are enzymes that hydrolyse beta-lactam antibiotics such as penicillins and cephalosporins. They also hydrolyse substrate analogues such as oxacillin and cloxacillin, with a biphasic kinetic as it has been reported for Bacillus cereus beta-lactamase. A molecular model of Bacillus cereus beta-lactamase was built and the conformational changes that the substrates benzylpenicillin and cloxacilline produced in the conformation of selected regions of the protein were analyzed. This study was performed using the docking of the substrates, their tetrahedral intermediates and the corresponding acids on the active site, followed by molecular dynamic and subsequent optimisation procedures. The most important changes were produced on Tyr105 and Tyr273, when the tetrahedral intermediate of cloxacillin was docked at the active site, these amino acids are partially responsible for the stabilisation of the substrates at the active site. These changes may explain the kinetic differences observed during the hydrolysis of substrates type S and type A by beta-lactamases class A.     

Selectivity of pyruvate kinase for Na+ and K+ in water/dimethylsulfoxide mixtures.

In aqueous media, muscle pyruvate kinase is highly selective for K+ over Na+. We now studied the selectivity of pyruvate kinase in water/dimethylsulfoxide mixtures by measuring the activation and inhibition constants of K+ and Na+, i.e. their binding to the monovalent and divalent cation binding sites of pyruvate kinase, respectively [Melchoir J.B. (1965) Biochemistry 4, 1518-1525]. In 40% dimethylsulfoxide the K0.5 app for K+ and Na+ were 190 and 64-fold lower than in water. Ki app for K+ and Na+ decreased 116 and 135-fold between 20 and 40% dimethylsulfoxide. The ratios of Ki app/K0.5 app for K+ and Na+ were 34-3.5 and 3.3-0.2, respectively. Therefore, dimethylsulfoxide favored the partition of K+ and Na+ into the monovalent and divalent cation binding sites of the enzyme. The kinetics of the enzyme at subsaturating concentrations of activators show that K+ and Mg2+ exhibit high selectivity for their respective cation binding sites, whereas when Na+ substitutes K+, Na+ and Mg2+ bind with high affinity to their incorrect sites. This is evident by the ratio of the affinities of Mg2+ and K+ for the monovalent cation binding site, which is close to 200. For Na+ and Mg2+ this ratio is approximately 20. Therefore, the data suggest that K+ induces conformational changes that prevent the binding of Mg2+ to the monovalent cation binding site. Circular dichroism spectra of the enzyme and the magnitude of the transfer and apparent binding energies of K+ and Na+ indicate that structural arrangements of the enzyme induced by dimethylsulfoxide determine the affinities of pyruvate kinase for K+ and Na+.     

Angiotensinase activity in hypothalamus and pituitary of hypothyroid, euthyroid and hyperthyroid adult male rats.

A local renin-angiotensin system (RAS) that may be involved in their regulatory functions has been identified in hypothalamus and pituitary. Altered thyroid status induces modifications in the secretory function of hypothalamus and pituitary. However, few studies have analyzed the role of the RAS in hypothalamus and, to our knowledge, there is no data on the pituitary RAS during thyroid dysfunction. In the present study, angiotensinase activities (glutamyl, aspartyl and alanyl aminopeptidase: GluAP, AspAP and AlaAP, respectively) were studied in hypothalamus and in the anterior and posterior lobes of pituitary of euthyroid, hypothyroid and hyperthyroid adult male rats. In the anterior pituitary, compared with euthyroid and hyperthyroid rats, hypothyroid animals showed a highly significant increase of GluAP and AspAP activities; the percentage increase in GluAP was markedly higher than the percentage increase in AspAP. This suggests an increased metabolism of angiotensin (Ang) I and Ang II to des-Asp 1-Ang I and Ang III, respectively. We also observed an increase of Ang III-degrading activity (AlaAP) in the hypothalamus of hyperthyroid rats in soluble fraction. Increased Ang I and Ang II metabolism in the anterior pituitary of hypothyroid rats and increased metabolism of Ang III in the hypothalamus of hyperthyroid animals may be related to alterations in the secretory function of hypothalamus and pituitary in these thyroid dysfunctions.     

Analysis of the effect of potato carboxypeptidase inhibitor pro-sequence on the folding of the mature protein.

Protein folding can be modulated in vivo by many factors. While chaperones act as folding catalysts and show broad substrate specificity, some pro-peptides specifically facilitate the folding of the mature protein to which they are bound. Potato carboxypeptidase inhibitor (PCI), a 39-residue protein carboxypeptidase inhibitor, is synthesized in vivo as a precursor protein that includes a 27-residue N-terminal and a seven-residue C-terminal pro-regions. In this work the disulfide-coupled folding of mature PCI in vitro has been compared with that of the same protein extended with either the N-terminal pro-sequence (ProNtPCI) or both N- and C-terminal pro-sequences (ProPCI), and also with the N-terminal pro-sequence in trans (ProNt + PCI). No significant differences can be observed in the folding kinetics or efficiencies of all these molecules. In addition, in vivo folding studies in Escherichia coli have been performed using wild-type PCI and three PCI mutant forms with and without the N-terminal pro-sequence, the mutations had been previously reported to affect folding of the PCI mature form. The extent to which the 'native-like' form was secreted to the media by each construction was not affected by the presence of the N-terminal pro-sequence. These results indicate that PCI does not depend on the N-terminal pro-sequence for its folding in both, in vitro and in vivo in E. coli. However, structural analysis by spectroscopy, hydrogen exchange and limited proteolysis by mass spectrometry, indicate the capability of such N-terminal pro-sequence to fold within the precursor form.     

Sex and reproduction of the alfonsino Beryx splendens (Pisces, Berycidae) from the Macaronesian archipelagos

This paper provides comparative information on the reproductive biology of the alfonsino, Beryx splendens Lowe, 1834, species with commercial interest in the Azores, Madeira and the Canary Islands. A total of 846 individuals from Azores (14.0–42.0 cm fork length), 621 from Madeira (17.2–50.0 cm fork length) and 643 from the Canaries (18.2–38.9 cm fork length) were used for the study. The alfonsino is gonochoric with no evidence of sexual dimorphism. Females are more abundant than males; this dominance probably reflects certain differences in the spatial distribution and/or the catchability of males and females in the Macaronesian archipelagos. The spawning season was distinct for the three Macaronesian areas, with an observed North–South variation in the reproductive period: September–March in the Azores, March–June in Madeira and July–September in the Canary Islands. The size at sexual maturity estimated for Madeira and the Canary Islands is similar (32 and 30 cm fork length, respectively), while for the Azores it is reached at smaller length (23 cm fork length). The differences observed in the size at sexual maturity can be explained by the different exploitation levels in each archipelago. Life‐history parameters of the alfonsino suggest that this species has a specialistic life‐history strategy and fisheries based on this species are more susceptible to growth overfishing and population depletion.     

Ras-mediated cell proliferation and cell death: some clues from the interleukin-2 receptor system

Oncoproteins of the Ras family have been extensively studied because of their implication in human cancer. Their roles have been primarily assigned to the commandment of cell proliferation and suppression of apoptosis, which has also been demonstrated by the involvement of Ras activation in the signal transduction pathways triggered by most cytokine receptors. Nevertheless, the functions of Ras proteins have been extended in the last years by the findings showing that they can also act as promoters or enhancers of apoptosis in various systems and conditions. These considerations have raised the issue as to how the signals delivered by Ras are regulated and translated in terms of cellular responses, suggesting that signal complementation may direct the final fate of cells. As an example, the interleukin-2 receptor system may represent a useful model in which the meaning of Ras signals may be evaluated in terms of interactions with other simultaneous signalling events, since knowledge of the biochemical events triggered by the interaction of interleukin-2 with its cell surface receptor in lymphocytes has allowed the proposal of a complete signalling model arranged in three independent channels, one of which is mediated by Ras.This work was supported by grants from CICYT and Pharmacia-Upjohn.