Activation of ribulose 1,5-diphosphate carboxylase by nicotinamide adenine dinucleotide phosphate and other chloroplast metabolites

Ribulose 1,5-diphosphate carboxylase, when activated by preincubation with 10 mm MgCl₂ and 1 mm bicarbonate in the absence of ribulose 1,5-diphosphate, can be further activated about 170% with 0.5 mm NADPH present in the preincubation mixture. NADP⁺, NADH, and NAD⁺ are ineffective. The activation by NADPH is comparable to that previously seen with 0.05 to 0.10 mm 6-phosphogluconate in that these specific preincubation conditions are required, but the effects of NADPH and 6-phosphogluconate are not additive. Moreover, where higher concentrations of 6-phosphogluconate inhibited the enzyme, higher concentrations of NADPH give a greater activation, saturating at about 1 mm and 200%. Under the specified conditions of preincubation, fructose 1,6-diphosphate has an activation curve similar to that of 6-phosphogluconate, peaking at 0.1 mm and 70%. Above this level, activation decreases, and inhibition is seen at still higher concentrations. Other metabolites tested produced smaller or no effects on the enzyme activity assayed under these conditions. When either reduced NADP or 6-phosphogluconate are present in the preincubation mixture, it becomes possible to determine the Km for bicarbonate using a Lineweaver-Burk plot, and the Km for bicarbonate under these conditions is 2.8 mm, corresponding to 0.3% CO₂ at pH 7.8 and 25 C.     

Activation and inhibition of ribulose 1,5-diphosphate carboxylase by 6-phosphogluconate

Ribulose 1,5-diphosphate carboxylase, when activated by preincubation with 1 mm bicarbonate and 10 mm MgCl₂ in the absence of ribulose 1,5-diphosphate, remains activated for 20 minutes or longer after reaction is initiated by addition of ribulose diphosphate. If as little as 50 μm 6-phosphogluconate is added during this preincubation period, 5 minutes before the start of the reaction, a further 188% activation is observed. However, addition of 6-phosphogluconate at the same time or later than addition of ribulose diphosphate, or at any time with 50 mm bicarbonate, gives inhibition of the enzyme activity. Possible relevance of these effects in vivo regulatory effects is discussed.     

The presence of ribulose 1,5-diphosphate carboxylase in the nonphotosynthetic endosperm of germinating castor beans

Ribulose 1,5-diphosphate carboxylase was detected in extracts of germinating castor bean (Ricinus communis var. Hale) endosperms. This is the first report of this enzyme in a nonphotosynthetic (no chlorophyll) plant tissue. Radioactive 3-phosphoglyceric acid has been identified as the principle product resulting from the enzymatic condensation of ¹⁴C-bicarbonate and ribulose-1,5-diP in endosperm extracts. The Km values of bicarbonate and ribulose-1,5-diP for the endosperm carboxylase are 1.14 × 10⁻²m and 7.5 × 10⁻⁵m, respectively. The carboxylase activity peaks at 4 days in endosperms of castor beans germinated in the dark. The specific activity of the carboxylase at this stage of germination is 4.3 μmoles of 3-phosphoglycerate formed/mg protein·hr. The presence of ribulose-1,5-diP carboxylase and other enzymes of the reductive pentose phosphate pathway show the potential of this pathway in castor bean endosperms.     

Deoxycytidylate deaminase. Properties of the enzyme from cultured kidney cells of baby hamster

dCMP deaminase was partially purified from BHK-21/C13 cells grown in culture. The molecular weight of the enzyme was estimated by gel filtration and gradient centrifugation to be 130000 and 115000 respectively. The enzyme had a pH optimum of 8.4. Its activity versus substrate concentration curve was sigmoid, the substrate concentration at half-maximal velocity being 4.4mm. dCTP activated the deaminase maximally at 40μm, gave a hyperbolic curve for activity versus dCMP concentration and a K_m value for dCMP of 0.91mm. dCTP activation required the presence of Mg²⁺ or Mn²⁺ ions. dTTP inhibited the deaminase maximally at 15μm; the inhibition required the presence of Mg²⁺ or Mn²⁺ ions. The enzyme was very heat-labile but could be markedly stabilized by dCTP at 0.125mm and ethylene glycol at 20% (v/v).     

The metabolism of cyclic nucleotides in the guinea-pig pancreas. Cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase

Both cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase were recovered mainly from the supernatant fractions of guinea-pig pancreas, but a higher proportion of the activity of the former was associated with the pellet fractions. The activities in the supernatant were not separated by gel filtration, but were clearly separated by subsequent chromatography on an anion-exchange resin. The activities of cyclic AMP phosphodiesterase and cyclic GMP phosphodiesterase had high-affinity (K_m 6.5±1.1μm and 31.9±3.9μm respectively) and low-affinity (K_m 0.56±0.05mm and 0.32±0.03mm respectively) components. The activity of neither enzyme was affected by the pancreatic secretogens, cholecystokinin-pancreozymin, secretin and carbachol. Removal of ions by gel filtration resulted in a marked reduction in cyclic nucleotide phosphodiesterase activity, which could be restored by addition of Mg²⁺. Mn²⁺ (3mm) was as effective as Mg²⁺ (3mm) in the case of cyclic AMP phosphodiesterase, but was less than half as effective in the case of cyclic GMP phosphodiesterase. The metal-ion chelators, EDTA and EGTA, also decreased activity. Ca²⁺ (1mm) did not affect the activity of cyclic nucleotide phosphodiesterase when the concentration of Mg²⁺ was 3mm. At concentrations of Mg²⁺ between 0.1 and 1mm, 1mm-Ca²⁺ was activatory, and at concentrations of Mg²⁺ below 0.1mm, 1mm-Ca²⁺ was inhibitory. These results are discussed in terms of the possible significance of cyclic nucleotide phosphodiesterase in the physiological control of cyclic nucleotide concentrations during stimulus–secretion coupling.     

The enzymes forming isopentenyl pyrophosphate from 5-phosphomevalonate (mevalonate 5-phosphate) in the latex of Hevea brasiliensis

1. Phosphomevalonate kinase and 5-pyrophosphomevalonate decarboxylase have been purified from the freeze-dried latex serum of the commercial rubber tree Hevea brasiliensis. 2. The phosphomevalonate kinase was acid- and heat-labile and required the presence of a thiol to maintain activity. 3. The 5-pyrophosphomevalonate decarboxylase was relatively acid-stable and more heat-stable than the phosphokinase. 4. Maximum activity of the phosphokinase was achieved at pH 7.2 with 0.2mm-5-phosphomevalonate (K_m 0.042mm), 2.0mm-ATP (K_m 0.19mm) and 8mm-Mg²⁺ at 40°C. The apparent activation energy was 14.8kcal/mol. 5. Maximum activity of 5-pyrophosphomevalonate decarboxylase was achieved at pH5.5–6.5 with 0.1mm-5-pyrophosphomevalonate (K_m 0.004mm), 1.5mm-ATP (K_m 0.12mm) and 2mm-Mg²⁺. The apparent activation energy was 13.7kcal/mol. The enzyme was somewhat sensitive to inhibition by its products, isopentenyl pyrophosphate and ADP.     

Light and calcium interactions in chlorella inhibited by sodium chloride

Analysis of NaCl toxicity in Chlorella sorokiniana showed decreased growth rates, increased dry weight per cell, increased intracellular Na⁺ and Cl⁻, more total chlorophyll per cell, a decreased chlorophyll a to chlorophyll b ratio, increased rates of O₂ evolution, and decreased rates of CO₂ fixation when the extracellular concentration of NaCl was increased from zero to 0.3 m. Cultures did not grow at concentrations greater than 0.3 m NaCl unless 10 mm calcium salts were present. Inclusion of that concentration of Ca²⁺ extended the tolerance to 0.5 m NaCl before growth stopped. Increasing the light intensity from 1.2 to 9.4 mw/cm² increased growth rates for cultures in 0.10 to 0.45 m NaCl. At 14 mw/cm² added Ca²⁺ reduced growth rates of cultures in 0.3 m NaCl compared to controls without added Ca²⁺. Maximal growth rates for cultures in NaCl media were achieved by addition of 10 mm CaSO₄ and maintenance of the light intensity at 9.4 mw/cm². The maximal growth rate of the organism was 9.6 doublings/day achieved at 2.7 mw/cm² for control cultures. In 0.3 m NaCl the growth rate was 4.3 doublings/day at 2.7 mw/cm² and 8.2 doublings/day at 9.4 mw/cm² with 10 mm CaSO₄ added.     

Effect of ammonium and other ions on the glucose-dependent active transport of l-histidine in slices of rat-brain cortex

1. The influence of cations on the active transport into cells of rat-brain-cortex slices of l-histidine, an amino acid that is not metabolized by this tissue, has been studied. 2. Like other amino acids, l-histidine accumulated in the cells in the presence of glucose in concentrations up to over double that in the incubation medium. 3. The active transport of l-histidine was highest in a medium containing Ca²⁺ (3mm). The addition of K⁺ (27mm) led to a marked decrease in the intracellular concentration of l-histidine, though the oxygen uptake of the slices was higher. 4. The active l-histidine transport was inhibited by NH₄⁺. The inhibitory effect increased with the NH₄⁺ concentration, being about 25% at 8mm, 65% at 20mm, and 90% at 27 and 50mm. The oxygen uptake of the brain slices was depressed by only 25% by the highest NH₄⁺ concentration used, and less by lower concentrations.     

myo-Inositol-1-Phosphatase from the Pollen of Lilium longiflorum Thunb

A Mg²⁺-dependent, alkaline phosphatase has been isolated from mature pollen of Lilium longiflorum Thunb., cv. Ace and partially purified. It hydrolyzes 1l- and 1d-myo-inositol 1-phosphate, myo-inositol 2-phosphate, and β-glycerophosphate at rates decreasing in the order named. The affinity of the enzyme for 1l- and 1d-myo-inositol 1-phosphate is approximately 10-fold greater than its affinity for myo-inositol 2-phosphate. Little or no activity is found with phytate, d-glucose 6-phosphate, d-glucose 1-phosphate, d-fructose 1-phosphate, d-fructose 6-phosphate, d-mannose 6-phosphate, or p-nitrophenyl phosphate. 3-Phosphosphoglycerate is a weak competitive inhibitor. myo-Inositol does not inhibit the reaction. Optimal activity is obtained at pH 8.5 and requires the presence of Mg²⁺. At 4 millimolar, Co²⁺, Fe²⁺ or Mn²⁺ are less effective. Substantial inhibition is obtained with 0.25 molar Li⁺. With β-glycerophosphate as substrate the K_m is 0.06 millimolar and the reaction remains linear at least 2 hours. In 0.1 molar Tris, β-glycerophosphate yields equivalent amounts of glycerol and inorganic phosphate, evidence that transphosphorylation does not occur.     

Polyphosphoinositide Phospholipase C in Plasma Membranes of Wheat (Triticum aestivum L.) : Orientation of Active Site and Activation by Ca and Mg

Polyphosphoinositide-specific phospholipase C activity was present in plasma membranes isolated from different tissues of several higher plants. Phospholipase C activities against added phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4,5-bisphosphate (PIP₂) were further characterized in plasma membrane fractions isolated from shoots and roots of dark-grown wheat (Triticum aestivum L. cv Drabant) seedlings. In right-side-out (70-80% apoplastic side out) plasma membrane vesicles, the activities were increased 3 to 5 times upon addition of 0.01 to 0.025% (w/v) sodium deoxycholate, whereas in fractions enriched in inside-out (70-80% cytoplasmic side out) vesicles, the activities were only slightly increased by detergent. Furthermore, the activities of inside-out vesicles in the absence of detergent were very close to those of right-side-out vesicles in the presence of optimal detergent concentration. This verifies the general assumption that polyphosphoinositide phospholipase C activity is located at the cytoplasmic surface of the plasma membrane. PIP and PIP₂ phospholipase C was dependent on Ca²⁺ with maximum activity at 10 to 100 μm free Ca²⁺ and half-maximal activation at 0.1 to 1 μm free Ca²⁺. In the presence of 10 μm Ca²⁺, 1 to 2 mm MgCl₂ or MgSO₄ further stimulated the enzyme activity. The other divalent chloride salts tested (1.5 mm Ba²⁺, Co²⁺, Cu²⁺, Mn²⁺, Ni²⁺, and Zn²⁺) inhibited the enzyme activity. The stimulatory effect by Mg²⁺ was observed also when 35 mm NaCl was included. Thus, the PIP and PIP₂ phospholipase C exhibited maximum in vitro activity at physiologically relevant ion concentrations. The plant plasma membrane also possessed a phospholipase C activity against phosphatidylinositol that was 40 times lower than that observed with PIP or PIP₂ as substrate. The phosphatidylinositol phospholipase C activity was dependent on Ca²⁺, with maximum activity at 1 mm CaCl₂, and could not be further stimulated by Mg²⁺.     

Glutamine synthetase of pea leaves: divalent cation effects, substrate specificity, and other properties

Purified glutamine synthetase from pea seedlings was most active with Mg²⁺ as the metal activator, but Mn²⁺ and Co²⁺ were 45 to 60% and 30 to 45% as effective, respectively, when assayed at the optimal pH for each cation. The Mg²⁺ saturation curve was quite sigmoid, and evidence indicates that MgATP is the active ATP substance. Co²⁺ also gave a sigmoidal saturation curve, but when Mn²⁺ was varied only slightly sigmoidal kinetics were seen. Addition of Mn²⁺, Ca²⁺, or Zn²⁺ at low concentrations sharply inhibited the Mg²⁺ -dependent activity, partially by shifting the pH optimum. Addition of Co²⁺ did not inhibit Mg²⁺-dependent activity. The nucleotide triphosphate specificity changed markedly when Co²⁺ or Mn²⁺ replaced Mg²⁺. Using the Mg²⁺-dependent assay, the Michaelis constant (Km) for NH₄⁺ was about 1.9 × 10⁻³ M. The Km for l-glutamate was directly proportional to ATP concentration and ranged from 3.5 to 12.4 mm with the ATP levels tested. The Km for MgATP also varied with the l-glutamate concentration, ranging from 0.14 mm to 0.65 mm. Ethylenediaminetetracetic acid activated the enzyme by up to 54%, while sulfhydryl reagents gave slight activation, occasionally up to 34%.     

C-reactive Protein Exists in an NaCl Concentration-dependent Pentamer-Decamer Equilibrium in Physiological Buffer

C-reactive protein (CRP) is an acute phase protein of the pentraxin family that binds ligands in a Ca²⁺-dependent manner, and activates complement. Knowledge of its oligomeric state in solution and at surfaces is essential for functional studies. Analytical ultracentrifugation showed that CRP in 2 mm Ca²⁺ exhibits a rapid pentamer-decamer equilibrium. The proportion of decamer decreased with an increase in NaCl concentration. The sedimentation coefficients s_20,w⁰ of pentameric and decameric CRP were 6.4 S and in excess of 7.6 S, respectively. In the absence of Ca²⁺, CRP partially dissociates into its protomers and the NaCl concentration dependence of the pentamer-decamer equilibrium is much reduced. By x-ray scattering, the radius of gyration R_G values ranged from 3.7 nm for the pentamer to above 4.0 nm for the decamer. An averaged K_D value of 21 μm in solution (140 mm NaCl, 2 mm Ca²⁺) was determined by x-ray scattering and modeling based on crystal structures for the pentamer and decamer. Surface plasmon resonance showed that CRP self-associates on a surface with immobilized CRP with a similar K_D value of 23 μm (140 mm NaCl, 2 mm Ca²⁺), whereas CRP aggregates in low salt. It is concluded that CRP is reproducibly observed in a pentamer-decamer equilibrium in physiologically relevant concentrations both in solution and on surfaces. Both 2 mm Ca²⁺ and 140 mm NaCl are essential for the integrity of CRP in functional studies and understanding the role of CRP in the acute phase response.     

5-Oxoprolinase (l-Pyroglutamate Hydrolase) in Higher Plants: Partial Purification and Characterization of the Wheat Germ Enzyme

5-Oxoprolinase has been found to be widely distributed in higher plants. This enzyme catalyzes the ATP-dependent hydrolysis of 5-oxo-l-proline (l-pyrollidone carboxylate, l-pyroglutamate) to glutamate. The enzyme has been purified almost 60 fold from wheat germ (Triticum aestivum L). This enzyme requires a divalent cation, either Mn²⁺ or Mg²⁺, and a combination of both appears to be the most effective. There is also an absolute requirement for a monovalent cation best fulfilled by either NH₄⁺ or K⁺. The K_m for ATP is 0.4 mm and for 5-oxo-l-proline is 14 μm. A small amount of activity is observed when other purine nucleotides such as ITP and GTP replace ATP. The substitution of the pyrimidine nucleotides CTP and UTP for ATP yield almost completely inactive preparations. The enzyme appears to have an active sulfhydryl group since there is an increase in activity in the presence of dithioerythritol. Preincubation with reagents such as N-ethylmaleimide or iodoacetamide lead to complete inactivation. The presence of this enzyme leads to the speculation of the possible presence of a γ-glutamyl cycle in higher plants.     

Separation and partial characterization of multiple ribonucleic Acid polymerases from soybean hypocotyl

Two major peaks of RNA polymerase activity have been routinely separated by diethylaminoethyl cellulose chromatography following solubilization from soybean (Glycine max L. var. Wayne) chromatin. The relative amounts of these two peaks depend upon the manner in which the chromatin is purified. Pelleting the chromatin through dense sucrose solutions results in not only a loss of total solubilized RNA polymerase activity but also a selective loss of the α-amanitin-sensitive form of the enzyme. Peak I elutes from a diethylaminoethyl cellulose column at a KCl concentration of approximately 0.27 m, is insensitive to α-amanitin and rifamycin, and has Mg²⁺ + Mn²⁺ optima of 5 mm and 1.25 mm, respectively. The enzyme is inhibited by KCl concentrations of about 0.03 m or greater. Peak II elutes from the column at a KCl concentration of approximately 0.35 m, is sensitive to α-amanitin, insensitive to rifamycin, and has Mg²⁺ + Mn²⁺ optima of 2 mm and 1.0 mm, respectively. Activity is inhibited by KCl concentrations of about 0.06 m or greater. Both enzymes prefer denatured calf thymus DNA, but peak II exhibits a stronger preference.     

CO(2) Metabolism in Corn Roots. III. Inhibition of P-enolpyruvate Carboxylase by l-malate

Phosphoenolpyruvate carboxylase was purified from corn root tips about 80-fold by centrifugation, ammonium sulfate fractionation, and anion exchange and gel filtration chromatography. The resulting preparation was essentially free from malate dehydrogenase, isocitrate dehydrogenase, malate enzyme, NADH oxidase, and pyruvate kinase activity. Kinetic analysis indicated that l-malate was a noncompetitive inhibitor of P-enolpyruvate carboxylase with respect to P-enolpyruvate (K_I = 0.8 mm). d-Malate, aspartate, and glutamate inhibited to a lesser extent; succinate, fumarate, and pyruvate did not inhibit. Oxaloacetate was also a noncompetitive inhibitor of P-enolpyruvate carboxylase with an apparent K_I of 0.4 mm. A comparison of oxaloacetate and l-malate inhibition suggested that the mechanisms of inhibition were different. These data indicated that l-malate may regulate CO₂ fixation in corn root tips by a feedback or end product type of inhibition.     

The sedimentation behaviour of ribonuclease-active and -inactive ribosomes from bacteria

1. The `30s' and `50s' ribosomes from ribonuclease-active (Escherichia coli B) and -inactive (Pseudomonas fluorescens and Escherichia coli MRE600) bacteria have been studied in the ultracentrifuge. Charge anomalies were largely overcome by using sodium chloride–magnesium chloride solution, I 0·16, made 0–50mm with respect to Mg²⁺. 2. Differentiation of enzymic and physical breakdown at Mg²⁺ concentrations less than 5mm was made by comparing the properties of E. coli B and P. fluorescens ribosomes. 3. Ribonuclease-active ribosomes alone showed a transformation of `50s' into 40–43s components. This was combined with the release of a small amount of `5s' material which may be covalently bound soluble RNA. Other transformations of the `50s' into 34–37s components were observed in both ribonuclease-active and -inactive ribosomes at 1·0–2·5mm-Mg²⁺, and also with E. coli MRE600 when EDTA (0·2mm) was added to a solution in 0·16m-sodium chloride. 4. Degradation of ribonuclease-active E. coli B ribosomes at Mg²⁺ concentration 0·25mm or less was coincident with the formation of 16s and 21s ribonucleoprotein in P. fluorescens, and this suggested that complete dissociation of RNA from protein was not an essential prelude to breakdown of the RNA by the enzyme. 5. As high Cs⁺/Mg²⁺ ratios cause ribosomal degradation great care is necessary in the interpretation of equilibrium-density-gradient experiments in which high concentrations of caesium chloride or similar salts are used. 6. The importance of the RNA moiety in understanding the response of ribosomes to their ionic environment is discussed.     

Nucleoside Diphosphate-sugar 4-Epimerases I. Uridine Diphosphate Glucose 4-Epimerase of Wheat Germ

Uridine diphosphate (UDP)-glucose 4-epimerase (EC 5.1.3.2) has been purified over 1000-fold from extracts of wheat germ by MnCl₂ treatment, (NH₄)₂SO₄ fractionation, Sephadex column chromatography, and adsorption onto and elution from calcium phosphate gel. The enzyme has a pH optimum of 9.0. Km values are 0.1 mm for UDP-d-galactose and 0.2 mm for UDP-d-glucose. NAD is required for activity; K_a = 0.04 mm. NADH is an inhibitor strictly competitive with NAD; K_i = 2 μm. Wheat germ also contains UDP-l-arabinose 4-epimerase (EC 5.1.3.5) and thymidine diphosphate (TDP)-glucose 4-epimerase which are distinct from UDP-glucose 4-epimerase.     

Studies of cardiac muscle with a high permeability to calcium produced by treatment with ethylenediaminetetraacetic acid

Thin strips of frog ventricle were isolated and bathed for 15 min in a solution containing 140 mM KCl, 5 mM Na₂ATP, 3 mM EDTA, and 10 mM Tris buffer at pH 7.0. The muscle was then exposed to contracture solutions containing 140 mM KCl, 5 mM Na₂ATP, 1 mM MgCl₂, 10 mM Tris, 3 mM EGTA, and CaCl₂ in amounts to produce concentrations of free calcium from 10^-4.8 M to 10^-9 M. The muscles developed some tension at approximately 10^-8 M, and maximum tension was achieved in 10^-5 M Ca⁺⁺. They relaxed in Ca⁺⁺ concentrations less than 10^-8 M. The development of tension by the EDTA-treated muscles was normalized by comparison with twitch tension at a stimulation rate of 9 per min before exposure to EDTA. In 10^-5 M Ca⁺⁺ tension was always several times the twitch tension and was greater than the contracture tension of a frog ventricular strip in KCl low Na-Ringer. Tension equal to half-maximum was produced at approximately 10^-6.2 M Ca⁺⁺. Intracellular recording of membrane potential indicated that after EDTA treatment the resting potential of cells in Ringer solution with 10^-5 M Ca or less was between 5 and 20 mv. Contracture solutions did not produce tension without prior treatment with EDTA. The high permeability of the membrane produced by EDTA was reversed and the normal resting and action potentials restored in 1 mM Ca-Ringer. Similar studies of EDTA-treated rabbit right ventricular papillary muscle produced a similar tension vs. Ca⁺⁺ concentration relation, and the high permeability state reversed with exposure to normal Krebs solution.     

Partial Purification and Properties of d-Glucosamine 6-Phosphate N-Acetyltransferase from Phaseolus aureus

d-Glucosamine-6-P N-acetyltransferase (EC 2.3.1.4) from mung bean seeds (Phaseolus aureus) was purified 313-fold by protamine sulfate and isoelectric precipitation, ammonium sulfate and acetone fractionation, and CM Sephadex column chromatography. The partially purified enzyme was highly specific for d-glucosamine-6-P. Neither d-glucosamine nor d-galactosamine could replace this substrate. The partially purified enzyme preparation was inhibited up to 50% by 2 × 10⁻²m EDTA, indicating the requirement of a divalent cation. Among divalent metal ions tested, Mg²⁺ was required for maximum activity of the enzyme. Mn²⁺ and Zn²⁺ were inhibitory, while Co²⁺ had no effect on the enzyme activity. The pH optimum of the enzyme in sodium acetate and sodium citrate buffers was found to be 5.2. The effect of Mg²⁺ on the enzyme in sodium acetate and sodium citrate buffers was particularly noticeable in the range of optimum pH. Km values of 15.1 × 10⁻⁴m and 7.1 × 10⁻⁴m were obtained for d-glucosamine-6-P and acetyl CoA, respectively. The enzyme was completely inhibited by 1 × 10⁻⁴mp-hydroxymercuribenzoate, and this inhibition was partially reversed by l-cysteine; indicating the presence of sulfhydryl groups at or near the active site of the enzyme.