Changes in Net O(2) Exchange Induced by Inorganic Nitrogen in the Blue-Green Alga Anacystis nidulans

The response of net O(2) exchange to light intensity by intact Anacystis nidulans cells in the presence of saturating NaHCO(3) concentrations followed a curve with an inflection near the light-compensation point. Addition of either KNO(3) or NH(4)Cl stimulated O(2) uptake in the dark and at light intensities below the light-compensation point. This resulted in steeper slopes of the curve calculated below and above the light-compensation point. At O(2) concentrations limiting dark respiration, addition of inorganic nitrogen had no effect on either dark respiration or O(2) exchange in the light. The apparent changes in photosynthetic yield observed under normal O(2) concentration disappeared when respiration was limited by O(2) availability, indicating that the effects of inorganic nitrogen on O(2) exchange at low light intensities are due to stimulation of respiration rather than to increases in photosynthetic yield.

Determination of intracellular nitrate

A sensitive procedure has been developed for the determination of intracellular nitrate. The method includes: (i) preparation of cell lysates in 2 M-H3PO4 after separation of cells from the outer medium by rapid centrifugation through a layer of silicone oil, and (ii) subsequent nitrate analysis by ion-exchange h.p.l.c. with, as mobile phase, a solution containing 50 mM-H3PO4 and 2% (v/v) tetrahydrofuran, adjusted to pH 1.9 with NaOH. The determination of nitrate is subjected to interference by chloride and sulphate when present in the samples at high concentrations. Nitrite also interferes, but it is easily eliminated by treatment of the samples with sulphamic acid. The method has been successfully applied to the study of nitrate transport in the unicellular cyanobacterium Anacystis nidulans.

Relationship between a 47-kDa cytoplasmic membrane polypeptide and nitrate transport in Anacystis nidulans

The polypeptide composition of cytoplasmic membranes of the cyanobacterium Anacystis nidulans changes in response to variations in the nitrogen source available to the cells, differing specifically in the amount of a polypeptide of 47-kDa molecular mass. Synthesis of the polypeptide and expression of nitrate transport activity are repressed by ammonium. Transfer of ammonium-grown cells to a medium containing nitrate as the sole nitrogen source results in parallel development of the 47-kDa polypeptide and nitrate transport activity of the cells. These results suggest the involvement of the 47-kDa cytoplasmic membrane polypeptide in nitrate transport by A. nidulans.

Regulated nitrate transport in the cyanobacterium Anacystis nidulans

Intracellular accumulation of nitrate, indicative of the operation of an active nitrate transport system, has been measured in intact cells of the cyanobacterium Anacystis nidulans. The ability of the cells to accumulate nitrate was effectively hindered by either ammonium addition or selective inhibition of CO2 fixation by DL-glyceraldehyde, with the effect of either compound being prevented by previously blocking ammonium assimilation. The results support the contention that nitrate utilization in cyanobacteria is regulated at the level of nitrate transport through the concerted action of ammonium assimilation and CO2 fixation.

Modulation of nitrate uptake in Anacystis nidulans by the balance between ammonium assimilation and CO2 fixation

Gradual inhibition of ammonium assimilation in Anacystis nidulans cells by increasing concentrations of 5-hydroxylysine resulted in a progressive enhancement of nitrate uptake. For 5-hydroxylysine-treated cells, the magnitude of the inhibition of nitrate uptake promoted by added ammonium was dependent on the ammonium assimilation capacity. In cells with a moderate ammonium assimilation activity, acceleration of CO2 fixation induced by bicarbonate addition antagonized the negative effect of ammonium, allowing full nitrate uptake activity. The results support the contention that nitrate utilization is under the feed-back control exerted by products of its own assimilation via ammonium, the inhibitory effect being potentiated by ammonium addition and alleviated by enhanced CO2 fixation. Results of amino acid analysis in cells exhibiting different capacities to utilize nitrate speak against these compounds as direct effectors of nitrate uptake.

Photosynthetic Assimilation of NO(3) by Intact Cells of the Cyanobacterium Anacystis nidulans: Influence of NO(3) and NH(4) Assimilation on CO(2) Fixation

Illuminated suspensions of Anacystis nidulans, supplied with saturating concentrations of CO(2) evolved O(2) at a greater rate when nitrate was simultaneously present. The extent of the stimulation of noncyclic electron flow induced by nitrate was dependent on light intensity, being maximal under light saturating conditions. Accordingly, nitrate depressed the rate of CO(2) fixation at limiting but not at saturating light, this depression reflecting the competition between both processes for assimilatory power. In contrast, ammonium stimulated CO(2) fixation at any light intensity assayed, the stimulation being dependent on the incorporation of ammonium to carbon skeletons. The positive effect of ammonium on CO(2) fixation also appeared to occur when nitrate was the nitrogen source, since with either nitrogen source an increase in the incorporation of newly fixed carbon into acid-soluble metabolites took place. From these results, the in vivo partitioning of assimilatory power between photosynthetic nitrogen and carbon assimilation and the quantitative and qualitative effects of inorganic nitrogen assimilation on CO(2) fixation are discussed.

Distinctive Light and CO(2)-Fixation Requirements of Nitrate and Ammonium Utilization by the Cyanobacterium Anacystis nidulans

The effect of light intensity on the rates of ammonium and nitrate uptake and of CO(2) fixation has been determined in intact Anacystis nidulans cells. Ammonium uptake became saturated at photon flux values of about 60 microeinsteins per square meter per second, whereas both nitrate uptake and CO(2) fixation reached saturation at about 250 microeinsteins per square meter per second, the rates of the two latter processes being tightly correlated at any light intensity assayed. Inhibition of ammonium assimilation resulted in the loss of correlation between CO(2) fixation and nitrate uptake, the latter process exhibiting then a reduced light requirement. The results establish a clear distinction between ammonium utilization and nitrate utilization with regard to their light requirement and to the nature of their dependence upon CO(2) fixation.

Inhibition of nitrate utilization by amino acids in intact Anacystis nidulans cells

In an attempt to establish the nature of the ammonium-assimilation products which mediate the inhibition by ammonium of nitrate uptake in cyanobacteria, the effect of different amino acids on nitrate utilization by intact Anacystis nidulans cells has been assayed. To exclude an indirect inhibition of nitrate uptake through the ammonium which the amino acids might release, the cells were pretreated with L-methionine-D,L-sulfoximine (MSX), a potent inactivator of glutamine synthetase. Under these conditions, several L-amino acids, but not the corresponding D-isomers, affected nitrate utilization to a variable extent, causing inhibitions ranging between 20 and 80% when added at 20 mM concentration. For most of the inhibitory amino acids, including L-isoleucine, L-leucine and L-valine, a correlation was found between their ability to act as amino group donors to alpha-ketoglutarate, in reactions catalyzed by A. nidulans cell-free extracts, and their inhibitory effect on nitrate utilization. L-Glutamine, L-asparagine and glycine, being effective inhibitors of nitrate utilization, were poor substrates for the transaminating activity to alpha-ketoglutarate, however. The possible role of the latter amino acids as mediators in the ammonium-promoted inhibition of nitrate uptake is discussed.

Dependence of nitrate utilization upon active CO2 fixation in Anacystis nidulans: a regulatory aspect of the interaction between photosynthetic carbon and nitrogen metabolism

Specific inhibition of photosynthetic CO2 fixation in Anacystis nidulans cells by D,L-glyceraldehyde resulted in the simultaneous inhibition of nitrate utilization, indicating a dependence of the latter process upon the provision of CO2-fixation products. This dependence was lost in cells treated with L-methionine-D,L-sulfoximine or azaserine, effective inhibitors of ammonium assimilation. In these cells, nitrate uptake could proceed at rates similar to those in control cells even if CO2 fixation was severely inhibited by D,L-glyceraldehyde. The results support the contention that CO2-fixation products participate in the control of nitrate uptake in A. nidulans by preventing the accumulation of certain ammonium derivatives which are negative effectors of nitrate uptake.

Bioelectrical tissue resistance during various methods of myocardial preservation

Twenty-one mongrel dogs underwent cardiopulmonary bypass (CPB) and ischemic cardiac arrest for 60 minutes. The bioelectrical tissue impedance, water content, and ultrastructure of the myocardium were studied and correlated with various types of myocardial protection applied. The animals were divided into three groups according to the myocardial protection applied: Group 1, moderate general hypothermia; Group 2, moderate general and local hypothermia; Group 3, moderate general hypothermia and crystalloid cardioplegia. The hearts from 3 normal dogs served as controls. After 35 minutes of ischemic arrest, the difference in bioelectrical impedance of the myocardium of the left ventricle between Groups 1 and 2 was highly significant (p less than 0.001); in Groups 2 and 3, a significant difference was observed after 50 minutes. The heart water content in all three groups increased over that found in the hearts of the control animals, but there was no significant difference between Groups 2 and 3. There was evidence by ultramicroscopic study of ischemic damage to the myocardium in all of the experimental groups. Damage was less evident in Groups 2 and 3. We conclude that measurement of myocardial bioelectrical tissue resistance is a sensitive indicator of myocardial damage due to ischemia and may be useful clinically in determining the need for and type of cardiac resuscitation, particularly following CPB.