Evidence for Activation of the Oxidative Pentose Phosphate Pathway during Photosynthetic Assimilation of NO(3) but Not NH(4) by a Green Alga

Rapamycin and dietary restriction induce metabolically distinctive changes in mouse liver

Dietary restriction (DR) is the gold standard intervention used to delay aging, and much recent research has focused on the identification of possible DR mimetics. Energy sensing pathways, including insulin/IGF1 signaling, sirtuins, and mammalian Target of Rapamycin (mTOR), have been proposed as pathways involved in the antiaging actions of DR, and compounds that affect these pathways have been suggested to act as DR mimetics, including metformin (insulin/IGF1 signaling), resveratrol (sirtuins), and rapamycin (mTOR). Rapamycin is a promising DR mimetic because it significantly increases both health span and life span in mice. Unfortunately, rapamycin also leads to some negative effects, foremost among which is the induction of insulin resistance, potentially limiting its translation into humans. To begin clarifying the mechanism(s) involved in insulin resistance induced by rapamycin, we compared several aspects of liver metabolism in mice treated with DR or rapamycin for 6 months. Our data suggest that although both DR and rapamycin inhibit lipogenesis, activate lipolysis, and increased serum levels of nonesterified fatty acids, only DR further activates β-oxidation of the fatty acids leading to the production of ketone bodies.

Subcellular and tissue localization of NAD kinases from Arabidopsis: compartmentalization of de novo NADP biosynthesis

The de novo biosynthesis of the triphosphopyridine NADP is catalyzed solely by the ubiquitous NAD kinase family. The Arabidopsis (Arabidopsis thaliana) genome contains two genes encoding NAD+ kinases (NADKs), annotated as NADK1, NADK2, and one gene encoding a NADH kinase, NADK3, the latter isoform preferring NADH as a substrate. Here, we examined the tissue-specific and developmental expression patterns of the three NADKs using transgenic plants stably transformed with NADK promoter::glucuronidase (GUS) reporter gene constructs. We observed distinct spatial and temporal patterns of GUS activity among the NADK::GUS plants. All three NADK::GUS transgenes were expressed in reproductive tissue, whereas NADK1::GUS activity was found mainly in the roots, NADK2::GUS in leaves, and NADK3::GUS was restricted primarily to leaf vasculature and lateral root primordia. We also examined the subcellular distribution of the three NADK isoforms using NADK-green fluorescent protein (GFP) fusion proteins expressed transiently in Arabidopsis suspension-cultured cells. NADK1 and NADK2 were found to be localized to the cytosol and plastid stroma, respectively, consistent with previous work, whereas NADK3 localized to the peroxisomal matrix via a novel type 1 peroxisomal targeting signal. The specific subcellular and tissue distribution profiles among the three NADK isoforms and their possible non-overlapping roles in NADP(H) biosynthesis in plant cells are discussed.

Heterotrophic and mixotrophic growth of Micractinium pusillum Fresenius in the presence of acetate and glucose: effect of light and acetate gradient concentration

The main objective of this study was to determine the importance of secondary mechanism of organic carbon utilization (mixotrophic and heterotrophic modes) in addition to CO2 fixation (photoautotrophic mode) in the green alga, Micractinium pusillum Fresenius (chlorophyta), isolated from a waste stabilization pond. The growth was studied in the presence of acetate and glucose. The incorporation rate of 14C- acetate was measured in the light and in the dark at different concentrations. Finally, in order to underline the role of photosynthesis and respiration processes in the acetate assimilation, the effect of two specific metabolic inhibitors, a specific inhibitor of photosystem II (DCMU) and an uncoupler respiratory (DNP), has been studied. The obtained results showed that M. pusillum grows in the presence of organic substrates, i.e., glucose and acetate, in the light (mixotrophic growth) as well as in the dark (Heterotrophic growth). The growth was much more important in the light than in the dark and more in the presence of glucose than of acetate. In the light, the presence of acetate led to a variation of growth parameters mumax, iotaopt, and beta. The effect of acetate gradient on the growth of the microalga was severe as soon as its concentration in the medium was higher. The acetate uptake followed a Michaelis-Menten kinetic in the light as well as in the dark. The capacity of assimilation was slightly higher in the dark. The utilization of DNP and DCMU indicates that acetate incorporation is an active process depending on both anabolic (photosynthesis) and catabolic (respiration) metabolisms, corroborating the model of the Michaelis-Menten kinetic.

In vitro reconstitution of electron transport from glucose-6-phosphate and NADPH to nitrite

An NADPH-dependent NO2--reducing system was reconstituted in vitro using ferredoxin (Fd) NADP+ oxidoreductase (FNR), Fd, and nitrite reductase (NiR) from the green alga Chlamydomonas reinhardtii. NO2- reduction was dependent on all protein components and was operated under either aerobic or anaerobic conditions. NO2- reduction by this in vitro pathway was inhibited up to 63% by 1 mm NADP+. NADP+ did not affect either methyl viologen-NiR or Fd-NiR activity, indicating that inhibition was mediated through FNR. When NADPH was replaced with a glucose-6-phosphate dehydrogenase (G6PDH)-dependent NADPH-generating system, rates of NO2- reduction reached approximately 10 times that of the NADPH-dependent system. G6PDH could be replaced by either 6-phosphogluconate dehydrogenase or isocitrate dehydrogenase, indicating that G6PDH functioned to: (a) regenerate NADPH to support NO2- reduction and (b) consume NADP+, releasing FNR from NADP+ inhibition. These results demonstrate the ability of FNR to facilitate the transfer of reducing power from NADPH to Fd in the direction opposite to that which occurs in photosynthesis. The rate of G6PDH-dependent NO2- reduction observed in vitro is capable of accounting for the observed rates of dark NO3- assimilation by C. reinhardtii.

Changes in intracellular amino acids and organic acids induced by nitrogen starvation and nitrate or ammonium resupply in the cyanobacterium Phormidium laminosum

In Phormidium laminosum cells, nitrogen starvation caused a decrease in the intracellular levels of all amino acids, except glutamate, and an increase in the total level of the analyzed organic acids. The addition of nitrate or ammonium to N-starved cells resulted in substantial increases in the pool size of most amino acids. Upon addition of ammonium the total level of organic acids diminished, whereas it increased upon addition of nitrate, after a transient decay during the first minutes. Nitrogen resupply stimulated amino acid synthesis, the effect being faster and higher when ammonium was assimilated. The data indicate that nitrate and ammonium assimilation induced an enhancement of carbon flow through the glycolytic and the tricarboxylic-acid pathways to amino acid biosynthesis, with a concurrent decrease in the carbohydrate reserves. The results suggest that the availability of carbon skeletons limited the rate of ammonium assimilation, whereas the availability of reducing equivalents limited the rate of nitrate assimilation.

Coordination of Chloroplastic Metabolism in N-Limited Chlamydomonas reinhardtii by Redox Modulation (II. Redox Modulation Activates the Oxidative Pentose Phosphate Pathway during Photosynthetic Nitrate Assimilation)

The onset of photosynthetic NO3- assimilation in N-limited Chlamydomonas reinhardtii increased the initial extractable activity of the glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory step of the oxidative pentose phosphate pathway. The total activated enzyme activity did not change upon NO3- resupply. The higher activity, therefore, represents activation of existing enzyme. No activation occurred during NH4+ assimilation. Incubation of extracts with DTT reversed the NO3- stimulation of G6PDH activity, indicating that the activation involved redox modulation of G6PDH. Phosphoribulosekinase, an enzyme activated by thioredoxin reduction, was inhibited at the onset of NO3- assimilation. A 2-fold stimulation of O2 evolution and a 70% decrease in the rate of photosynthetic CO2 assimilation accompanied the enzyme activity changes. There was an immediate drop in the NADPH and an increase in NADP upon addition of NO3-, whereas NH4+ caused only minor fluctuations in these pools. The response of C. reinhardtii to NO3- indicates that the oxidative pentose phosphate pathway was activated to oxidize carbon upon the onset of NO3- assimilation, whereas reduction of carbon via the reductive pentose phosphate pathway was inhibited. This demonstrates a possible role for the Fd-thioredoxin system in coordinating enzyme activity in response to the metabolic demands for reducing power and carbon during NO3- assimilation.