Effects of internal ph on the nonselective cation channel from the mouse collecting tubule

We investigated the effects of internal pH on Ca-activated, nucleotide-inhibited nonselective cation channels in the basolateral membranes of mouse collecting tubules, using the inside-out variant of the patch clamp technique. pH modulated the channel open probability (P _o ), giving a bell-shaped curve peaking at pH 6.8/7.0: P _o at pH 6.0 was 11±6% of P _o at pH 7.2 and 32 ±7% at pH 8.0. The open and closed time distributions, best fitted to the sum of two exponentials, were differently sensitive to acid and alkaline conditions. Low pH reduced the short and long open times to 38 and 24% of their pH 7.2 values, while high pH produced a 4-fold increase in the long closed time. As previously reported, 4-acetamido-4′-isothiocyanatostilbene-2,2′-disulfonic acid (SITS) induced a quasi-permanent opening of the channel. The inhibition of the channel produced by high pH disappeared in the presence of SITS, while the inhibition produced by low pH was unaffected. These results suggest that the pH dependence of the channel is due to two separate mechanisms. pH was without effect on the ATP-evoked inhibition of the channel, while high pH profoundly reduced the steepness of the AMP inhibition curve, without altering the half-maximal inhibitory AMP concentration.     

Inhibition of Low pH-induced Elongation in Avena Coleoptiles by Abscisic Acid

An angular position-sensing transducer was used to make continuous measurements of acid-induced elongation of Avena sativa coleoptile segments. Elongation rates at pH 4.5 (5 mm succinate buffer) were about 5-fold greater than those at pH 6.0. Buffered 0.1 mm abscisic acid produced a partial decrease of the growth rate. Pretreatments with abscisic acid buffered at pH 6.0 usually caused a further reduction of the elongation response when the coleoptile segments were subsequently placed in buffer at pH 4.5 containing abscisic acid. Abscisic acid did not completely prevent the pH effect in any of these experiments, and the brief latent period of the pH response was not affected by abscisic acid treatments. At pH 4.5, where the inhibitory effect of ABA was maximum, low pH-induced elongation was also inhibited by KCN and HgCl₂. These results suggest that pH-(4.5) induced elongation in this system may be dependent on some metabolic processes and that abscisic acid-induced inhibition of this elongation may involve an interaction with these processes.     

Effects of organic acids on ion uptake and retention in barley roots

Effects of several organic acids on ion uptake and retention and on respiration in barley roots having low and high KCl contents were assayed by measurements of K⁺, Na⁺, Ca²⁺, Cl⁻, and oxygen uptake. Organic acids with high pK_a values increase the permeability of roots to ions and decrease respiration when present in sufficient concentrations at pH 5 but have no inhibitory effects at pH 7. Absence of respiratory inhibition in short times and at lower organic acid concentrations, under conditions that immediately produce a permeability increase, indicate that the permeability change is not a result of respiratory inhibition. Effects of formate, acetate, propionate, and glutarate are attributed to entry of undissociated acid molecules into the effective membranes. Lack of a permeability increase with succinate, which has lower distribution coefficients to lipid solvents than do the aliphatic acids, can be explained by failure of sufficient amounts of the hydrophilic succinic acid molecules to penetrate the membranes involved. These experiments suggest that undissociated acid in root membranes can increase permeability of the roots.     

Catalytic and ligand-binding properties of rat intestinal alkaline phosphatase.

Rat intestinal alkaline phosphatase is a dimeric enzyme with identical subunits and thus possesses two presumably identical active sites. Binding studies with P_i and l-phenylalanine and pre-steady-state “burst” titrations confirm the existence of two active sites per molecule of enzyme. The sites appear to be nonequivalent with respect to P_i binding, both at low pH, where an enzyme (E)-P_i covalent complex is formed, and at high P_i, where an E-P_i noncovalent complex predominates. The binding affinity of the first site is 100-fold greater than that of the second, i.e., there is negative cooperativity. The K_i value for competitive inhibition of substrate hydrolysis by P_i corresponds to the higher affinity site. The negative cooperativity appears not to be an artifact resulting from contaminating P_i in the purified enzyme preparation. l-Phenylalanine does not bind to the enzyme unless P_i is present, as expected from the previously proposed mechanism of uncompetitive inhibition by the amino acid. No negative cooperativity is seen in l-phenylalanine binding, but the number of moles of amino acid bound at saturation depends on the degree of saturation by P_i The enzyme is also inhibited uncompetitively by NADH, which can compete with l-phenylalanine for the same site on alkaline phosphatase.     

Selection of Protease-Positive and Protease-Negative Variants of Streptococcus cremoris

Protease-negative variants were shown to outcompete the wild-type strains of Streptococcus cremoris E₈, HP, and Wg₂ at pH values higher than 6.0 in milk. For S. cremoris E₈ this process was studied in more detail. At lower pH values the wild type had a selective advantage. This pH-dependent selection was not found in all media tested. The poor growth of the protease-negative variant at low pH was not due to lower internal pH values. By growing S. cremoris E₈ and Wg₂ in acidified milk (pH 5.9) the proteolytic activity of the cultures could be stabilized. In continuous cultures under amino acid limitation the wild type S. cremoris E₈ and HP strains had a selective advantage over the protease-negative variants at low dilution rates (D < 0.2) at all pH values of the medium. This was apparently due to a lower affinity-constant (K_s) of the protease-positive variants for amino acids. Finally, a high fraction of protease-positive variants could be maintained in continuous cultures by using a growth medium with low concentrations of casein as a nitrogen source. At high dilution rates nearly all cells were protease positive.     

Comparative studies of the photosynthesis of the submerged macrophyte Elodea canadensis and the filamentous algae Cladophora glomerata and Spirogyra sp.

The photosynthetic and respiratory responses of Elodea canadensis Michx., Cladophora glomerata (L.) Kütz. and Spirogyra sp. to oxygen, temperature, HCO₃⁻¹ concentration, pH and irradiance were determined. Photosynthesis was inhibited by O₂ in all three species under all conditions and inhibition was greatert in E. canadensis. This inhibition was not caused solely by an accelerated rate of dark respiration and this suggests that photorespiration may be an important factor controlling productivity, particularly in E. canadensis.Photosynthetic performance was impaired much more in E. canadensis than in either of the filamentous algae conditions of high pH and low CO₂ concentrations. In the field photosynthesis of these algae may increase the pH and reduce the CO₂ content of the water sufficiently to exert deleterious effects on macrophyte photosynthesis. Such a plant-induced change in water quality could give C. glomerata a competitive advantage over E. canadensis and be a factor in the replacement of the vascular plant by the alga in some waters.     

Inhibition by oxalates of spinach chloroplast phenolase in unfrozen and frozen states

Spinach chloroplast phenolase was inhibited by oxalic acid and its salts. Complete inhibitions were induced instantly in the acidic region (e.g. by 1 and 5 mM oxalate at pH 5 and 5.5, respectively), and in the neutral region pre-incubation of the enzyme with oxalates could also lead to complete loss of activity. The inhibition mode was non-competitive for phenol substrate with K_i of 0.9 mM pH 6.8. Reduction of enzyme activity in a crude extract of chloroplasts induced by freezing at neutral pH was due to the presence of ammonium oxalate. With 0.5 mM oxalate, the inhibition attained 75% under frozen conditions, whilst no inhibition could be detected in the enzyme which had not been frozen. Free oxalic acid and K⁺ and Na⁺ salts also caused freezing inhibition. Glyoxylic and oxamic acids acted as inhibitors with less efficiency. With a pure mushroom tyrosinase (phenolase), essentially the identical results were obtained using the same conditions.     

Activation of NADP-specific isocitrate dehydrogenase by chelating agents.

The effects of different metal chelating agents on the activity of the NADP-linked isocitrate dehydrogenase from pig heart have been studied. Addition of ethylene glycolbis(β-aminoethyleter) N,N′-tetraacetic acid, N-hydroxyethylenediamine triacetic acid, and ethylenediamine tetraacetic acid (EDTA) under certain conditions could enhance the activity by a factor of nearly 3. Moreover, the time lag occurring before the reaction rate approached a constant value at suboptimal metal-ion concentrations was abolished by the metal chelating agents. S^0.5 for isocitrate increased slightly in the presence of the metal-chelating agents. The substrate inhibition occurring at high NADP concentrations was abolished by the activator. The pH optimum was the same in the absence and presence of EDTA. The extent of activation increased on a relative basis with increasing pH. Studies of the sedimentation behavior of the enzyme under different conditions suggested that the effect of the metal-chelating agents could not be accounted for by aggregation or depolymerization of the enzyme. NADPH affects the enzyme activity in a similar way, although less efficiently than the metal chelating agents. The results indicate that most organic metal complexes can activate the enzyme. It has previously been suggested that isocitrate complexed with a metal ion is the real substrate for the enzyme. If this holds true, the activation found with other organic metal complexes can be accounted for by a reduction in the apparent K_m for the isocitrate metal complex and by an increase in the maximum rate of the reaction by removal of the substrate inhibition at high NADP concentrations.     

Light-Induced Proton Gradient Formation in Intact Cells of Dunaliella salina

A light-induced proton gradient (ΔpH) increase as exhibited by an increase of 9-aminoacridine fluorescence quenching is demonstrated between the external medium and the interior of the halophytic green alga Dunaliella salina. The formation and maintenance of the ΔpH is sensitive to electron transport inhibitors and to uncouplers. It is inhibited by p-chloromercuribenzenesulfonic acid (50% inhibition at 3 micromolar), which does not affect photosynthetic O₂ evolution. It is concluded that the observed ΔpH is located across the plasmalemma or the chloroplast envelope. The formation and maintenance of the light-induced proton gradient requires the presence of Na⁺. Substitution of NaCl by KCl or glycerol results in inhibition of the ΔpH formation. The proton gradient is also sensitive to ATPase and energy transfer inhibitors. It is suggested that a Na⁺/H⁺ pump mechanism may be involved in the formation of the proton gradient in intact Dunaliella cells.     

Growth and metabolism of the obligate photolithotroph Chlorobium thiosulfatophilum in the presence of added organic nutrients

Growth of Chlorobium vibrioforme f. thiosulfatophilum NCIB 8327 could be monitored by measurement of turbidity (E₆₀₀); absorbance at 745 and 665 nm; increase in methanol-extractable pigment (E₆₆₀); fixation of ¹⁴CO₂; and titration of thiosulphate and sulphide in the medium. Growth could be inhibited by formate, methionine, tryptophan, tyrosine, threonine, serine and glycine, but not by 14 other amino acids, shikimic acid, some alcohols, sugars or acetate. Inhibition could some-times be relieved by the presence of other amino acids. This was probably partly due to restoration of normal internal amino acid requirements by “feeding”, and partly because uptake of amino acids appeared to show some competition for two or more low specificity uptake systems. Numerous ¹⁴C-labelled amino acids, formate and glucose were shown to be photoassimilated by Chlorobium, and the labelling patterns obtained provided information on its pathways of intermediary biosynthesis. Growth inhibition by threonine could be related to the probable presence of a normal branched pathway for the synthesis of the aspartate family of amino acids, with an aspartokinase enzyme subject to strong inhibition by threonine and lysine, separately and in combination.     

Salicylhydroxamic Acid (SHAM) Inhibition of the Dissolved Inorganic Carbon Concentrating Process in Unicellular Green Algae

Rates of photosynthetic O₂ evolution, for measuring K_0.5(CO₂ + HCO₃⁻) at pH 7, upon addition of 50 micromolar HCO₃⁻ to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K₁(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO₂ uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O₂ evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO₃), and the rate of ¹⁴CO₂ fixation with 100 micromolar [¹⁴C] HCO₃⁻. Salicylhydroxamic acid inhibition of O₂ evolution and ¹⁴CO₂-fixation was reversed by higher levels of NaHCO₃. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO₂ accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related.     

Sulphate/proton cotransport in plasma-membrane vesicles isolated from roots of Brassica napus L.: increased transport in membranes isolated from sulphur-starved plants

The characteristics of sulphate uptake into right-side-out plasma-membrane vesicles isolated from roots of Brassica napus L., Metzger, cv. Drakkar, and purified by aqueous polymer two-phase partitioning, were investigated. Sulphate uptake into the vesicles was driven by an artificially imposed pH gradient (acid outside), and could be observed for 5–10 min before a plateau was reached and no further net uptake occurred. The uptake was partially inhibited in the presence of depolarizing agents and little uptake was observed in the absence of an imposed pH gradient. Uptake was strongly pH-dependent, being greatest at more acidic pH. After imposition of a pH gradient, the capacity for uptake decreased slowly (t1/2>10 min). The uptake had a high-affinity component which was strongly dependent on the external proton concentration (K _m=10μM at pH 5.0, 64 μM at pH 6.5). The K _m for protons varied from 0.4–1.9 μM as the sulphate concentration was reduced from 33 to 1 μM. A low-affinity component was observed which could be resolved at low temperatures (0 °C). Microsomal membranes that partitioned into the lower phase of the two-phase system gave no indication of high-affinity sulphate transport. Sulphate uptake into plasma-membrane vesicles isolated from sulphur-starved plant material was approximately twofold greater than that observed in those isolated from sulphate-fed plant material. Isolated vesicles therefore mirror the well-known in-vivo response of roots, indicating an increase in the number of transporters to be, at least in part, the underlying cause of derepression.     

Inhibition of photosynthesis by ethylene-a stomatal effect

Ethylene at hormonally significant levels inhibited net photosynthesis of the cultivated peanut (Arachis hypogaea L.) as measured by gas analysis. Upon the removal of ethylene, the inhibition was naturally overcome at the concentration-exposure duration combinations tested. Increased length of exposure of 1 microliter of ethylene per liter of air up to 6 hours increased the degree of net photosynthesis inhibition (68% reduction after 6-hour exposure). Significantly greater inhibition of photosynthesis by ethylene was detected on peanut genotypes having higher photosynthetic efficiency. In contrast to peanut, hormonal concentrations of ethylene only moderately inhibited sweet potato, Jerusalem artichoke, and sunflower photosynthesis and was without effect on beans, peas, Irish potato, Mimosa pudica, and white clover. No inhibition could be found by ethylene on ribulose 1,5-biphosphate carboxylase activity in vitro. Photosynthesis was lowered at all CO₂ concentrations below ambient at an O₂ concentration of 1.5%, indicating that the action of ethylene was not affected by low O₂; concomitantly, an increase in the CO₂ compensation point occurred. Diffusion resistance measurements of leaf water vapor loss made on ethylene-treated peanut leaves showed a measurable decrease in leaf conductance which correlated with net photosynthesis decrease. Ethylene influenced the conductance of abaxial stomata more so than adaxial.     

Fertilization acid release in Urechis eggs: I. The nature of the acid and the dependence of acid release and egg activation on external pH

The amount of fertilization acid produced by eggs of Urechis caupo, monitored by automatically back-titrating egg suspensions with base, depends linearly on the pH of the seawater. Above pH 7.0, at which no acid is released (Paul, M., Dev. Biol.43, 299–312, 1975), acid release increased approximately 0.34 pmole/egg/0.1 pH unit. Activation (germinal vesicle breakdown) depended on the amount of acid release in natural seawater; it did not occur if eggs released <1.5 pmole acid/egg. When fertilization acid is released into HCO^?₃-free seawater and the pH permitted to decrease, the supernatant can be tested for the presence of a volatile acid, such as CO₂, by bubbling with N₂ and comparing the increase in pH as volatile acid is driven off with experiments in which HCl or CO₂ is substituted for fertilization acid. An increase in pH of <0.2 pH units occurred on N₂ bubbling when fertilization acid or HCl was used to acidify HCO^?₃-free seawater compared to an increase of >0.5 pH units when CO₂ was used. Therefore, most, if not all, of Urechis fertilization acid is not volatile, and since Paul (1975) showed that it is not a nonvolatile weak acid, it must be H⁺.     

Esterification and hydrolysis of vitamin A in the liver of brook trout (Salvelinus fontinalis) and the influence of a coplanar polychlorinated biphenyl

Recent reports of extremely low retinoid stores in fish living in contaminated river systems prompted an initial investigation of the mechanisms of hepatic storage and mobilization in brook trout. Enzyme characterization in microsomes revealed a lecithin:retinol acyltransferase activity (LRAT) optimum in the alkaline range (pH 9.0; V_max=0.6 nmol per mg prot. h⁻¹; K_m=10.2 μM) which is not known to occur in mammals, in addition to a secondary optimum at pH 6.5 typical of mammals. Acyl CoA:retinol acyltransferase (ARAT) kinetic parameters were quite different to those of mammals. The substrate affinity of trout ARAT (K_m=1.6 μM) was approximately 22-fold greater than that of the rat while maximal velocity (V_max=0.2 nmol per mg prot. h⁻¹) was 18-fold less. Retinyl ester hydrolase activity (REH) was optimal under acid conditions (pH 4.2; V_max=6.6 nmol per mg prot. h⁻¹; K_m=0.6 mM), was inhibited by a bile salt analogue and was greater in males than females. This REH was tentatively categorized as a bile salt-independent, acid retinyl ester hydrolase (BSI-AREH). REH was inhibited in a dose-dependent manner following in vivo exposure to a representative environmental contaminant the coplanar polychlorinated biphenyl (PCB), 3,3′,4,4′-tetrachlorobiphenyl (TCBP). Inhibition may be an indirect effect because enzyme activity was not affected by in vitro exposure of control microsomes. REH inhibition in the brook trout may affect the uptake of retinyl esters (REs) from chylomicron remnants as well as the mobilization of stored REs.     

Gibberelic acid-mediated activation of monophenolase in de-embryonated half-seeds of wheat (Triticum Aestivum)

Stimulation of monophenolase activity was observed when de-embryonated prewashed half-seeds of wheat were imbibed in a solution of gibberellic acid (GA₃, 10^t?5 M). Crude extracts, prepared from GA₃-treated half-seeds, showed ca a two-fold increase in monophenolase activity over the controls at pH 6.6, while a dramatic rise in enzyme activity (seven- to nine-fold) was observed at pH 9.0. The partially purified (NH₄)₂SO₄ fraction precipitate (30–50% saturation) also showed enhancement of enzyme activity at pH 9.0 in GA₃-treated half-seeds, while in controls, there was negligible activity at this pH. Administration of five amino acid analogues (1 mM each) to half-seeds showed no significant inhibition of GA₃-stimulated monophenolase activity, but proved very effective in decreasing (86% inhibition) the GA₃-induced amylase activity. This indicated that the hormone-regulated monophenolase activity was not dependent on de novo protein synthesis. Treatment of half-seeds with GA₃ modified the monophenolase and altered the electrophoretic pattern. The enzyme was relatively thermostable at 55° and the pH optimum was shifted from pH 7.0 to 9.0. In addition, the GA₃-treated half-seeds showed relatively high stability of enzyme activity in the presence of (NH₄)₂S0₄ ions. These alterations in the GA₃-stimulated nomophenolase suggest there is activation of preformed enzyme molecules. The appearance of slow-migrating multiple forms on acrylamide gels, in response to GA₃ treatment, is probably due to the association of fast-migrating forms. Such oligomerization could result in a conformational change leading to enzyme activation. This may be an adaptive mechanism so that the enzyme can function with varying temperature, pH and ionic strength during early stages of seed germination.     

Invertases in Oat Seedlings: SEPARATION, PROPERTIES, AND CHANGES IN ACTIVITIES IN SEEDLING SEGMENTS

The soluble invertase activity in etiolated Avena seedlings was highest at the apex of the coleoptile and much lower in the primary leaf, mesocotyl, and root. The activity in all parts of the seedling consisted of two invertases (I and II) which were separated by chromatography on diethylaminoethylcellulose. Both enzymes appeared to be acid invertases, but they differed in molecular size, pH optimum, and the kinetic parameters K_m and V_max of their action on sucrose, raffinose, and stachyose. Invertase II had low stability at pH 3.5 and below, and exhibited high sensitivity to Hg²⁺, with complete inhibition by 2 micromolar HgCl₂. Segments of coleoptiles incubated in water lost about two-thirds of the total invertase activity after 16 hours. The loss of activity was due primarily to a decrease in the level of invertase II. The loss of invertase was decreased by indoleacetic acid, 2,4-dichlorophenoxyacetic acid, and α-naphthaleneacetic acid but not by β-naphthaleneacetic acid and p-chlorophenoxyisobutyric acid. Conditions that inhibited auxin-induced growth of the segments (20 millimolar CaCl₂ and 200 millimolar mannitol) also blocked the auxin effect on invertase loss.     

Biosynthesis of Fatty acids by a soluble extract from developing soybean cotyledons

Fractionation of developing soybean cotyledons into cellular components demonstrates that most of the activity necessary to incorporate acetate-1-¹⁴C into lipid remains in the supernatant from a 198,000g spin for 1 hr. The system studied is dependent upon ATP, CoA, and CO₂. Concentrations of ATP greater than 4 × 10⁻³m are inhibitory, while 1 × 10⁻⁴m CoA is needed for optimal activity. Avidin inhibition of acetate incorporation into lipid could be reversed by biotin. Studies indicated that NADPH is a better source of reducing power than NADH. The system studied is inhibited by p-chloromercuribenzoic acid and this inhibition can be reversed by an excess of GSH. The system studied shows maximum activity in tris buffer at pH 8.6 or in glycine buffer, pH 9.4.     

On the amino acids involved in the ATPase site of mitochondrial F1 and the implication of the subunit C.

The data on the pH dependence of the K_m for Mg-ATP and the V_m of the ATPase of pig heart mitochondrial F₁ indicate the presence of two groups of different pK's which modify the enzyme activity. The first pK at pH 9.6 ± 0.2 may be related to the possible presence of arginine and/or tyrosine residues in the ATPase site; the second pK at pH 7.2 ± 0.2 could be due to the presence of a histidine residue in the ATPase site or to the involvement of amino groups in the ATPase site. The inhibition induced by photooxidation in the presence of Rose Bengal is not pH dependent in the pH range corresponding to the pK of histidine. The inhibition induced by diethylpyrocarbonate cannot be reversed by hydroxylamine and the characteristics of this inhibition rather correspond to the reaction of the inhibitor with amino groups. Pyridoxal phosphate also inhibits the ATPase activity of F₁ by reaction with amino groups. The presence of ATP or phosphate partially protects against the inhibition induced by diethylpyrocarbonate or pyridoxal phosphate, which indicates that amino groups may be directly or indirectly involved in the binding of nucleotide and phosphate to F₁. Glutaraldehyde also inhibits the enzyme by reacting with amino groups and inducing a crosslinking of the subunits. The disappearance of subunit C is well correlated with the decrease of ATPase activity, indicating that subunit C is essential in the ATPase activity.     

Malonic Semialdehyde Reductase from the Archaeon Nitrosopumilus maritimus Is Involved in the Autotrophic 3-Hydroxypropionate/4-Hydroxybutyrate Cycle

The recently described ammonia-oxidizing archaea of the phylum Thaumarchaeota are highly abundant in marine, geothermal, and terrestrial environments. All characterized representatives of this phylum are aerobic chemolithoautotrophic ammonia oxidizers assimilating inorganic carbon via a recently described thaumarchaeal version of the 3-hydroxypropionate/4-hydroxybutyrate cycle. Although some genes coding for the enzymes of this cycle have been identified in the genomes of Thaumarchaeota, many other genes of the cycle are not homologous to the characterized enzymes from other species and can therefore not be identified bioinformatically. Here we report the identification and characterization of malonic semialdehyde reductase Nmar_1110 in the cultured marine thaumarchaeon Nitrosopumilus maritimus. This enzyme, which catalyzes the reduction of malonic semialdehyde with NAD(P)H to 3-hydroxypropionate, belongs to the family of iron-containing alcohol dehydrogenases and is not homologous to malonic semialdehyde reductases from Chloroflexus aurantiacus and Metallosphaera sedula. It is highly specific to malonic semialdehyde (K_m, 0.11 mM; V_max, 86.9 μmol min⁻¹ mg⁻¹ of protein) and exhibits only low activity with succinic semialdehyde (K_m, 4.26 mM; V_max, 18.5 μmol min⁻¹ mg⁻¹ of protein). Homologues of N. maritimus malonic semialdehyde reductase can be found in the genomes of all Thaumarchaeota sequenced so far and form a well-defined cluster in the phylogenetic tree of iron-containing alcohol dehydrogenases. We conclude that malonic semialdehyde reductase can be regarded as a characteristic enzyme for the thaumarchaeal version of the 3-hydroxypropionate/4-hydroxybutyrate cycle.