Allosteric regulation of biosynthetic threonine deaminase from Escherichia coli: effects of isoleucine and valine on active-site ligand binding and catalysis

The sigmoidal steady-state kinetics of biosynthetic threonine deaminase from Escherichia coli is allosterically controlled by isoleucine and valine, the end-products of branched-chain amino acid biosynthesis. A basis for the regulation of threonine deaminase by heterotropic effectors has been studied by capitalizing on the intrinsic fluorescence of the essential pyridoxal phosphate cofactor in this enzyme in kinetic and equilibrium binding studies with the substrate analog D-threonine. D-Threonine binds cooperatively to four sites on the free enzyme, with an average dissociation constant of 19.8 mM. However, in the presence of saturating valine, or isoleucine, the D-threonine binding isotherms are noncooperative and characterized by dissociation constants of 3.9 and 24.8 mM, respectively. The rate of association of D-threonine with threonine deaminase in the presence of the regulatory ligands was biphasic. Analysis of the data in terms of a two-step scheme whereby the internal aldimine Schiff base in the initial encounter complex undergoes transimination with D-threonine to form an external aldimine yielded estimates for overall binding constants that were in good agreement with those determined from equilibrium binding isotherms. These analyses indicate that the positive allosteric effector valine acts solely to alter the binding of D-threonine to the active sites of threonine deaminase by shifting the equilibrium between a low-affinity and high-affinity state, consistent with predictions from a simple two-state model. However, isoleucine has a compound effect on the enzyme. The negative allosteric ligand promotes decreases in the rate and equilibrium constants for encounter complex formation, consistent with its preferential binding to the low-affinity state of the enzyme. In addition, however, isoleucine promotes a decrease in the transimination rate and equilibrium constants. Since transimination is generally considered to be protein-catalyzed in pyridoxal phosphate requiring enzymes, it is proposed that isoleucine affects both binding and catalysis in threonine deaminase, which provides a possible explanation for the inadequacy of a simple two-state model to describe the allosteric regulation of this enzyme.

Cooperative binding of the feedback modifiers isoleucine and valine to biosynthetic threonine deaminase from Escherichia coli

Control of the regulatory enzyme threonine deaminase from Escherichia coli is achieved by isoleucine inhibition and valine activation. The mechanism by which these heterotropic effectors regulate the enzyme was investigated by measuring the binding of isoleucine and valine by spectroscopic, kinetic, calorimetric and equilibrium dialysis techniques. The addition of isoleucine or valine to threonine deaminase resulted in large changes in the intrinsic fluorescence of the two tryptophans per polypeptide chain. Slightly cooperative binding isotherms for isoleucine were obtained in potassium phosphate, pH 7.5, yielding an average dissociation constant of 4.91 microM, which was confirmed by equilibrium dialysis measurements. Valine binding was much more cooperative, and yielded an average dissociation constant of 122 microM. Titration calorimetry experiments indicated that cooperative heterotropic ligand binding was exothermic, and yielded a stoichiometry of four isoleucine bound per tetrameric enzyme, with an average enthalpy of -10.70 kcal/mol. Valine also bound to four sites per tetramer, with an average enthalpy of -7.45 kcal/mol. The effect of ligands on the fluorescence and circular dichroism spectra of the essential pyridoxal phosphate cofactor indicates that isoleucine and valine bind to effector sites that are distinct from the active sites in threonine deaminase. Shifts in the kinetic properties of threonine deaminase promoted by isoleucine and valine binding are to a first approximation consistent with analyses of effector binding isotherms in terms of a simple two-state model, and suggest that isoleucine regulates threonine deaminase by preferentially binding to the low activity T state, whereas valine binds preferentially to the high activity R state. Finally, analyses of heterotropic effector binding isotherms suggest that active site ligands may have significant affinity for the regulatory sites, which gives rise to underestimates for the allosteric equilibrium constants determined from substrate analog binding isotherms.

Energetics of cooperative ligand binding to the active sites of biosynthetic threonine deaminase from Escherichia coli

The sigmoidal kinetics of alpha-ketobutyrate production catalyzed by threonine deaminase are shifted in the presence of the feedback inhibitor isoleucine and the activator valine to control carbon flow through branched-chain amino acid biosynthesis in Escherichia coli. As an initial effort toward developing a molecular mechanism for cooperativity and feedback inhibition in this enzyme, the binding of the substrate analogs 2-aminobutyrate and alanine were measured. Binding isotherms were determined in potassium phosphate at pH 7.5 by detection of an 8-10-fold increase in intrinsic fluorescence of the external aldimine Schiff base of these analogs with the essential pyridoxal phosphate cofactor of the enzyme. Both 2-aminobutyrate and alanine bind cooperatively to four sites on threonine deaminase, with an average dissociation constant of 12.7 and 43.8 mM, respectively. The feedback inhibitor isoleucine decreases the average affinity for the ligands and increases the degree of cooperativity. The activator valine decreases the degree of cooperativity, but gives rise to a slight increase in the average dissociation constant for 2-aminobutyrate and alanine, possibly due to a competitive effect with active site ligands. The temperature dependence of the average affinity of ligands for the active sites indicates that binding is entropically controlled, with average values for delta H0 of +6.0 kcal/mol for alanine and +4.87 kcal/mol for 2-aminobutyrate. Since none of the ligands under investigation had any effect on the tetrameric quaternary structure of the enzyme as judged by sedimentation equilibrium, an initial attempt to describe cooperative ligand association with threonine deaminase was undertaken by analysis of the binding isotherms in terms of a two-state model in which the enzyme equilibrates between two conformations, T and R, that differ in their affinity for ligands. This analysis suggests that cooperative binding to the enzyme is largely the result of the high preferential affinity of ligands for the R state. In addition, estimates for the allosteric equilibrium constant L range from 2 to 5, in the absence of modifiers, to 35 in the presence of isoleucine, and 0.9 in the presence of valine. To a first approximation, the data are consistent with isoleucine and valine regulating threonine deaminase by shifting the allosteric equilibrium between the different affinity forms.

Aromatic amine dehydrogenase, a second tryptophan tryptophylquinone enzyme

Aromatic amine dehydrogenase (AADH) catalyzes the oxidative deamination of aromatic amines including tyramine and dopamine. AADH is structurally similar to methylamine dehydrogenase (MADH) and possesses the same tryptophan tryptophylquinone (TTQ) prosthetic group. AADH exhibits an alpha 2 beta 2 structure with subunit molecular weights of 39,000 and 18,000 and with a quinone covalently attached to each beta subunit. Neither subunit cross-reacted immunologically with antibodies to the corresponding subunits of MADH, and the N-terminal amino acid sequence of the beta subunit of AADH exhibited no homology with the highly conserved beta subunits of MADH. The absorption spectra for the oxidized, semiquinone, and reduced forms of AADH have been characterized, and extinction coefficients for the absorption maxima of each redox form have been determined. These spectra are very similar to those for MADH, indicating the likelihood of a TTQ cofactor. This was verified by the near identity of the vibrational frequencies and intensities in the resonance Raman spectra for the oxidized forms of AADH and MADH. A stable semiquinone of AADH could be observed during a reductive titration with dithionite, whereas titration with tyramine proceeded directly from the oxidized to the reduced form. AADH was very stable against denaturation by heat and exposure to guanidine. The individual subunits could be separated by gel filtration after incubation in guanidine hydrochloride, and partial reconstitution of activity was observed on recombination of the subunits. Steady-state kinetic analysis of AADH yielded a Vmax of 17 mumol/min/mg and a Km for tyramine of 5.4 microM. Substrate inhibition by tyramine was observed. AADH was irreversibly inhibited by hydrazine, phenylhydrazine, hydroxylamine, semicarbazide, and aminoguanidine. Isonicotinic acid hydrazide (isoniazid) and isonicotinic acid 2-isopropyl hydrazide (iproniazid) were reversible noncompetitive inhibitors of AADH and exhibited K(i) values of 8 and 186 microM, respectively. The similarities and differences between AADH and other amine oxidizing enzymes are also discussed.

Equilibrium folding studies of tetrameric R67 dihydrofolate reductase

R67 dihydrofolate reductase (DHFR) is an R-plasmid encoded enzyme that confers resistance to the antibacterial drug trimethoprim. This enzyme is not homologous in sequence or structure to chromosomal DHFRs. Equilibrium folding of tetrameric R67 DHFR was studied and found to be fully reversible. Formation of an inactive intermediate was assayed by loss of enzyme activity. Denaturation of the intermediate was monitored by concurrent changes in fluorescence and circular dichroism signals. Both transitions are protein concentration dependent. A simple model fitting these data is tetramer<==>2 dimers<==>4 unfolded monomers. No evidence for folded monomer was found. Global fitting of all the folding data yielded a delta GH2O of -9.63 kcal/mol for the initial transition and a delta GH2O of -12.35 kcal/mol for the second transition. In addition, thermal unfolding of tetrameric R67 DHFR was found to be reversible A folding intermediate also occurred during thermal unfolding as evidenced by the asymmetric endotherms and a delta Hcalorimetric/delta H(van't Hoff) ratio of 2.1.

Automated monitoring of outcomes: application to treatment of drug abuse

This paper suggests a new approach for lowering follow-up costs, improving the delivery of health care, and monitoring treatment outcomes. An automated telephone follow-up system that calls, identifies, and interviews clients is an alternative method for monitoring patients that may be both reliable and cost-effective. To test the viability of such a system, the authors monitored a patient population that has historically been shown to be difficult to follow: recovering drug users and alcoholics. Forty-two subjects were asked to call the computer and complete interviews on a weekly basis for five months. Clients answered 25 recorded questions by pressing the keys on their telephone pads. The computer automatically analyzed the clients' answers and estimated a probability of relapse for each client. In addition, the computer automatically called subjects who failed to complete interviews at the scheduled times. The study showed that self-reported data collected by a computer are as reliable as data obtained through a written questionnaire and that clients are more willing to respond to computer interviews than to mailed written questionnaires. This study also provides preliminary data on the predictive ability of a questionnaire for predicting relapse.

Structural and physicochemical analysis of the reaction between the anti-lysozyme antibody D1.3 and the anti-idiotopic antibodies E225 and E5.2

The reaction between the mouse (BALB/c) anti-idiotopic monoclonal antibodies E225 and E5.2 and idiotopes on the (BALB/c) anti-lysozyme monoclonal antibody D1.3 has been characterized by titration calorimetry, by equilibrium sedimentation and by the determination of binding association and dissociation rates. The reaction between E5.2 and D1.3 is driven by a large negative enthalpy and its rate and equilibrium association constants are comparable to those observed in other antigen-antibody reactions. In contrast, the reaction between E225 and D1.3 is entropically driven and characterized by slow association kinetics (1 x 10(3) M-1 sec-1) and a resulting low equilibrium constant (Ka = 2 x 10(5) M-1). A correlation of these properties with the three-dimensional structure of the Fab225-FabD1.3 complex, previously determined by X-ray diffraction methods to 2.5 A resolution, indicates that conformational changes of several D1.3 contacting residues, located in its complementarity determining regions, may explain these features of the reaction.

An efficient approach to identify ilvA mutations reveals an amino-terminal catalytic domain in biosynthetic threonine deaminase from Escherichia coli

High-level expression of the regulatory enzyme threonine deaminase in Escherichia coli strains grown on minimal medium that are deficient in the activities of enzymes needed for branched-chain amino acid biosynthesis result in growth inhibition, possibly because of the accumulation of toxic levels of alpha-ketobutyrate, the product of the committed step in isoleucine biosynthesis. This condition affords a means for selecting genetic variants of threonine deaminase that are deficient in catalysis by suppression of growth inhibition. Strains harboring mutations in ilvA that decreased the catalytic activity of threonine deaminase were found to grow more rapidly than isogenic strains containing wild-type ilvA. Modification of the ilvA gene to introduce additional unique, evenly spaced restriction enzyme sites facilitated the identification of suppressor mutations by enabling small DNA fragments to be subcloned for sequencing. The 10 mutations identified in ilvA code for enzymes with significantly reduced activity relative to that of wild-type threonine deaminase. Values for their specific activities range from 40% of that displayed by wild-type enzyme to complete inactivation as evidenced by failure to complement an ilvA deletion strain to isoleucine prototrophy. Moreover, some mutant enzymes showed altered allosteric properties with respect to valine activation and isoleucine inhibition. The location of the 10 mutations in the 5' two-thirds of the ilvA gene is consistent with suggestions that threonine deaminase is organized functionally with an amino-terminal domain that is involved in catalysis and a carboxy-terminal domain that is important for regulation.

NIH conference. New insights into common variable immunodeficiency

Common variable immunodeficiency (CVI) is a heterogenous immunodeficiency syndrome characterized by hypogammaglobulinemia, recurrent bacterial infections, and various immunologic abnormalities. In addition to recurrent infections, patients with this syndrome also have an increased incidence of autoimmune disease and malignancy. Because the spectrum of associated diseases is broad, patients with CVI are seen by various medical specialists. This review discusses the pathogenesis, clinical manifestations, diagnosis, and treatment of CVI.

Titration of histidine 62 in R67 dihydrofolate reductase is linked to a tetramer<-->two-dimer equilibrium

R67 dihydrofolate reductase (DHFR) is an R-plasmid encoded protein that confers clinical resistance to the antibacterial drug trimethoprim. To determine whether an acidic titration in kinetic pH profiles is related to titration of histidines 62, 162, 262, and 362, the stability of tetrameric R67 DHFR has been monitored as a function of pH. For the pH range 5-8, tetrameric R67 DHFR reversibly dissociates into dimers, as monitored by ultracentrifugation and molecular sieving techniques. From the crystal structures of dimeric and tetrameric R67 DHFR [Matthews et al. (1986) Biochemistry 25, 4194-4204] (Narayana, Matthews, and Xuong, personal communication), symmetry-related histidines 62, 162, 262, and 362 occur at the two dimer-dimer interfaces and protonation of these residues could destabilize tetrameric R67 DHFR. Ionization of these histidines was confirmed by monitoring the chemical shifts of the C2 proton in NMR experiments, and best fits of an incomplete titration curve yield a pKa of 6.77. Since tryptophans 38, 138, 238, and 338 also occur at the dimer-dimer interfaces, fluorescence additionally monitors the tetramer-two dimers equilibrium. When fluorescence was monitored over the pH range 5-8, a protein concentration dependence of fluorescence was observed and global fitting of three titration curves yielded Kd = 9.72 nM and pKa = 6.84 for the linked reactions: [formula: see text] Modification of H62, H162, H262, and H362 by diethyl pyrocarbonate stabilizes dimeric R67 DHFR and causes a 200-600-fold decrease in catalytic efficiency. Decreased catalytic activity in dimeric R67 DHFR is presumably due to loss of the putative single active site pore found in tetrameric R67 DHFR.

Street youth: social imbalance and health risks

Street youth are children and adolescents who express, through their bodies and lives, misery and social abandonment. Paradoxically, they are also the embodiment of resistance and survival strategies in a very unfavourable environment. The current deterioration of citizens' rights and the social inequalities reflect the consequences of vicious cycles of economic and political crises in Brazil. Poverty conditions reach levels of misery and create strategies of survival that create multiple high-risk health situations. Many other factors make up the complex dynamics of these conflicts in which the limit of rejection and social abandonment are expressed in the violence of extermination. The proposals and solutions cannot be dissociated from the reality of this country and of the street youth lifestyle. Effective health education within this social context is difficult, but possible with the development of appropriate audio-visual material and the dissemination of preventive health information. A commitment to action must permeate all levels of decision making within the health system and in our society, for it is the essence of democracy and human rights.

T-cell abnormalities in common variable immunodeficiency

Common variable immunodeficiency (CVI) is a heterogeneous condition marked by a number of different immunologic defects. One group of patients, perhaps 60% of the CVI group as a whole, is characterized by T cells that produce reduced amounts of IL-2 (mRNA and protein product), IL-4, and IL-5 (mRNA) when stimulated with phytohemagglutinin. This defect does not extend to all lymphokines, however, because the cells produce normal amounts of interferon-gamma (mRNA and protein product) when exogenous IL-2 is present. Recently, we have reexamined the T cell lymphokine production defect using a panoply of T-cell activation stimuli and have shown that the defect is a subtle one that depends on activation of the cell via the CD3-T-cell receptor complex. Because T cells proliferate normally when stimulated via this receptor, this finding suggests the presence of a "downstream" defect, perhaps one involving the factors that are necessary for activation of lymphokine genes. A second form of CVI, in this case involving approximately 30% of the CVI group as a whole, is characterized by a reduced CD4/CD8 ratio and elevated numbers of CD8+ T cells bearing the CD57 marker. Although the CD4+ T cells in this patient group elaborate normal amounts of IL-2 under various activation conditions, their CD8+ T cells produce increased amounts of interferon-gamma. Furthermore, the CD8+ T cells in this case act as "suppressor" T cells, which suppress IgG production but not IgM production of purified (normal) SAC+, IL-2-induced B cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative complementation in aspartate transcarbamylase. Analysis of hybrid enzyme molecules containing different arrangements of polypeptide chains from wild-type and inactive mutant catalytic subunits

A comprehensive set of hybrid molecules of aspartate transcarbamylase (ATCase) from Escherichia coli has been constructed of wild-type and mutationally altered catalytic chains. The mutant enzymes that were virtually devoid of activity contained a replacement of Gly-128 in the catalytic polypeptide chains by either Asp or Arg. The kinetic properties of these hybrid enzyme-like molecules were analyzed to evaluate the basis for the unusual quaternary constraint demonstrated by an intersubunit hybrid containing one wild-type catalytic subunit, one inactive mutant subunit (containing the Gly to Asp replacement), and three wild-type regulatory subunits. A similar intersubunit hybrid was constructed from the wild-type catalytic subunit and the mutant in which Gly-128 was replaced by Arg, and it too demonstrated a pronounced decrease in activity relative to that expected for a hybrid containing three active sites. Moreover, neither of these hybrid holoenzymes exhibited the cooperativity with respect to aspartate that is characteristic of wild-type ATCase. In contrast, hybrid holoenzymes containing at least one wild-type chain in each catalytic subunit showed cooperativity. Also, hybrid enzymes containing different arrangements of five, four, three, or two wild-type catalytic chains with an appropriate complement of mutant chains had specific activities proportional to the number of wild-type chains in the holoenzymes. Exceptions were observed only in hybrids in which one of the two subunits in the holoenzyme was composed completely of mutant catalytic chains. For these hybrids the negative complementation was manifested as a much lower enzyme activity than expected from the number of wild-type chains in the enzyme and the loss of cooperativity. Thus, the activity and allosteric properties of these hybrids is dependent on the arrangement of catalytic chains in the holoenzyme, in contrast to results obtained for hybrids containing native and chemically modified catalytic chains. Intrasubunit hybrid catalytic trimers containing one or two wild-type chains exhibited one-third and two-thirds the activity of the intact wild-type catalytic subunit, respectively, indicating the dominant negative effect that was seen in intersubunit hybrid holoenzymes is absent within trimers.

Cloning, expression, purification, and characterization of biosynthetic threonine deaminase from Escherichia coli

Feedback inhibition of the regulatory enzyme threonine deaminase by isoleucine provides an important level of enzymic control over branched chain amino acid biosynthesis in Escherichia coli. Cloning ilvA, the structural gene for threonine deaminase, under control of the trc promoter results in expression of active enzyme upon induction by isopropyl 1-thio-beta-D-galactoside to levels of approximately 20% of the soluble protein in cell extracts. High level expression of threonine deaminase has facilitated the development of a rapid and efficient protocol for the purification of gram quantities of enzyme with a specific activity 3-fold greater than previous preparations. The catalytic activity of threonine deaminase is absolutely dependent on the presence of pyridoxal phosphate, and the tetrameric molecule is isolated containing 1 mol of cofactor/56,000-Da chain. Wild-type threonine deaminase demonstrates a sigmoidal dependence of initial velocity on threonine concentration in the absence of isoleucine, consistent with a substrate-promoted conversion of the enzyme from a low activity to a high activity conformation. The enzymic dehydration of threonine to alpha-ketobutyrate measured by steady-state kinetics, performed at 20 degrees C in 0.05 M potassium phosphate, pH 7.5, is described by a Hill coefficient, nH, of 2.3 and a K0.5 of 8.0 mM. The negative allosteric effector L-isoleucine strongly inhibits the enzyme, yielding a value for nH of 3.9 and K0.5 of 74 mM whereas enzyme activity is greatly increased by L-valine, which yields nearly hyperbolic kinetics characterized by a value for nH of 1.0 and a K0.5 of 5.7 mM. Thus, these effectors promote dramatic and opposing effects on the transition from the low activity to the high activity conformation of the tetrameric enzyme.

Heterotropic effectors promote a global conformational change in aspartate transcarbamoylase

The sigmoidal dependence of activity on substrate concentration exhibited by the regulatory enzyme aspartate transcarbamoylase (ATCase) of Escherichia coli is generally attributed to a ligand-promoted change in the quaternary structure of the enzyme. Although a global conformational change in ATCase upon the binding of ligands to some of the six active sites is well documented, a corresponding alteration in the structure of the wild-type enzyme upon the addition of the inhibitor, CTP, or the activator, ATP, has not been detected. Such evidence is essential for testing whether heterotropic, as well as homotropic, effects can be accounted for quantitatively in terms of coupled equilibria involving a conformational change in the enzyme and preferential binding of ligands to one conformation or the other. This evidence has now been obtained with a mutant form of ATCase in which Lys 143 in the regulatory chain was replaced by Ala, thereby perturbing interactions at the interface between the regulatory and catalytic chains in the enzyme and destabilizing the low-activity, compact (T) conformation relative to the high-activity, swollen (R) state. Difference sedimentation velocity experiments involving measurements of the changes caused by the binding of the bisubstrate analogue N-(phosphonacetyl)-L-aspartate demonstrated that the sedimentation coefficient of the mutant enzyme was intermediate between that observed for the T and R states of wild-type ATCase. We interpret the results as indicating that the [T]/[R] ratio in phosphate buffer at pH 7.0 is reduced from about 2 X 10(2) for the wild-type enzyme to 2.7 for r143Ala ATCase.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in stability and allosteric properties of aspartate transcarbamoylase resulting from amino acid substitutions in the zinc-binding domain of the regulatory chains

Changes in subunit interaction energies linked to the allosteric transition of the regulatory enzyme aspartate transcarbamoylase (ATCase; EC 2.1.3.2) from Escherichia coli are localized in part at interfaces between the six catalytic (c) and six regulatory (r) polypeptide chains. Site-directed mutagenesis has been used to construct enzymes with amino acid substitutions in a limited region of the zinc-binding domain of the r chains. Substitution of Ser or His for r114 Cys, one of four cysteines binding the structural zinc ion in the regulatory chain, leads to incorrectly folded chains as shown by the inability to detect stable assembled holoenzyme in cell extracts. Replacement of r111 Asn by Ala at the interface between an r chain and a c chain in the apposing catalytic trimer causes a complete loss of the homotropic and heterotropic effects characteristic of wild-type ATCase. Moreover, sedimentation velocity experiments demonstrated that this mutant enzyme exists in the R ("relaxed") conformation in the absence of active site ligands due to preferential destabilization of the T ("taut") conformation relative to the R state. In contrast, replacement of r113 Asn by Ala at the interface between adjacent r and c chains leads to an increase in the cooperativity of the enzyme. When r139 Lys is replaced by Met, Vmax is reduced by 50% compared to wild-type ATCase, whereas it is increased about 2-fold when r142 Glu is replaced by Asp. Amino acid substitutions in this domain significantly affect subunit interaction energy as measured by rate of subunit exchange when holoenzymes are incubated with isolated catalytic subunits, thus permitting measurements of the effect of the bisubstrate analog N-(phosphonacetyl)-L-asparatate in weakening intersubunit interactions. Subunit exchange increased about 9-fold for the r142 Glu----Asp mutant and almost 20-fold for the r142 Glu----Ala mutant in the presence of the ligand.

Regulation of arginine metabolism in Saccharomyces cerevisiae. Association of arginase and ornithine transcarbamoylase

Association of arginase and ornithine transcarbamoylase (OTCase) has been proposed to play an essential role in the regulation of arginine metabolism in Saccharomyces cerevisiae (Wiame, J.-M. (1971) Curr. Top. Cell. Reg. 4, 1-39). In this report multienzyme complex formation is directly demonstrated in the presence of the active-site ligands for OTCase and arginase. Using equilibrium sedimentation, a dissociation constant for complex formation was determined to be 2.3 X 10(-8) M in the presence of ornithine and agmatine, active-site ligands for OTCase and arginase, respectively. A molecular stoichiometry in the complex of one molecule of OTCase to one molecule of arginase was verified using transmission electron microscopy. The dimensions of the complex were determined by negative staining and rotary and unidirectional shadowing techniques to be 102 A wide by 81 A high. These dimensions are quantitively consistent with dimensions of the individual enzymes (Duong, L. T., Eisenstein, E., Green, S. M., Ornberg, R. L., and Hensley, P. (1986) J. Biol. Chem. 261, 12807-12813). The enzymatic activity of OTCase is virtually completely inhibited when associated with arginase, reflecting the dramatic modulation of enzyme activity as a consequence of the acquisition of quaternary structure in this multienzyme complex.

The quaternary structure of ornithine transcarbamoylase and arginase from Saccharomyces cerevisiae

Electron microscopic studies employing negative staining, rotary shadowing, and unidirectional shadowing have revealed the subunit architecture of ornithine transcarbamoylase and arginase from Saccharomyces cerevisiae. These techniques have confirmed the quaternary structure of these enzymes, and have permitted an estimate of the shape and dimensions of each of the individual enzymes as well as those of the corresponding subunits to be determined. Both enzymes are trimers exhibiting 3-fold rotational symmetry with subunits which are oblate ellipsoids of revolution. The overall dimensions determined for ornithine transcarbamoylase are a height of 39 A and a width of 102 A. The consequent subunit dimensions are 59 X 59 X 39 A, yielding a subunit axial ratio of 1.51. Arginase has a height of 42 A and a width of 97 A. The subunit dimensions are 56 X 56 X 42 A, yielding a subunit axial ratio of 1.33. The hydrodynamic behavior of the two enzymes is fully consistent with this molecular architecture, suggesting that the structures proposed here are similar to those of the proteins in aqueous solution.

Ligand binding-promoted conformational changes in yeast ornithine transcarbamoylase

It has been proposed that regulatory multienzyme complex formation between yeast ornithine transcarbamoylase (OTCase) and arginase is triggered by a conformational change promoted by the binding of ornithine to a regulatory site in OTCase (Wiame, J.-M. (1971) Curr. Top. Cell. Regul. 4, 1-38). To isolate the binding of ornithine to the proposed regulatory site, the active site was blocked with the high affinity (Ki = 13 +/- 1.4 nM) bisubstrate analogue, delta-N-phosphonacetyl-L-ornithine (PALO). The binding of PALO to the active site produces large changes in the absorption (delta A290-296 = 0.010/mg of enzyme) and in the fluorescence (25% quenching) of the protein. These changes both saturate at one PALO/polypeptide chain. The binding of PALO also changes the rate constant for diffusional acrylamide quenching by 43% and increases the midpoint for the thermal denaturation of the enzyme by 13 degrees C. Finally, PALO binding results in a +2.8% change in the sedimentation coefficient demonstrating that these spectral and energetic changes are associated with a gross structural change in the enzyme. In an effort to detect ligand binding to the proposed effector site on OTCase, ornithine was added to the enzyme saturated with PALO, and consequent conformational changes were tested for using methodologies identical to those which demonstrated active site ligand binding-promoted conformational changes. In no instance were any additional differences observed. Hence, strong support for isosteric effector binding-promoted conformational changes cannot be presented. We conclude that active site ligand binding events themselves are responsible for conformational changes which promote enzyme-enzyme association of OTCase with arginase.

Purification and characterization of ornithine transcarbamoylase from Saccharomyces cerevisiae

Ornithine transcarbamoylase (OTCase) has been purified in 100-mg quantities from a plasmid-containing, enzyme-overproducing strain of Saccharomyces cerevisiae. The specific activity of the homogeneous enzyme is 2.5-fold above that previously reported. The molecular weight and partial specific volume of OTCase were determined by sedimentation equilibrium in solutions containing H2O and D2O. Data from two rotor speeds were simultaneously fit using nonlinear least squares analysis with multiple independent variables giving a molecular weight of 110,000 +/- 2,200 and a partial specific volume of 0.732 +/- 0.006 ml g-1. The ultraviolet absorption spectrum of OTCase gives a specific absorbance at 280 nm of 0.36. This low value is consistent with a small number of aromatic residues. Amino acid analysis, fluorescence, and multicomponent analysis yield 1 tryptophan, 4 tyrosine, and 24 phenylalanine/polypeptide chain. From an analysis of the circular dichroic spectrum, it was determined that OTCase contained 22% alpha-helix, 43% beta-sheet, 8% beta-turn, and 27% random structure. The fluorescence of the single tryptophan/polypeptide chain has an emission maximum at 320 nm, indicating a hydrophobic environment.

Evaluation of the growth hormone exercise test in normal and growth hormone-deficient children

A group of 168 short but otherwise normal children (group A) and 25 children deficient in growth hormone (GH) (group B) wwere studied with an exercise stimulation test to determine the expected error of this method. In group A, 125 (74.4%) had maximum GH responses greater than 15 ng/ml, 23 (13.7%) had responses between 10 and 15 ng/ml, and 20 (11.9%) had responses less than 10 ng/ml. The mean +/- SD values were 8.4 +/- 0.4 ng/ml at 0 time, 26.3 +/- 15.0 at 20 minutes, and 10.7 +/- 8.3 at 40 minutes. The mean maximum response was 27.7 +/- 14.3 ng/ml. In group B, 22 (88%) had maximum responses less than 10 ng/ml and 3 (12%) had responses between 10 and 15 ng/ml. Patients with maximum responses less than 10 ng/ml have a high probability of being GH-deficinet, whereas patients with responses between 10 and 15 ng/ml are less likely to be GH-deficient. No patients with responses greater than 15 ng/ml were GH-deficient.