Photosynthesis and nitrogen utilization in exponentially growing nitrogen-limited cultures of Lemna gibba

The photosynthetic performance and nitrogen utilization of Lemna gibba L. G3 adapted to limited nitrogen supply was studied. The plants were adapted to two levels of nitrogen limitation where the nitrogen addition rates were calculated to sustain relative growth rates (RGR) of 0.15 day^?1 and 0.25 day^?1, respectively. The photosynthetic performance of these cultures was compared to nitrogen-sufficient cultures with an average RGR of 0.32 day^?1. Plants transferred from nitrogen-sufficient conditions attained RGR values corresponding to the nitrogen addition rates after 6 to 10 days. Light-saturated net photosynthesis declined during adaptation according to the drop in growth rate, and a concomitant decrease in the respiration rate was recorded. The efficiency of net photosynthesis on a dry weight basis increased with increased nitrogen supply, whereas it was the same in all cultures when expressed on a chlorophyll basis. The light compensation point was unaffected by the nitrogen regime. Limited nitrogen supply resulted in an increased proportion of dry matter in the roots, which led to decreased leaf area ratios. The net assimilation rates also decreased, but not to the same extent as the leaf area ratios. Growth-limiting amounts of nitrogen were added to the cultures once daily, and the net influx of N was higher than the requirement for N, also in adapted cultures with a steady growth rate. This resulted in transient, periodic fluctuations in the NO₃^?, NH₄⁺ and amino acid pools. Also the rates of NO₃^? reduction and NH₄⁺ assimilation fluctuated as did the amino acid assimilation which paralleled NH₄⁺ assimilation. The role of flux rates over the plasmalemma and tonoplast for control of nitrogen assimilation rates are discussed.     

Purification and properties of indole 2,3-dioxygenase from maize leaves

An indole 2,3-dioxygenase was purified ca 38-fold from maize leaves. The enzyme had an MW of about 98000, an optimum pH of 5.0 and the energy of activation was 9.1 kcal/mol. The K_max for indole was 1.4 × 10^?4 M. The enzyme was inhibited by diethyldithiocarbamate, salicylaldoxime and sodium dithionite. The inhibition by diethyldithiocarbamate was specifically reversed by Cu²⁺. The dialysed enzyme was stimulated by Cu²⁺. Four atoms of oxygen were utilized in the disappearance of 1 mole of indole. Inhibition of the enzyme by -SH compounds and -SH group inhibitors, and their partial removal by Cu²⁺ only, suggested the involvement of -SH groups in binding of Cu²⁺ at the catalytic site.     

Effects of chemical modification of carboxyl groups on the voltage-clamped nerve fiber of the frog

Summary Voltage-clamped single nerve fibers of the frogRana esculenta were treated with the carboxyl group activating reagent N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) in the presence of different primary amines and without added amine. Carboxyl groups form stable amide bonds with primary amines in the presence of EEDQ. EEDQ treatment reduced the sodium current considerably and irreversibly, regardless of the presence of a primary amine in the Ringer's solution. The potassium current was also reduced. After modification the reduced sodium currents inactivated slowly and incompletely. The descending branch of the sodium current-voltage relation,I _Na(E), was shifted along the voltage axis in the depolarizing direction. The size of the shift was strongly dependent on the amine present during modification with EEDQ. The voltage-dependence of sodium inactivation,h _x (E), was shifted to more positive values of membrane potential by EEDQ in the presence of ethylenediamine (11 mV) and glucosamine (3 mV). In contrast, a small shift to more negative potentials occurred in the presence of taurine (–3 mV) or without the addition of an amine (–2 mV). A tenfold increase of the calcium concentration still shifted theI _Na(E) andh _x (E) curves of the chemically modified fibers. However, these shifts were smaller than those observed on untreated fibers. The currents remaining after the modification were completely blocked by tetrodotoxin; no change of the reversal potential occurred.     

Sulfhydryl Groups in Receptor Binding of Thyrotropin-Releasing Hormone to Rat Amygdala

Abstract: Binding of [³H]-[3-Me-His²]thyrotropin-releasing hormone ([³H]MeTRH) to TRH receptors in rat amygdala was decreased by sulfhydryl reagents in a time-, temperature-, and concentration-dependent manner. A pronounced reduction in receptor density, with little or no change in binding affinity, was apparent following disulfide bond reduction by dithiothreitol (DTT), alkylation of thiol groups by N -ethylmaleimide (NEM), and their oxidation by 5,5'-dithiobis (2-nitrobenzoic acid). Heavy metals (Cd²⁺, Hg²⁺), which complex with reactive -SH residues, also potently inhibited binding. The pharmacological specificity of residual [³H]MeTRH binding in chemically modified amygdala membranes was the same as that in control preparations. Sequential exposure to thiol reagents, in the presence or absence of cations, revealed possible additive effects. Pretreatment of membranes with TRH (10^--⁸-^-10^--⁶ M ), and its continued presence during modification, afforded protection against DTT and NEM. These results indicate the possible importance of thiol groups in the maintenance of TRH receptor conformation.     

Amino Acids in Plasma and CSF and Monoamine Metabolites in CSF: Interrelationship in Healthy Subjects

Plasma and cerebrospinal fluid (CSF) concentrations of amino acids were measured in 65 healthy volunteers (50 men and 15 women). The CSF levels of the monoamine metabolites homovanillic acid (HVA), 3-methoxy-4-hydroxyphenylethylene glycol (MOPEG), and 5-hydroxyindoleacetic acid (5-HIAA) were also determined. Sex differences were observed in both plasma and CSF amino acid levels as well as in the relationship between these concentrations. No significant correlations were observed between the CSF levels of HVA and 5-HIAA, and the concentrations of their precursor amino acids in either plasma or CSF. The MOPEG level in CSF correlated positively with the plasma concentrations of several amino acids.     

pH Selectivity of N-Ethylmaleimide Reactions with Opiate Receptor Complexes in Rat Brain Membranes

N-Ethylmaleimide (NEM) decreases opiate agonist binding presumably by blocking crucial sulfhydryl (SH) groups at receptor binding sites. At physiological pH, NEM decreased GTP and manganese regulation but increased sodium effects on [3H]D-Ala2-Met5-enkephalinamide (D-Ala enk) binding to rat brain membranes. To determine the apparent pK values of putative SH groups in opiate receptors that react with NEM, rat brain membranes were incubated with 100-250 microM NEM in buffers ranging from pH 4.5 to 8.0. Results showed that lowering pH below 6.5 reduced the NEM effect on opiate receptor functions and that the apparent pK values of NEM-reacting SH groups in binding and regulatory sites ranged between 5.4 to 6.0. Most of the total SH groups in brain membranes continued to react with NEM at low pH, so that when nonspecific SH groups were blocked by incubating membranes at pH 4.5 with NEM, opiate receptors became sensitive to very low concentrations (1 microM) of NEM.     

Sequence of Guinea Pig Myelin Basic Protein

This paper proposes a tentative amino acid sequence of guinea pig myelin basic protein obtained by comparison of peptide fragments of the guinea pig and bovine proteins. Analyses of the tryptic peptides confirmed the known sequence differences in the NH2-terminal half of the molecule and showed that in the COOH-terminal half of the guinea pig protein Ser131 was missing, Ala136 - His137 was deleted, Leu140 was replaced by Phe, and an extra Ala was inserted somewhere within sequence 142-151 (tryptic peptide T23 ). Sequence determination of guinea pig tryptic peptides corresponding to residues 130-134 ( T20 ), 135-138 ( T21 ), and 142-151 ( T23 ) of the bovine protein confirmed the above sequence changes and placed the extra Ala between Gly142 and His143 . The sequence of the region corresponding to bovine residues 130-143 is thus Ala-Asp-Tyr-Lys-Ser-Lys-Gly-Phe-Lys-Gly-Ala-His. No species differences were observed in the amino acid compositions of the remaining tryptic peptides obtained from the COOH-terminal half of the molecule. Based upon these results, the guinea pig basic protein contains 167 amino acid residues and has a molecular weight of 18,256.     

Population genetics theory of concerted evolution and its application to the immunoglobulin V gene tree

Summary The previous simple model for treating concerted evolution of multigene families has been revised to be compatible with various new observations on the immunoglobulin variable region family and other families. In the previous model, gene conversion and unequal crossing-over were considered, and it was assumed that genes are randomly arranged on the chromosome; neither subdivision nor correlation of gene identity and chromosomal distance were considered. Although this model satisfactorily explains the observed amino acid diversity within and between species, it fails to predict the very ancient branching of the mouse immunoglobulin heavy chain V-gene family. By incorporating subdivided structure and genetic correlation with chromosomal distance into the simple model, the data of divergence may be satisfactorily explained, as well as the rate of nucleotide substitution and the amino acid diversity. The rate at which a V-gene is duplicated or deleted by conversion or by unequal crossing-over is estimated by the new model to be on the order of 10^–6 per year. The model may be applicable to other multigene families, such as those coding for silkmoth chorion or mammalian kallikrein.Contribution no. 1560 from the National Institute of Genetics, Mishima, 411 Japan     

腺苷酸激酶在我国九个民族中的多态分布

用淀粉胶电泳及特异染色的方法,对我国9个民族的腺苷酸激酶(AK)多态分布进行了测定。9个民族中维吾尔族AK表型分布具有多态性。维吾尔族中AK_1~1基因频率为0.965,AK_1~2基因频率为0.035,而侗、回、白、土家、苗、彝、藏、满等8个民族的AK_1~2均未达到多态水平。在9个民族中AK_1表型分布均符合Hardy-Weinberg平衡,并且未发现其它罕见表型。     

Analysis of two chloride requirements for sodium-dependent amino acid and glucose transport by intestinal brush-border membrane vesicles of fish

Intestinal brush border vesicles of a Mediterranean sea fish (Dicentrarchus labrax) were prepared using the Ca²⁺-sedimentation method. The transport of glucose, glycine and 2-aminoisobutyric acid is energized by an Na⁺ gradient ($\text{out > in}$). In addition, amino acid uptake requires Cl^? in the extravesicular medium (2-aminoisobutyric acid more than glycine). This Na⁺- and Cl^?-dependent uptake is electrogenic, since it can be stimulated by negative charges inside the vesicles. The specific Cl^? requirement of glycine and 2-aminoisobutyric acid transport is markedly influenced by pH, a change from 6.5 to 8.4 reducing the role played by Cl^?. In the presence of Cl^?, the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of 2-aminoisobutyric acid uptake is reduced and its $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ is enhanced. Cl^? affects also a non-saturable Na⁺-dependent component of this amino acid uptake. Amino acid transport is also increased by intravesicular Cl^? (2-aminoisobutyric acid less than glycine). This effect is more concerned with glucose uptake, which can be then multiplied by 2.3. A concentration gradient ($\text{in > out}$) as well as the presence of Na⁺ in the incubation medium seems to enter into this requirement. This intravesicular Cl^? effect is not influenced by pH between 6.5 and 8.4.