The role of nickel in acetyl-CoA synthesis by the bifunctional enzyme CO dehydrogenase/acetyl-CoA synthase: enzymology and model chemistry

 CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) is one of the four known nickel enzymes. It is a bifunctional protein that catalyzes the oxidation of CO to CO₂ at a nickel iron-sulfur cluster (Cluster C) and a remarkable condensation reaction between a methyl group (donated from a methylated corrinoid iron-sulfur protein), carbon monoxide, and coenzyme A to form acetyl-CoA at a separate nickel iron-sulfur cluster (Cluster A). This review focuses on the current understanding of the structure and function of Cluster A and on related model chemistry. It describes studies that uncovered the first example of a biological organometallic reaction sequence. The mechanism of acetyl-CoA synthesis includes enzymebound methylnickel, iron-carbonyl, and acylmetal intermediates. Discovery of the methylnickel species constituted the first example of an alkylnickel species in biology and unveiled a new biological role for nickel. Received: 10 April 1996 / Accepted: 4 July 1996     

Nature of the lectin-induced activation of plasma membrane Mg2+ATPase.

The Mg2+ATPase activity of liver plasma membranes decreases markedly with increasing temperature above 30 degrees. This negative temperature dependency is counteracted by the binding of wheat germ agglutinin, concanavalin A, or Ricinus communis agglutinin (at concentrations greater than or equal 0.5 mg/ml) to membranes prior to assay of the enzyme. With one of these lectins bound, the enzyme has a single energy of activation between 20 degrees and 45 degrees. The binding of dimeric succinyl concanavalin A, soybean agglutinin, fucose-binding lectin from Lotus tetragonolobus, or the leucoagglutinin from Phaseolus vulgaris does not alter the temperature dependency of the enzyme. The latter two lectins, however, do prevent the concanavalin A-induced activation of the enzyme at 37 degrees. At saturating substrate concentrations, the enzyme is not inhibited by any of the lectins tested over a wide range of concentrations. Cytochalasin B and colchicine separately or in combination have little influence on the lectin-induced enhancement of enzyme activity. Chlorpromazine and vinblastine sulfate each partially prevent the activation and in combination do so completely. Treatment of the membranes with the detergent Lubrol-PX or phospholipase A prevents activation of the enzyme by concanavalin A. The results are consistent with a restriction by the lectin of an environment which is normally too disordered for maximal enzyme activity above 30 degrees.     

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Angiotensin-converting enzyme: zinc- and inhibitor-binding stoichiometries of the somatic and testis isozymes.

The blood pressure regulating somatic isozyme of angiotensin-converting enzyme (ACE) consists of two homologous, tandem domains each containing a putative metal-binding motif (HEXXH), while the testis isozyme consists of just a single domain that is identical with the C-terminal half of somatic ACE. Previous metal analyses of somatic ACE have indicated a zinc stoichiometry of 1 mol of Zn2+/mol of ACE and inhibitor-binding studies have found 1 mol of inhibitor bound/mol of enzyme. These and other data have indicated that only one of the two domains of somatic ACE is catalytically active. We have repeated the metal and inhibitor-binding analyses of ACE from various sources and have determined protein concentration by quantitative amino acid analysis on the basis of accurate polypeptide molecular weights that are now available. We find that the somatic isozyme in fact contains 2 mol of Zn2+ and binds 2 mol of lisinopril (an ACE inhibitor) per mol of enzyme, whereas the testis isozyme contains 1 mol of Zn2+ and binds 1 mol of lisinopril. In the case of somatic ACE, the second equivalent of inhibitor binds to a second zinc-containing site as evidenced by the ability of a moderate excess of inhibitor to protect both zinc ions against dissociation. However, active site titration with lisinopril assayed by hydrolysis of furanacryloyl-Phe-Gly-Gly revealed that 1 mol of inhibitor/mol of enzyme abolished the activity of either isozyme, indicating that the principal angiotensin-converting site likely resides in the C-terminal (testicular) domain of somatic ACE and that binding of inhibitor to this site is stronger than to the second site.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma membrane lipids of human diploid fibroblasts from normal individuals and patients with cystic fibrosis.

The lipid composition of isolated plasma membranes of human skin fibroblasts is described for the first time. Plasma membranes from a number of strains of fibroblasts from patients with cystic fibrosis and matched normals were isolated by a recently described procedure and analysed for major phospholipid classes, cholesterol and fatty acids. No differences in the quantities of these compounds were detected between cells of the two different origins. The fetal calf serum used to supplement the growth medium contained relatively more palmitoleate and oleate but less stearate than the membranes. There were also no consistent differences between cystic fibrosis and normal membranes in terms of the fatty acid compositions of their individual phospholipid classes. Consistent with this lack of chemical change in the lipids of membranes of cystic fibrosis cells, the degree of fluorescence polarization of diphenylhexatriene, an index of fluidity, was also unchanged.     

Gas chromatography-mass spectrometry for probing the structure and mechanism of action of enzyme active sites. The role of Glu-270 in carboxypeptidase A.

A new technique for the study of the mechanism of enzymes has been developed. An enzyme, modified by an active-site directed reagent, is digested by one or more proteases. The resulting mixture of oligopeptides is then analyzed directly by gas chromatography-mass spectrometry without the use of separation or isolation procedures. A comparison with unmodified enzyme identifies the modified residue as well as quantifies the reaction. This approach has been applied to the identification of Glu-270 in the active site of carboxypeptidase A using a carbodiimide as modification reagent. Studies on the possible incorporation of 18O (from 18O-enriched water) into Glu-270 or other acidic residues near the active site of carboxypeptidase A show that the oxygens of the carboxyl groups of these residues are not exchangeable.     

Intracellular localization of GDP-l-fucose: Glycoprotein and CMP-sialic acid: Apolipoprotein glycosyltransferases in rat and pork livers

A GDP-l-fucose:glycoprotein fucosyltransferase which transfers l-fucose to terminal β-N-acetyl-d-glucosaminyl residues of sialidase-, β-galactosidase-treated α₁-acid glycoprotein and a CMP-sialic acid:glycoprotein sialyltransferase acting on sialidase-treated apolipoprotein-Ala₁ from human very low density lipoprotein have been shown to be concentrated in rat liver Golgi apparatus preparations at enrichments of 40- and 45-fold, respectively, and in pork liver Golgi-rich fractions at enrichments of 35- and 20-fold, respectively. A second fucosyltransferase acting on sialidase-treated α₁-acid glycopretein was absent from rat liver and was enriched only 13-fold in a pork liver Golgi-rich fraction. The smooth-surfaced microsome fraction was the only other rat liver subcellular fraction with appreciable levels of the GDP-l-fucose: β-N-acetyl-d-glucosaminide fucosyltransferase and the lipoprotein sialyltransferase (enrichments of 2.6- and 5.2-fold, respectivley). This enrichment could not be attributed to the plasma membrane content of the smooth microsome fraction since plasma membrane fractions from rat liver were shown to have relatively low concentrations of these two transferases (enrichments of 0.3 or less). Rat liver plasma membrane was also shown to have similarly low relative specific activities for three other glycosyltransferases (sialyl-, galactosyl-, and N-acetylglucosaminyl-). The accurate determination of the glycosyltransferase activities of the plasma membrane fraction required the use of relatively low concentrations of plasma membrane and relatively high concentrations of nucleotide-sugars in order to avoid interference by the high nucleotide-sugar pyrophosphatase and hydrolase activities of this fraction.     

Binding of [3H]cytochalasin B and [3H]colchicine to isolated liver plasma membranes

The binding to isolated hepatocyte plasma membranes of radioactively labelled inhibitors of microfilamentous and microtubular protein function ([³H]cytochalasin B and [³H]colchicine, respectively) was studied as one means of assessing the degree of association of these proteins with cell surface membranes. [³H]Cytochalasin B which behaved identically to the unlabelled compound with respect to binding to these membranes was prepared by reduction of cytochalasin A with NaB³H₄. The binding was rapid, readily reversible, proportional to the amount of membrane and relatively insentive to changes of pH or ionic strength. At 10^?6 M [³H]cytochalasin B, glucose or p-chloromercuribenzoate, an inhibitor of glucose transport inhibited binding by about 20%; treatment of membranes with 0.6 M KI which depolymerizes F actin to G actin caused about 60% inhibition of binding. These two types of inhibition were additive indicating two separate classes of binding sites, one associated with sugar transport and one with microfilaments. Filamentous structures with the diameter of microfilaments (50 Å) were seen in electron micrographs of thin sections of the membranes. At concentrations greater than 10^?5 M [³H]cytochalasin B, binding was proportional to drug concentration, characteristic of non-specific adsorption or partitioning. Intracellular membranes of the hepatocyte also bound [³H]cytochalasin B, those of the smooth endoplasmic reticulum to a greater extent than plasma membranes.[³H]Colchicine bound to plasma membranes in proportion to the amount of membrane and at a rate compatible with binding to tubulin. However, other properties of the binding including effects of temperature, drug concentration and antisera against tubulin were different from those of binding to tubulin. Hence, no evidence was obtained for association of microtubular elements with these membranes. Despite this there appeared to be an interdependence between microtubule and microfilament inhibitors: vinblastine sulfate stimulated [³H]cytochalasin B binding and cytochalasin B stimulated ³H colchicine binding. [³H]Colchicine also bound to intracellular membranes, especially smooth microsomes.     

Purification and functional reconstitution of the cystic fibrosis transmembrane conductance regulator (CFTR).

Circumstantial evidence has accumulated suggesting that CFTR is a regulated low-conductance Cl- channel. To test this postulate directly, we have purified to homogeneity a recombinant CFTR protein from a high-level baculovirus-infected insect cell line. Evidence of purity included one- and two-dimensional gel electrophoresis, N-terminal peptide sequence, and quantitative amino acid analysis. Reconstitution into proteoliposomes at less than one molecule per vesicle was accomplished by established procedures. Nystatin and ergosterol were included in these vesicles, so that nystatin conductance could serve as a quantitative marker of vesicle fusion with a planar lipid bilayer. Upon incorporation, purified CFTR exhibited regulated chloride channel activity, providing evidence that the protein itself is the channel. This activity exhibited the basic biophysical and regulatory properties of the type of Cl- channel found exclusively in CFTR-expressing cell types and believed to underlie cAMP-evoked secretion in epithelial cells.     

Purification and characterization of recombinant human testis angiotensin-converting enzyme expressed in Chinese hamster ovary cells.

Enzymatically active human testis angiotensin-converting enzyme (ACE) was expressed in Chinese hamster ovary (CHO) cells stably transfected with each of three vectors: p omega-ACE contains a full-length testis ACE cDNA under the control of a retroviral promoter; and pLEN-ACEVII and pLEN-ACE6/5, in which full-length and membrane anchor-minus testis ACE cDNAs, respectively, are under the control of the human metallothionein IIA promoter and SV40 enhancer. In every case, active recombinant human testis ACE (hTACE) was secreted in a soluble form into the culture media, up to 2.4 mg/liter in the media of metal-induced, high-producing clones transfected with one of the pLEN vectors. In addition, membrane-bound recombinant enzyme was recovered from detergent extracts of cell pellets of CHO cells transfected with either p omega-ACE or pLEN-ACE-VII. Recombinant converting enzyme was purified to homogeneity by single-step affinity chromatography of conditioned media and detergent-extracted cell pellets in 85 and 70% overall yield, respectively. Purified hTACE from all sources comigrated with the native testis isozyme on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with M(r) approximately 100 kDa. The native and recombinant proteins cross-reacted equally with anti-human kidney ACE antiserum on Western blotting. The catalytic activity of recombinant angiotensin-converting enzyme, in terms of angiotensin I and 2-furanacryloyl-Phe-Gly-Gly hydrolysis, chloride activation, and lisinopril inhibition, was essentially identical to that of the native enzyme. The facile recovery in high yield of fully active hTACE from the media of stably transfected CHO cells provides a suitable system for investigating structure-function relationships in this enzyme.