A sulphate metabolizing centre in Euglena mitochondria

The mitosome of the anaerobic parasitic protist Entamoeba histolytica: A peculiar and minimalist mitochondrion-related organelle

The simplest class of mitochondrion-related organelles (MROs) is the mitosome, an organelle present in a few anaerobic protozoan parasites such as Entamoeba histolytica, Giardia intestinalis, and Cryptosporidium parvum. E. histolytica causes amoebiasis in humans, deemed as one of the important, yet neglected tropical infections in the world. Much of the enigma of the E. histolytica mitosome circles around the obvious lack of a majority of known mitochondrial components and functions exhibited in other organisms. The identification of enzymes responsible for sulfate activation (AS, IPP, and APSK) and a number of lineage-specific proteins such as the outer membrane beta-barrel protein (MBOMP30), and transmembrane domain-containing proteins that bind to various organellar proteins (ETMP1, ETMP30, EHI_170120, and EHI_099350) showcased the remarkable divergence of this organelle compared to the other MROs of anaerobic protozoa. Here, we summarize the findings regarding the biology of the mitosomes in E. histolytica, from their discovery up to the present understanding of its roles and interactions. We also include current advances and future perspectives on the biology, biochemistry, and evolution of the mitosomes of E. histolytica.

A Uniquely Complex Mitochondrial Proteome from Euglena gracilis

Euglena gracilis is a metabolically flexible, photosynthetic, and adaptable free-living protist of considerable environmental importance and biotechnological value. By label-free liquid chromatography tandem mass spectrometry, a total of 1,786 proteins were identified from the E. gracilis purified mitochondria, representing one of the largest mitochondrial proteomes so far described. Despite this apparent complexity, protein machinery responsible for the extensive RNA editing, splicing, and processing in the sister clades diplonemids and kinetoplastids is absent. This strongly suggests that the complex mechanisms of mitochondrial gene expression in diplonemids and kinetoplastids occurred late in euglenozoan evolution, arising independently. By contrast, the alternative oxidase pathway and numerous ribosomal subunits presumed to be specific for parasitic trypanosomes are present in E. gracilis. We investigated the evolution of unexplored protein families, including import complexes, cristae formation proteins, and translation termination factors, as well as canonical and unique metabolic pathways. We additionally compare this mitoproteome with the transcriptome of Eutreptiella gymnastica, illuminating conserved features of Euglenida mitochondria as well as those exclusive to E. gracilis. This is the first mitochondrial proteome of a free-living protist from the Excavata and one of few available for protists as a whole. This study alters our views of the evolution of the mitochondrion and indicates early emergence of complexity within euglenozoan mitochondria, independent of parasitism.

Metabolic quirks and the colourful history of the Euglena gracilis secondary plastid

Euglena spp. are phototrophic flagellates with considerable ecological presence and impact. Euglena gracilis harbours secondary green plastids, but an incompletely characterised proteome precludes accurate understanding of both plastid function and evolutionary history. Using subcellular fractionation, an improved sequence database and MS we determined the composition, evolutionary relationships and hence predicted functions of the E. gracilis plastid proteome. We confidently identified 1345 distinct plastid protein groups and found that at least 100 proteins represent horizontal acquisitions from organisms other than green algae or prokaryotes. Metabolic reconstruction confirmed previously studied/predicted enzymes/pathways and provided evidence for multiple unusual features, including uncoupling of carotenoid and phytol metabolism, a limited role in amino acid metabolism, and dual sets of the SUF pathway for FeS cluster assembly, one of which was acquired by lateral gene transfer from Chlamydiae. Plastid paralogues of trafficking-associated proteins potentially mediating fusion of transport vesicles with the outermost plastid membrane were identified, together with derlin-related proteins, potential translocases across the middle membrane, and an extremely simplified TIC complex. The Euglena plastid, as the product of many genomes, combines novel and conserved features of metabolism and transport.

Unique Features of Entamoeba Sulfur Metabolism; Compartmentalization, Physiological Roles of Terminal Products, Evolution and Pharmaceutical Exploitation

Sulfur metabolism is essential for all living organisms. Recently, unique features of the Entamoeba metabolic pathway for sulfated biomolecules have been described. Entamoeba is a genus in the phylum Amoebozoa and includes the causative agent for amoebiasis, a global public health problem. This review gives an overview of the general features of the synthesis and degradation of sulfated biomolecules, and then highlights the characteristics that are unique to Entamoeba. Future biological and pharmaceutical perspectives are also discussed.

Biochemistry and Physiology of Heavy Metal Resistance and Accumulation in Euglena

Free-living microorganisms may become suitable models for removal of heavy metals from polluted water bodies, sediments, and soils by using and enhancing their metal accumulating abilities. The available research data indicate that protists of the genus Euglena are a highly promising group of microorganisms to be used in bio-remediation of heavy metal-polluted aerobic and anaerobic acidic aquatic environments. This chapter analyzes the variety of biochemical mechanisms evolved in E. gracilis to resist, accumulate and remove heavy metals from the environment, being the most relevant those involving (1) adsorption to the external cell pellicle; (2) intracellular binding by glutathione and glutathione polymers, and their further compartmentalization as heavy metal-complexes into chloroplasts and mitochondria; (3) polyphosphate biosynthesis; and (4) secretion of organic acids. The available data at the transcriptional, kinetic and metabolic levels on these metabolic/cellular processes are herein reviewed and analyzed to provide mechanistic basis for developing genetically engineered Euglena cells that may have a greater removal and accumulating capacity for bioremediation and recycling of heavy metals.

Bio-recovery of non-essential heavy metals by intra- and extracellular mechanisms in free-living microorganisms

Free-living microorganisms may become suitable models for recovery of non-essential and essential heavy metals from wastewater bodies and soils by using and enhancing their accumulating and/or leaching abilities. This review analyzes the variety of different mechanisms developed mainly in bacteria, protists and microalgae to accumulate heavy metals, being the most relevant those involving phytochelatin and metallothionein biosyntheses; phosphate/polyphosphate metabolism; compartmentalization of heavy metal-complexes into vacuoles, chloroplasts and mitochondria; and secretion of malate and other organic acids. Cyanide biosynthesis for extra-cellular heavy metal bioleaching is also examined. These metabolic/cellular processes are herein analyzed at the transcriptional, kinetic and metabolic levels to provide mechanistic basis for developing genetically engineered microorganisms with greater capacities and efficiencies for heavy metal recovery, recycling of heavy metals, biosensing of metal ions, and engineering of metalloenzymes.

Mitosomes in Entamoeba histolytica contain a sulfate activation pathway

Hydrogenosomes and mitosomes are mitochondrion-related organelles in anaerobic/microaerophilic eukaryotes with highly reduced and divergent functions. The full diversity of their content and function, however, has not been fully determined. To understand the central role of mitosomes in Entamoeba histolytica, a parasitic protozoon that causes amoebic dysentery and liver abscesses, we examined the proteomic profile of purified mitosomes. Using 2 discontinuous Percoll gradient centrifugation and MS analysis, we identified 95 putative mitosomal proteins. Immunofluorescence assay showed that 3 proteins involved in sulfate activation, ATP sulfurylase, APS kinase, and inorganic pyrophosphatase, as well as sodium/sulfate symporter, involved in sulfate uptake, were compartmentalized to mitosomes. We have also provided biochemical evidence that activated sulfate derivatives, adenosine-5'-phosphosulfate and 3'-phosphoadenosine-5'-phosphosulfate, were produced in mitosomes. Phylogenetic analysis showed that the aforementioned proteins and chaperones have distinct origins, suggesting the mosaic character of mitosomes in E. histolytica consisting of proteins derived from alpha-proteobacterial, delta-proteobacterial, and ancestral eukaryotic origins. These results suggest that sulfate activation is the major function of mitosomes in E. histolytica and that E. histolytica mitosomes represent a unique mitochondrion-related organelle with remarkable diversity.

Sulfate assimilation in eukaryotes: fusions, relocations and lateral transfers

Background

The sulfate assimilation pathway is present in photosynthetic organisms, fungi, and many bacteria, providing reduced sulfur for the synthesis of cysteine and methionine and a range of other metabolites. In photosynthetic eukaryotes sulfate is reduced in the plastids whereas in aplastidic eukaryotes the pathway is cytosolic. The only known exception is Euglena gracilis, where the pathway is localized in mitochondria. To obtain an insight into the evolution of the sulfate assimilation pathway in eukaryotes and relationships of the differently compartmentalized isoforms we determined the locations of the pathway in lineages for which this was unknown and performed detailed phylogenetic analyses of three enzymes involved in sulfate reduction: ATP sulfurylase (ATPS), adenosine 5'-phosphosulfate reductase (APR) and sulfite reductase (SiR).

Results

The inheritance of ATPS, APR and the related 3'-phosphoadenosine 5'-phosphosulfate reductase (PAPR) are remarkable, with multiple origins in the lineages that comprise the opisthokonts, different isoforms in chlorophytes and streptophytes, gene fusions with other enzymes of the pathway, evidence a eukaryote to prokaryote lateral gene transfer, changes in substrate specificity and two reversals of cellular location of host- and endosymbiont-originating enzymes. We also found that the ATPS and APR active in the mitochondria of Euglena were inherited from its secondary, green algal plastid.

Conclusion

Our results reveal a complex history for the enzymes of the sulfate assimilation pathway. Whilst they shed light on the origin of some characterised novelties, such as a recently described novel isoform of APR from Bryophytes and the origin of the pathway active in the mitochondria of Euglenids, the many distinct and novel isoforms identified here represent an excellent resource for detailed biochemical studies of the enzyme structure/function relationships.

Molecular mechanisms of resistance to heavy metals in the protist Euglena gracilis

The biochemical mechanisms of resistance to several heavy metals, which are associated with their accumulation (binding by high-affinity chelating molecules such as thiol-compounds together with their compartmentalization into organelles), are analyzed for the photosynthetic, free-living protist Euglena gracilis. The complete understanding of these mechanisms may facilitate the rational design of strategies for bioremediation of heavy metal polluted water and soil systems.

Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.

Sulfur assimilation and glutathione metabolism under cadmium stress in yeast, protists and plants

Glutathione (gamma-glu-cys-gly; GSH) is usually present at high concentrations in most living cells, being the major reservoir of non-protein reduced sulfur. Because of its unique redox and nucleophilic properties, GSH serves in bio-reductive reactions as an important line of defense against reactive oxygen species, xenobiotics and heavy metals. GSH is synthesized from its constituent amino acids by two ATP-dependent reactions catalyzed by gamma-glutamylcysteine synthetase and glutathione synthetase. In yeast, these enzymes are found in the cytosol, whereas in plants they are located in the cytosol and chloroplast. In protists, their location is not well established. In turn, the sulfur assimilation pathway, which leads to cysteine biosynthesis, involves high and low affinity sulfate transporters, and the enzymes ATP sulfurylase, APS kinase, PAPS reductase or APS reductase, sulfite reductase, serine acetyl transferase, O-acetylserine/O-acetylhomoserine sulfhydrylase and, in some organisms, also cystathionine beta-synthase and cystathionine gamma-lyase. The biochemical and genetic regulation of these pathways is affected by oxidative stress, sulfur deficiency and heavy metal exposure. Cells cope with heavy metal stress using different mechanisms, such as complexation and compartmentation. One of these mechanisms in some yeast, plants and protists is the enhanced synthesis of the heavy metal-chelating molecules GSH and phytochelatins, which are formed from GSH by phytochelatin synthase (PCS) in a heavy metal-dependent reaction; Cd(2+) is the most potent activator of PCS. In this work, we review the biochemical and genetic mechanisms involved in the regulation of sulfate assimilation-reduction and GSH metabolism when yeast, plants and protists are challenged by Cd(2+).

Cadmium accumulation in the chloroplast of Euglena gracilis

Intracellular distribution of Cd, cysteine, glutathione, and Cd-induced thiol peptides in Euglena gracilis cultured under photoheterotrophic conditions was studied. After 3 days of culture with 0.2 mM CdCl2, 62% of the Cd accumulated by cells was equally distributed between the cytosolic and chloroplastic fractions. However, after 8 days, metal content increased in the crude chloroplastic fraction to 40% of total and decreased to 19% in the cytosol; in Percoll-purified chloroplasts the estimated content of Cd raised to 62%. Accumulation of Cd in chloroplasts could be mediated by a transporter of free Cd2+, since uptake of added CdCl2 in isolated chloroplasts exhibited a hyperbolic type of kinetics with a Km of 57 &mgr;M and Vmax of 3.7 nmol (mg protein)-1 min-1. The contents of cysteine and glutathione markedly increased in both chloroplasts (7-19 times) and cytosol (4-9 times) by exposure to Cd2+, although they were always higher in the cytosol. Thiol-containing peptides induced by Cd were mainly located in the cytosol after 3 days, and in the chloroplasts after 8 days of culture. The data suggested that Cd was compartmentalized into chloroplasts in a process that may involve the transport of free Cd and the participation of thiol-peptides.

Differential subcellular localization and expression of ATP sulfurylase and 5'-adenylylsulfate reductase during ontogenesis of Arabidopsis leaves indicates that cytosolic and plastid forms of ATP sulfurylase may have specialized functions

ATP sulfurylase and 5'-adenylylsulfate (APS) reductase catalyze two reactions in the sulfate assimilation pathway. Cell fractionation of Arabidopsis leaves revealed that ATP sulfurylase isoenzymes exist in the chloroplast and the cytosol, whereas APS reductase is localized exclusively in chloroplasts. During development of Arabidopsis plants the total activity of ATP sulfurylase and APS reductase declines by 3-fold in leaves. The decline in APS reductase can be attributed to a reduction of enzyme during aging of individual leaves, the highest activity occurring in the youngest leaves and the lowest in fully expanded leaves. By contrast, total ATP sulfurylase activity declines proportionally in all the leaves. The distinct behavior of ATP sulfurylase can be attributed to reciprocal expression of the chloroplast and cytosolic isoenzymes. The chloroplast form, representing the more abundant isoenzyme, declines in parallel with APS reductase during aging; however, the cytosolic form increases over the same period. In total, the results suggest that cytosolic ATP sulfurylase plays a specialized function that is probably unrelated to sulfate reduction. A plausible function could be in generating APS for sulfation reactions.

Interactions of arsenate, sulfate and phosphate with yeast mitochondria

In the presence of K(+), addition of ATP or ethanol to yeast mitochondria triggers the depletion of the transmembrane potential (DeltaPsi) and this is prevented by millimolar concentrations of phosphate (PO(4)). Different monovalent and polyvalent anions were tested for their protective effects on mitochondria from Saccharomyces cerevisiae. Only arsenate (AsO(4)) and sulfate (SO(4)) were as efficient as PO(4) to protect mitochondria against the K(+) mediated swelling, depletion of the DeltaPsi, and decrease in the ratio of uncoupled state to state 4 respiration rates. Protection by PO(4), SO(4) or AsO(4) was inhibited by mersalyl, suggesting that these anions interact with a site located in the matrix side. In addition, the effects of SO(4) and AsO(4) on the F(1)F(0)-ATPase were tested: both SO(4) and AsO(4) inhibited the synthesis of ATP following competitive kinetics against PO(4) and non-competitive kinetics against ADP. The mersalyl sensitive uptake of (32)PO(4) was not inhibited by SO(4) or AsO(4), suggesting that the synthesis of ATP was inhibited at the F(1)F(0)-ATPase. The hydrolysis of ATP was not inhibited, only a stimulation was observed when AsO(4) or sulfite (SO(3)) were added. It is suggested that the structure and charge similarities of PO(4), AsO(4) and SO(4) result in undiscriminated binding to at least two sites located in the mitochondrial matrix: at one site, occupation by any of these three anions results in protection against uncoupling by K(+); at the second site, in the F(1)F(0)-ATPase, AsO(4) and SO(4) compete for binding against PO(4) leading to inhibition of the synthesis of ATP.

A new pathway for the synthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol

A new pathway is proposed for the biosynthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol. The pathway begins at UDP-glucose and involves the formation therefrom of UDP-4-ketoglucose-5-ene to which is subsequently added sulphite (or its metabolic equivalent). Evidence consistent with this pathway, rather than with the previously proposed 'sulphoglycolytic' route, was obtained from experiments with pea chloroplast preparations. The evidence included the failure of potential inhibitors of the sulphoglycolytic pathway to alter the rate of synthesis of sulpholipid and the stimulation of the incorporation of 35SO4(2-) into the latter by UTP. Radioactivity was effectively incorporated into sulpholipid from UDP-[14C]glucose and this radiolabelling was stimulated by the addition of methyl alpha-glucose-enide or of an enzyme system known to be forming (although not accumulating) UDP-4-ketoglucose-5-ene. This new pathway is also consistent with other data in the literature.

Purification and properties of a phenol sulphotransferase from Euglena using L-tyrosine as substrate

A purification procedure based on (NH4)2SO4 precipitation, and chromatography on Affi-Gel Blue, DEAE-cellulose, hydroxyapatite and Bio-Gel P-60 yields a stable 6400-fold-purified active monomeric phenol (tyrosine) sulphotransferase of 26 kDa from W10BSmL, an aplastidic mutant of Euglena gracilis var. bacillaris. The apparent Km for adenosine 3'-phosphate 5'-phosphosulphate (PAPS) is 15 microM (60 microM tyrosine as substrate); adenosine 5'-phosphosulphate is inactive. L-Tyrosine gave the lowest apparent Km (33 microM) (with PAPS at 30 microM), but tyrosine esters, tyrosinamide, L-p-hydroxyphenylglycine and a number of tyrosine dipeptides were also active, with higher Km values. Nitrophenols (m- and p-) and chlorophenols (o-, m- and p-) were active, with higher Km values than for tyrosine. D-Tyrosine was inactive as a substrate, as was D-p-hydroxyphenylglycine and a number of other tyrosine derivatives lacking the carboxy carbonyl or the amino group, or having extra ring substituents or the hydroxy group in the wrong position. Adenosine 3',5'-bisphosphate and tyrosine O4-sulphate, products of the enzyme reaction with PAPS and tyrosine as substrates, showed competitive (Ki = 20 microM) and uncompetitive (Ki = 500 microM) inhibition kinetics respectively. This appears to be the first phenol sulphotransferase to accept tyrosine as substrate. This membrane-bound enzyme may be involved in tyrosine transport as well as detoxification.

Purification and characterization of O-acetylserine (thiol) lyase from spinach chloroplasts

O-Acetylserine (thiol) lyase, the last enzyme in the cysteine biosynthetic pathway, was purified to homogeneity from spinach leaf chloroplasts. The enzyme has a molecular mass of 68,000 and consists of two identical subunits of Mr 35,000. The absorption spectrum obtained at pH 7.5 exhibited a peak at 407 nm due to pyridoxal phosphate, and addition of O-acetylserine induced a considerable modification of the spectrum. The pyridoxal phosphate content was found to be 1.1 per subunit of 35,000, and the chromophore was displaced from the enzyme by O-acetylserine, leading to a progressive inactivation of the holoenzyme. Upon gel filtration chromatography on Superdex 200, part of the chloroplastic O-acetylserine (thiol) lyase eluted in association with serine acetyltransferase at a position corresponding to a molecular mass of 310,000 (such a complex called cysteine synthase has been characterized in bacteria). The activity of O-acetylserine (thiol) lyase was optimum between pH 7.5 and 8.5. The apparent Km for O-acetylserine was 1.3 mM and for sulfide was 0.25 mM. The calculated activation energy was 12.6 kcal/mol at 10 mM O-acetylserine. The overall amino-acid composition of spinach chloroplast O-acetylserine (thiol) lyase was different than that determined for the same enzyme (cytosolic?) obtained from a crude extract of spinach leaves. A polyclonal antibody prepared against the chloroplastic O-acetylserine (thiol) lyase exhibited a very low cross-reactivity with a preparation of mitochondrial matrix and cytosolic proteins suggesting that the chloroplastic isoform was distinct from the mitochondrial and cytosolic counterparts.

Subcellular Distribution of O-Acetylserine(thiol)lyase in Cauliflower (Brassica oleracea L.) Inflorescence

The subcellular localization of O-acetyiserine(thiol)lyase (EC 4.2.99.8) in nongreen tissue from higher plants has been studied using purified proplastids, mitochondria, and protoplasts from cauliflower (Brassica oleracea L.) buds as a source of subcellular fractions. O-Acetylserine(thiol)lyase has been detected in both organelles (proplastids and mitochondria) and a cytosolic extract obtained by protoplast fractionation. We confirmed these observations, demonstrating that a form of the enzyme different in global charge and separated from others by anion-exchange chromatography corresponded to each subcellular location. Our observations are consistent with the need for cysteine biosynthesis in each subcellular compartment where the synthesis of proteins occurs.

Purification and properties of two forms of ATP sulfurylase from Euglena

Two forms of ATP sulfurylase have been purified to homogeneity from mitochondria (ATPSm) and cells (ATPSc) of Euglena gracilis Klebs var. bacillaris Cori (aplastidic mutant W10BSmL). Both forms are monomeric, ATPSc is 52.3 kDa and ATPSm is 55 kDa. The pI is 7.9 for ATPSc and 5.8 for ATPSm. Therefore, ATPSm binds to DEAE-cellulose at pH 7.4; ATPSc does not. After cleavage by CNBr, the two forms of ATP sulfurylase show different sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) patterns, suggesting that they differ in amino acid sequence. ATPSm is mainly associated with the mitochondrial membrane and ATPSc is mainly soluble in the cells. Both enzymes require similar conditions in the molybdolysis assay, but show different pH optima when sulfate is used as substrate. ATPSc is more sensitive to adenosine 5'-phosphosulfate (APS) inhibition than ATPSm in the SO2-4 incorporation reaction. In the reverse reaction, ATPSc requires much higher concentrations of PPi and MgCl2 to saturate the reaction than ATPSm. The data indicate that the two enzymes are quite distinct and may have different roles in cell metabolism.

Purification and properties of adenosine 5'-phosphosulphate sulphotransferase from Euglena

Adenosine 5'-phosphosulphate sulphotransferase (APSST) was extracted from Euglena gracilis Klebs var. bacillaris mutant W10BSmL by freezing and thawing and was purified about 10,000-fold (to homogeneity) with 10.5% recovery by (NH4)2SO4 precipitation, Sephadex G-100 chromatography, Reactive Blue-agarose, Reactive Dye-agarose, DEAE-cellulose, preparative isoelectric focusing and non-inactivating SDS/PAGE. The active APSST, with a molecular mass of 102 kDa and multiple forms from pI 5.0 to 5.5, is a tetramer held together by covalent (probably disulphide) bonds. An apparent Km of the purified enzyme for adenosine 5'-phosphosulphate (APS) of 0.1 microM is obtained when dithiothreitol is used as the thiol. The enzyme is stimulated by Mg2+, Ca2+ or Ba2+, and uses almost any thiol; dithiothreitol and dithioerythritol give the highest activity. In the absence of APS, the enzyme is inactivated (and is rendered monomeric) by thiols but is protected from thiol inactivation by AMP, adenosine 5'-phosphoramidate (APA) or adenosine 5'-monosulphate (AMS), which also inhibit APSST activity somewhat. The enzyme resists inactivation by SDS in the absence of thiols; SDS stimulates APSST activity at low concentration, but high concentrations are inhibitory.

Localization of ATP Sulfurylase and O-Acetylserine(thiol)lyase in Spinach Leaves

The intracellular compartmentation of ATP sulfurylase and O-acetylserine(thiol)lyase in spinach (Spinacia oleracea L.) leaves has been investigated by isolation of organelles and fractionation of protoplasts. ATP sulfurylase is located predominantly in the chloroplasts, but is also present in the cytosol. No evidence was found for ATP sulfurylase activity in the mitochondria. Two forms of ATP sulfurylase were separated by anion-exchange chromatography. The more abundant form is present in the chloroplasts, the second is cytosolic. O-Acetylserine(thiol)lyase activity is located primarily in the chloroplasts and cytosol, but is also present in the mitochondria. Three forms of O-acetylserine(thiol)lyase were separated by anion-exchange chromatography, and each was found to be specific to one intracellular compartment. The cytosolic ATP sulfurylase may not be active in vivo due to the unfavorable equilibrium constant of the reaction, and the presence of micromolar concentrations of inorganic pyrophosphate in the cytosol, therefore its role remains unknown. It is suggested that the plant cell may be unable to transport cysteine between the different compartments, so that the cysteine required for protein synthesis must be synthesized in situ, hence the presence of O-acetylserine(thiol)lyase in the three compartments where proteins are synthesized.

Formation of tyrosine O-sulfate by mitochondria and chloroplasts of Euglena

Mitochondria that have been purified from cells of light-grown wild-type Euglena gracilis Klebs var. bacillaris Cori or dark-grown mutant W10BSmL and incubated with 35SO4(2-) and ATP accumulate a labeled compound in the surrounding medium. This compound is also labeled when mitochondria are incubated with [14C]tyrosine and nonradioactive sulfate under the same conditions. This compound shows exact coelectrophoresis with synthetic tyrosine O-sulfate at pH 2.0, 5.8, and 8.0, and yields sulfate and tyrosine on acid hydrolysis. Treatment with aryl sulfatase from Aerobacter aerogenes yields sulfate and tyrosine but no tyrosine methyl ester; no hydrolysis of tyrosine methyl ester to tyrosine is observed under identical conditions, ruling out methyl esterase activity in the aryl sulfatase preparation. Thus the compound is identified as tyrosine O-sulfate. No tyrosine O-sulfate is found outside purified developing chloroplasts of Euglena incubated with 35SO4(2-) and ATP, but both chloroplasts and mitochondria accumulate labeled tyrosine-O-sulfate externally when incubated with adenosine 3'-phosphate 5'-phospho[35S]-sulfate (PAP35S). Since tyrosine does not need to be added, it must be provided from endogenous sources. Labeled tyrosine O-sulfate is found in the free pools of light-grown Euglena cells grown on 35SO4(2-) or in dark-grown cells incubated with 35SO4(2-) in light, but none is found in the medium after cell growth. No labeled tyrosine O-sulfate is found in Euglena proteins (including those in extracellular mucus) after growth or incubation of cells with 35SO4(2-) or after incubation of organelles with 35SO4(2-) and ATP or PAP35S, ruling out sulfation of the tyrosine in protein or incorporation of free-pool tyrosine O-sulfate into protein. The system forming tyrosine O-sulfate is membrane-bound and may be involved in transporting tyrosine out of the organelles.