Purification and Kinetic Mechanism of Human Brain Soluble Catechol-O-Methyltransferase

The soluble form of human brain catechol-O-methyltransferase (EC 2.1.1.6, COMT) has been purified approximately 4,000-fold from a 250,000 X g supernatant solution. The purified enzyme exhibits a molecular weight near 27,500 and a pI value equal to approximately pH 5.0. Initial velocity and product inhibition studies are consistent with an ordered reaction mechanism for soluble COMT. Tropolone, a dead-end inhibitor, exhibited a competitive pattern of inhibition when dopamine (DA) was the varied substrate and an uncompetitive pattern when S-adenosyl-L-methionine (SAM) was the varied substrate. These observations strongly suggest that the soluble form of COMT from human brain catalyzes the O-methylation of catecholamines via an ordered reaction mechanism in which SAM is the leading substrate. Since the membrane-bound form of COMT catalyzes the O-methylation of catecholamines through an identical reaction mechanism, these data provide further evidence that two forms of COMT, while being localized in distinct subcellular compartments, are quite similar in their molecular structure.     

“Thiocyanate gold”: small (2–3 nm) colloidal gold for affinity cytochemical labeling in electron microscopy

Summary Reduction of HAuCl₄ by NaSCN or KSCN produces colloidal gold particles of 2.6 nm in diameter and homogeneous in size (coefficient of variation 15%). The Au_SCN sol forms protein-gold complexes. The amount of protein required to form an Au_SCN-protein complex is best determined in the electron microscope, where serial dilutions of protein with gold sol are inspected for the presence of aggregates.By immuno-electron microscopy SCN-gold complexed to protein A is active and visible as is shown by revealing -amylase in rat pancreatic acinar cells.     

Kinetic evidence for differential agonist and antagonist binding to bovine hippocampal synaptic membrane opioid receptors

To examine the kinetics of opioid receptor binding, the agonists [D-Ala2-D-Leu5]enkephalin (DADL) and [D-Ala2-MePhe4-Gly-ol5]enkephalin (DAGO) and the antagonists diprenorphine and naltrexone were used with bovine hippocampal synaptic plasma membranes. By computer modeling of equilibrium binding displacement curves utilizing the LIGAND program, we found opioid peptides bind with high affinity to single populations of synaptic plasma membranes receptors, whereas opiate alkaloids bind to multiple sites. Initial kinetic experiments revealed that agonist rates of association were radioligand concentration-independent. Pseudo first-order rate constants for DADL, DAGO, diprenorphine, and naltrexone association were estimated to be 5.63 X 10(5), 5.08 X 10(5), 4.60 X 10(6), and 2.3 X 10(6) mol-1 X s-1, respectively. After preincubation of 0.2-1 nM radioligand for variable time intervals, dissociation was initiated by addition of 1 microM unlabeled ligand. If saturation binding was achieved before dissociation was initiated, then nearly monophasic dissociation of DADL, DAGO, and diprenorphine and a biphasic off-rate for naltrexone were observed. When association times were reduced to pre-equilibrium intervals, the kinetics of dissociation of agonists became biphasic and association time-dependent, but that for antagonists did not change significantly. Comparisons by both graphical methods and computerized nonlinear regression analyses of rate constants revealed that the fraction of the rapid component of agonist dissociation decreases and that of the slow component is elevated with increasing receptor occupancy. In the presence of 100 mM NaCl, DADL dissociation became association time-independent. These data are consistent with the idea that the Na+ effect is brought about by a change of receptor to an antagonist-like conformation. On the basis of both association and dissociation kinetic data, opioid agonists appear to interact in a multistep process in which a rapid, reversible association is followed by the formation of a more tightly bound complex.     

Characterization of Membrane-Bound and Soluble Catechol-O-Methyltransferase from Human Frontal Cortex

Abstract: Catechol- O -methyltransferase (COMT; E.C. 2.1.1.6) from human frontal cortex occurs in both a soluble and membrane-bound form. Attempts to solubilize the membrane-bound transferase by repeated washing or by extraction into solutions of high ionic strength were unsuccessful. The finding that Triton X-100 was capable of solubilizing membrane-bound COMT suggested that the membrane-bound transferase is an integral membrane protein. The membrane-bound and soluble enzymes did not differ in their requirements for magnesium ions or in their pH-activity profiles; both enzymes showed an optimum near pH 8.0 when assayed in phosphate buffer. In addition, the two enzymes did not differ in the degree of inhibition caused by CaCl₂, both enzymes displaying 65% inhibition at 2.5 m M CaCl₂. The competitive inhibitors tropolone and nordihydroguaiaretic acid displayed K _i values for the membrane-bound transferase five- to 10-fold lower than those observed for the soluble transferase. Solubilization of membrane-bound COMT in Triton X-100 resulted in an increase in the apparent K _m value of the membrane-bound transferase for dopamine. The increase in K _m appeared to be due to apparent competitive inhibition by Triton X-100 and reached a limiting value of approximately 80 μM. These results confirm that membrane-bound COMT is an integral membrane protein that may be structurally distinct from soluble COMT.     

Use of chemostat for selection ofStreptomyces hygroscopicus mutants altered in regulation of maltose utilization

Summary Streptomyces hygroscopicus mutants showing altered fermentation kinetics were isolated using a selection procedure in a chemostat. Several mutants were obtained which differed in their capacity to produce the macrolide antibiotic turimycin.     

Mutants ofStreptomyces hygroscopicus deregulated in amylase and α-glucosidase formation

Summary Two mutants, M36 and M39, of turimycin-producingS. hygroscopicus JA 6599/PR1 obtained by directed selection in a chemostat displayed altered pattern of amylase and -glucosidase production as revelaed by both constitutive enzyme formation and higher enzyme levels.     

Nonhomologous recombination in mammalian cells: role for short sequence homologies in the joining reaction.

Although DNA breakage and reunion in nonhomologous recombination are poorly understood, previous work suggests that short sequence homologies may play a role in the end-joining step in mammalian cells. To study the mechanism of end joining in more detail, we inserted a polylinker into the simian virus 40 T-antigen intron, cleaved the polylinker with different pairs of restriction enzymes, and transfected the resulting linear molecules into monkey cells. Analysis of 199 independent junctional sequences from seven constructs with different mismatched ends indicates that single-stranded extensions are relatively stable in monkey cells and that the terminal few nucleotides are critical for cell-mediated end joining. Furthermore, these studies define three mechanisms for end joining: single-strand, template-directed, and postrepair ligations. The latter two mechanisms depend on homologous pairing of one to six complementary bases to position the junction. All three mechanisms operate with similar overall efficiencies. The relevance of this work to targeted integration in mammalian cells is discussed.     

Protonated state of methotrexate, trimethoprim, and pyrimethamine bound to dihydrofolate reductase

13C nuclear magnetic resonance (NMR) of methotrexate, trimethoprim, and pyrimethamine enriched 90% with 13C at C2 has provided a sensitive means of detecting the state of protonation of the heterocyclic rings of these inhibitors. In each case, protonation of N1 causes an upfield movement of the chemical shift of C2 by more than 6 ppm. By this method it has been shown that, at pH values up to 9.2, methotrexate is bound to bovine liver dihydrofolate reductase with N1 of the inhibitor protonated, just as in the case of the complex with reductase from Streptococcus faecium and Lactobacillus casei. Furthermore, trimethoprim bound to reductase from any of the three sources, and pyrimethamine bound to either of the bacterial reductases also have N1 protonated even at pH values up to 10. This implies that in all cases there is a strong interaction between protonated N1 of the inhibitor and the carboxylate group of the active site aspartate or glutamate. In every case pKa of the bound inhibitor is increased by several units, a finding in accord with crystallographic evidence that inhibitor bound to L. casei reductase is in a hydrophobic environment and that N1 is not hydrogen-bonded to water. It was confirmed by titration of protein fluorescence that trimethoprim has greater affinity for bacterial reductase than for vertebrate (bovine) reductase, and that this selectivity is more marked in ternary complexes in which NADPH is also bound to the active site. However, the data cited above indicate that this difference in affinities is not due to a weaker ionic interaction between protonated N1 of trimethoprim and the bovine enzyme. Instead, binding of the trimethoprim side chain to hydrophobic sites on the enzyme must provide less binding energy in the case of the mammalian enzyme.     

Regulation of insulin receptor kinase activity by insulin mimickers and an insulin antagonist

Three agents which mimic insulin action in intact cells (concanavalin A, wheat germ agglutinin, and polyclonal insulin receptor antibody), mimicked insulin's ability to stimulate the kinase activity of purified insulin receptors. In contrast, monoclonal insulin receptor antibody, an antagonist of insulin action, did not stimulate the phosphorylation of the insulin receptor either in intact IM-9 cells or in purified receptor preparations. This antibody, however, antagonized the ability of insulin to stimulate the phosphorylation of the receptor both in intact cells and in the purified receptor. These studies with insulin mimickers and an insulin antagonist are consistent with a role for the kinase activity of the receptor mediating the actions of insulin.