Fully automated assay of glycohemoglobin with the Abbott IMx analyzer: novel approaches for separation and detection.

We describe a novel assay for measuring glycohemoglobin directly from anticoagulated whole blood with the Abbott IMx analyzer. The glycohemoglobin is labeled with a soluble polyanionic affinity reagent and the anionic complex is then captured with a cationic solid-phase matrix. Glycohemoglobin is quantified by measuring the quenching by heme of the static fluorescence from an added fluorophore. The assay is standardized to report both percent total glycohemoglobin (%GHb) and percent hemoglobin A1c (%HbA1c). Glucose, bilirubin, triglycerides, labile fraction, and hemoglobin variants do not interfere in the assay. Within- and between-run CVs are approximately 4-5%, with total CVs of approximately 6.5%. Highly significant linear correlations (r > 0.97) were obtained in comparison studies with two major assay methodologies. The time to obtain one result is approximately 10 min (including assay of a control), 56 min for 22 results. We describe the development, standardization, and validation of this new method.

Quantifying the MB isoenzyme of creatine kinase with the Abbott "IMx" immunoassay analyzer.

In this two-step automated assay of the MB isoenzyme of creatine kinase (CK-MB), developed for the Abbott "IMx" immunoassay analyzer, monoclonal anti-CK-MB antibody immobilized onto latex microparticles and polyclonal anti-CK-MM antibody coupled to alkaline phosphatase are used. Within-run CVs ranged from 3.9% to 9.0%, between-run CVs from 0.0% to 5.6%, and the sensitivity was 0.2 microgram/L. Twenty-four results can be obtained in about 37 min. Analytical recovery of CK-MB added to human serum or plasma ranged from 89% to 109%. Icteric, lipemic, or hemolyzed samples did not interfere with CK-MB recovery. Cross-reactivity with CK-MM and CK-BB was 0.012% and 0.001%, respectively. The normal reference interval was 0-5 micrograms/L. IMx CK-MB results correlated well with CK-MB enzyme activity as determined by electrophoresis (n = 203; r = 0.97; slope = 0.90; y-intercept = -4.29) and with commercial immunoassays. We think that this assay will be useful for confirmation of acute myocardial infarction, both in critical-care units and in the clinical laboratory.

Effect of Al3+ plus F- on the catecholamine-stimulated GTPase activity of purified and reconstituted Gs.

The effects of Al3+ and F- on the catecholamine-stimulated GTPase cycle were studied by using reconstituted phospholipid vesicles that contained purified beta-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, Gs. Al3+/F- activated reconstituted Gs to levels previously reported for detergent-solubilized, purified Gs, although both activation and deactivation were faster in the reconstituted preparation. Under these conditions, Al3+/F- did not inhibit by more than 15% the beta-adrenergic agonist-stimulated GTPase activity of the vesicles nor did it significantly inhibit the rates of GTP binding, GTP hydrolysis, or GDP release. When Mg2+ (50 mM) was used instead of agonist to promote GTP hydrolysis in the receptor-Gs vesicles, Al3+/F- was found to inhibit GTP gamma S binding, GDP release, and steady-state GTPase activity to unstimulated levels. These data can be interpreted as indicating that the receptor catalyzes nucleotide exchange by Gs faster or more efficiently than does Mg2+.

The avian beta-adrenergic receptor: primary structure and membrane topology.

Partial amino acid sequence information allowed the isolation of cDNA clones encoding the turkey erythrocyte beta-adrenergic receptor. Antisera raised against synthetic peptides encoded by the cDNA crossreacted with the purified receptor and appropriate tryptic fragments, confirming the identity of the cDNA. The receptor is composed of 483 amino acids and has a molecular mass of 54 kDa. Its sequence suggests that it is arranged predominantly in seven membrane-spanning sequences and a long cytoplasmic carboxyl-terminal domain. The extracellular amino-terminal domain contains a consensus sequence for N-glycosylation. The beta-adrenergic receptor displays overall structural similarity and weak sequence homology with rhodopsin. Because both proteins act by regulating GTP-binding proteins, a compact structure based on seven membrane-spanning regions may be a general model for receptors that act on G proteins.

Catecholamine-stimulated GTPase cycle. Multiple sites of regulation by beta-adrenergic receptor and Mg2+ studied in reconstituted receptor-Gs vesicles.

The regulation of the intermediary steps of the catecholamine-stimulated GTPase cycle by beta-adrenergic agonists and Mg2+ was investigated using unilamellar phosphatidylethanolamine-phosphatidylserine vesicles that contained purified beta-adrenergic receptor and the stimulatory GTP-binding protein of the adenylate cyclase system, Gs. The steady-state turnover number of the agonist-stimulated GTPase, normalized according to the receptor-responsive pool of Gs, was 0.8 min-1 for untreated vesicles and 1.7 min-1 for vesicles that had been treated with dithiothreitol to activate the receptors. The binding and release of [alpha-32P]GTP, [3H] GTP, and [gamma-32P]GTP were used to measure the binding and hydrolysis of GTP and the release of GDP. Agonist-liganded receptor stimulated both the binding of GTP and the release of the GDP product, and GDP release per se did not appear to be the mechanism by which receptor stimulated the binding of GTP. Both processes displayed apparent first order rate constants of about 0.5 min-1 for untreated vesicles and both rates increased about 5-fold after dithiothreitol treatment. Both processes were formally catalytic with respect to receptor, in that several (up to 8) molecules of Gs were stimulated per molecule of receptor. The hydrolysis of Gs X GTP to Gs X GDP was unaltered by agonist and occurred with a rate constant of about 4 min-1. The rates of these partial reactions were consistent with the overall rate of steady-state hydrolysis and with the ability of the agonist-liganded receptor to promote the formation of sufficient Gs X GTP to fully stimulate adenylate cyclase in a native membrane. The Mg2+ dependence of agonist-stimulated, steady-state GTPase activity appeared to consist of at least two, distinct Mg2+-requiring processes. Very low concentrations of Mg2+ (approximately 20 nM) were required for hydrolysis of Gs X GTP, and 10 microM Mg2+ was required to maximize the initial rate of agonist-stimulated [alpha-32P] GTP binding.

GTPase activity of the stimulatory GTP-binding regulatory protein of adenylate cyclase, Gs. Accumulation and turnover of enzyme-nucleotide intermediates.

The GTPase activity of the stimulatory guanine nucleotide-binding regulatory protein (Gs) of hormone-sensitive adenylate cyclase was investigated using purified rabbit hepatic Gs and either [alpha-32P]- or [gamma-32P] GTP as substrate. The binding of [35S]guanosine 5'-O-(thiotriphosphate) (GTP gamma S) was used to quantitate the total concentration of Gs. 1) GTPase activity was a saturable function of the concentration of GTP, with Km = 0.3 microM. MgCl2 monotonically increased the activity. The maximum observed turnover number was about 1.5 min-1. 2) During steady-state hydrolysis, 20-40% of total Gs could be trapped as a Gs-GDP complex and 1-2% could be trapped as Gs-GTP. The hydrolysis of Gs-GTP to Gs-GDP occurred with t 1/2 less than or equal to 5 s at 30 degrees C and t 1/2 approximately 1 min at 0 degrees C. Hydrolysis of Gs-GTP was inhibited by 1.0 mM EDTA in the absence of added Mg2+. 3) The rate of formation of Gs-GDP and the initial GTPase rate varied in parallel as functions of the concentrations of either GTP or MgCl2 (above 0.1 mM Mg2+). The ratio of the rate of accumulation of Gs-GDP to the GTPase rate was constant at 0.3-0.4. 4) The rate of dissociation of assayable Gs-GDP was biphasic. The initial phase accounted for 60-80% of total assayable Gs-GDP and was characterized by a t 1/2 of about 1 min. 5) Lubrol 12A9 potently inhibited the GTPase reaction and the dissociation of Gs-GDP in parallel, and inhibition of product release may account for the inhibition of steady-state hydrolysis. 6) The beta and gamma subunits of Gs markedly inhibited the dissociation of GDP from Gs in contrast to their ability to stimulate the dissociation of GTP gamma S. 7) GDP, GTP gamma S, and guanyl-5'-yl imidodiphosphate (Gpp(NH)p) competitively inhibited the accumulation of Gs-GDP. GTP gamma S and Gpp(NH)p inhibited the GTPase reaction noncompetitively, GDP displayed mixed inhibition, and Pi did not inhibit. These data are interpretable in terms of the coexistence of two specific mechanistic pathways for the overall GTPase reaction.

Reconstitution of catecholamine-stimulated guanosinetriphosphatase activity.

beta-Adrenergic receptors were partially purified from turkey erythrocyte membranes by alprenolol-agarose chromatography to 0.25-2 nmol/mg of protein, and the stimulatory guanosine 5'-triphosphate (GTP) binding protein of adenylate cyclase (Gs) was purified from rabbit liver. These proteins were reconstituted into phospholipid vesicles by addition of phospholipids and removal of detergent by gel filtration. This preparation hydrolyzes GTP to guanosine 5'-diphosphate (GDP) plus inorganic phosphate (Pi) in response to beta-adrenergic agonists. The initial rate of isoproterenol-stimulated hydrolysis is approximately 1 mol of GTP hydrolyzed min-1 X mol-1 of Gs. This low rate may be limited by the hormone-stimulated binding of substrate, since it is roughly equal to the rate of binding of the GTP analogue guanosine 5'-O-(3-[35S] thiotriphosphate) [( 35S]GTP gamma S) to Gs in the vesicles. Activity in the absence of agonist, or in the presence of agonist plus a beta-adrenergic antagonist, is 8-25% of the hormone-stimulated activity. Guanosinetriphosphatase (GTPase) is not saturated at 10 microM GTP, and the response to GTP is formally consistent either with the existence of multiple Km's or of a separate stimulatory site for GTP. The GTPase activity of Gs in vesicles is also stimulated by 50 mM MgCl2 in the presence or absence of receptor. Significant GTPase activity is not observed with Lubrol-solubilized Gs, although [35S]-GTP gamma S binding is increased by Lubrol solubilization.

Heterogeneity of binding sites for the pyruvate dehydrogenase component on the dihydrolipoyl transacetylase core of bovine kidney pyruvate dehydrogenase complex.

We have characterized the dissociation equilibrium constant (Kd) and the rate constants of association and dissociation for the binding of the pyruvate dehydrogenase component (PDH) to the dihydrolipoyl transacetylase component of kidney pyruvate dehydrogenase complex. We have found about 7 high-affinity sites (Kd = 1.5 X 10(-11) M) and about 13 low-affinity sites (Kd = 2.5 X 10(-8) M) with negative cooperativity for binding of PDH at the weaker sites. The high-affinity sites show a much higher rate constant for association of PDH with the core than do the low-affinity sites. Catalytic turnover strengthens binding of PDH at high-affinity sites when the complex is preincubated in the presence of thiamin pyrophosphate (TPP). In the absence of TPP, appreciably tighter binding occurs at the low-affinity sites (Kd = 5.9 X 10(-10) M). TPP weakens PDH binding at these sites with a half-maximal effect at about 5 microM TPP. In addition to TPP, increased ionic strength or 1-3 mM Mg2+ (with an enhanced effect of Mg2+ at higher ionic strengths) weakens PDH binding at low-affinity sites with a corresponding increase in the rate constant for dissociation. Further studies will be required to determine whether site heterogeneity contributes to dynamic processes in the function and regulation of the pyruvate dehydrogenase complex.

Specificity of the pyruvate dehydrogenase kinase for pyruvate dehydrogenase component bound to the surface of the kidney pyruvate dehydrogenase complex and evidence for intracore migration of pyruvate dehydrogenase component.

Using the bovine kidney pyruvate dehydrogenase complex we have investigated the mechanism whereby about three pyruvate dehydrogenase (active form) kinase molecules, tightly bound to the dihydrolipoyl transacetylase core, can rapidly phosphorylate and inactivate about 20 pyruvate dehydrogenase (active form) (PDHa) tetramers which are also bound to the 60-subunit core. Evidence is presented that PDHa kinase activity is not serviced by a process of dissociation and reassociation of PDHa. Rapid inactivation of a full complement of PDHa occurs at a rate exceeding the rate of dissociation of PDHa, indicating that a PDHa must move to the fixed kinase subunits without dissociating from the dihydrolipoyl transacetylase core. Consistent with that concept, at low concentrations of complex where a significant portion of PDHa is free, bound PDHa was inactivated at a rate equivalent to that at higher concentrations of complex, and free PDHa was phosphorylated more slowly at a rate closely approximated by the rate of association of free PDHa with the transacetylase core. Thus, with a low number of PDHa molecules bound, PDHa either is preferentially positioned for phosphorylation and inactivation by PDHa kinase or can rapidly become so positioned without dissociating from the transacetylase core.