Incorporation of nitrate nitrogen in rice seedlings transferred to anaerobic conditions

Summary Incorporation of¹⁵NO₃- into amino acids was studied in 3-day-old aerobic rice seedlings (with coleoptile and root) subjected for 24h to anaerobic conditions. The incorporation of¹⁵N into glutamate, glutamine and alanine accounted for 89% and 84% of total incorporation in coleoptile and root, respectively. These findings indicate that, after the primary incorporation of¹⁵N into glutamate and glutamine, the main fate of nitrate nitrogen in rice seedlings subjected to anoxia is alanine.     

The crystal structure of the monomeric human SOD mutant F50E/G51E/E133Q at atomic resolution. The enzyme mechanism revisited.

The crystal structure of the engineered monomeric human Cu,ZnSOD triple mutant F50E/G51E/E133Q (Q133M2SOD) is reported at atomic resolution (1.02 A). This derivative has about 20 % of the wild-type activity. Crystals of Q133M2SOD have been obtained in the presence of CdCl2. The metal binding site is disordered, with both cadmium and copper ions simultaneously binding to the copper site. The cadmium (II) ions occupy about 45 % of the copper sites by binding the four histidine residues which ligate copper in the native enzyme, and two further water molecules to complete octahedral coordination. The copper ion is tri-coordinate, and the fourth histidine (His63) is detached from copper and bridges cadmium and zinc. X-ray absorption spectroscopy performed on the crystals suggests that the copper ion has undergone partial photoreduction upon exposure to the synchrotron light. The structure is also disordered in the disulfide bridge region of loop IV that is located at the subunit/subunit interface in the native SOD dimer. As a consequence, the catalytically relevant Arg143 residue is disordered. The present structure has been compared to other X-ray structures on various isoenzymes and to the solution structure of the same monomeric form. The structural results suggest that the low activity of monomeric SOD is due to the disorder in the conformation of the side-chain of Arg143 as well as of loop IV. It is proposed that the subunit-subunit interactions in the multimeric forms of the enzyme are needed to stabilize the correct geometry of the cavity and the optimal orientation of the charged residues in the active channel. Furthermore, the different coordination of cadmium and copper ions, contemporaneously present in the same site, are taken as models for the oxidized and reduced copper species, respectively. These properties of the structure have allowed us to revisit the enzymatic mechanism.     

The development of lysosomal apparatus. II. Incorporation,subcellular distribution,and intraparticulate hydrolysis of 131 I-albumin by liver of mice at perinatal stages

Mouse fetuses at 15th and 18th day of gestation, and newborns aging 0, 5, 10, and 15 days were injected i.p. with 131 I-albumin (RISA). The degree of incorporation by liver and the intraparticulate hydrolysis of RISA in 27,000g × 10 min. particles increased after birth. By this time, changes in subcellular distribution of RISA and marker enzymes were also observed. Following an i.p. injection of India ink, numerous hepatic cells stained with carbon particles were observed at the light microscope from day 5 of life. The results suggest that the lysosomal apparatus acquires full capacity to incorporate colloidal particles and to degrade foreign macromolecules in the first week of life.     

Morphogenic Regulation of Pathotoxin Synthesis in Cochliobolus carbonum

The fungus Cochliobolus carbonum causes leaf spot disease of maize. Highly virulent isolates of the pathogen produce a host-selective, peptide toxin that is active against susceptible genotypes of maize. Prior to infection, spores must germinate and differentiate appressoria, structures specialized for leaf penetration. Analysis of spore germination fluids by plasma desorption mass spectrometry, which allowed detection of as little as 0.5 ng toxin, revealed that spores induced to form appressoria in vitro synthesized and released the toxin at a time coincident with maturation of appressoria. Spores incubated under conditions that did not induce appressorium formation failed to produce toxin. These observations indicate that synthesis of the host-selective toxin, which is essential for successful pathogenesis of maize by C. carbonum, is regulated by infection-related morphogenesis.     

NMRD studies on phthalate dioxygenase: evidence for displacement of water on binding substrate

 Water proton T ₁ ^–1 measurements at magnetic fields between 0.01 and 50 MHz [nuclear magnetic relaxation dispersion (NMRD) measurements] have been performed on solutions of phthalate dioxygenase (PDO) reconstituted at the catalytic iron site with copper(II) or manganese(II). The data show evidence of a weakly coordinated water molecule in CuPDO; in the presence of the substrate, phthalate, this water appears to become even less tightly bound, and an additional tightly coordinated water can be detected. In PDO reconstituted with manganese, one tightly coordinated water is detected in the presence and in the absence of phthalate. An attempt is made to reconcile these data with low-temperature near-IR magnetic circular dichroism and X-ray absorption data, which show that PDO reconstituted with iron or cobalt is six-coordinate in the absence of substrate and five-coordinate in the presence of substrate. Received: 24 April 1996 / Accepted: 17 July 1996     

Fecundity and fertility in a freshwater population of the neotropical amphidromous shrimp Macrobrachium acanthurus from the southeastern Atlantic

The neotropical amphidromous shrimp Macrobrachium acanthurus is one of various freshwater crustaceans heavily exploited in the southwestern Atlantic. Fecundity (nº early embryos female^?1) was examined during 2007 at four different localities (Iguape, Registro, Sete Barras, and Eldorado) along a stretch of river extending over 85?km (Ribeira de Iguape, São Paulo State, Brazil). Also, fertility (nº hatched larvae female^?1) was examined at one locality (Registro) during 2009–2010. Fecundity (mean?±?SD: 5191?±?2635; range: 1086–13,014 embryos female^?1) did not vary throughout the segment of river studied. Fecundity increased with female body size (carapace length, CL). However, fecundity scaled negatively with shrimp body size; females produce disproportionably fewer eggs with a unit increase in CL. The conditions explaining the negative allometric relationship between fecundity and female body size in M. acanthurus remain to be addressed. Nevertheless, natural food constraints limiting the ability of large but not small females to acquire enough resources to produce and fill their gonads with oocytes represents a plausible explanation for the negative scaling of fecundity with body size. Fertility varied between 545 and 12,465 hatched larvae female^?1 with an average (±SD) of 3981 (± 2693) and increased isometrically with a unit increase in female body size. M. acanthurus has an average fecundity and fertility that represents one of the extremes regarding the trade-off between fecundity/fertility and egg-size reported for caridean shrimps. All of this information needs to be considered in assessing shrimp stocks and establishing a sustainable management plan for this exploited species in the southwestern Atlantic.     

Rationalization of the reduction potentials within the series of the high potential iron-sulfur proteins

An analysis of the factors affecting reduction potentials within a series of high potential iron-sulfur proteins (HiPIP) has been performed by calculating the different contributions to the variation of electrostatic energy upon addition of one electron to the oxidized form of the protein. Molecular dynamics calculations were used to generate model structures of HiPIPs for which X-ray data are not available, starting from the known structures of highly homologous proteins. We have calculated and analyzed the contributions to the electrostatic energy deriving from the net charges present on the surface of the protein, the partial charges present on the uncharged residues, the polarizability of the protein atoms, the solvent dipoles and polarizabilities. A positive correlation with the reduction potentials was found only for the contribution due to the net charge of the protein which, in the absence of other factors such as differences in the coordination properties and in reorganizational energy upon reduction, is proposed to represent the determining effect for the large variation in reduction potential within this series of biological electron carriers.     

Chemical biology: Paper alert

A selection of interesting papers that were published in the two months before our press date in major journals most likely to report significant results in chemical biology.     

Investigation of the system copper(II) carbonic anhydrase and HCO3-/CO2

Copper(II) substituted human and bovine carbonic anhydrases B in the presence of bicarbonate have been investigated in solution through water-solvent proton nuclear magnetic resonance (nmr) at variable magnetic fields. HCO3-, contrary to all the other monoanionic inhibitors, partially reduces the water proton relaxation rates. This has been accounted for on the basis of the availability within the active cavity of two coordination positions partially overlapping. 13C-nmr measurements on both CO2 and HCO3- confirm that HCO3- binds the metal, whereas CO2 interacts with the paramagnetic center at nonbonding distance. The upper limit for the CO2 in equilibrium HCO3- interconversion has been estimated to be 10 sec-1.