Defects in Human Methionine Synthase in cblG Patients

Inborn errors resulting in isolated functional methionine synthase deficiency fall into two complementation groups, cblG and cblE. Using biochemical approaches we demonstrate that one cblG patient has greatly reduced levels of methionine synthase while in another, the enzyme is specifically impaired in the reductive activation cycle. The biochemical data suggested that low levels of methionine synthase activity in the first patient may result from mutations in the catalytic domains of the enzyme, reduced transcription, or generation of unstable message or protein. Using Northern analysis, we demonstrate that the molecular basis for the biochemical phenotype in this patient is associated with greatly diminished steady-state levels of methionine synthase mRNA. The biochemical data on the second patient cell line implicated mutations specific to reductive activation, a function that is housed in the C-terminal AdoMet-binding domain and the intermediate B12-binding domain, in the highly homologous bacterial enzyme. We have detected two mutations in a compound heterozygous state, one that results in conversion of a conserved proline (1173) to a leucine residue and the other a deletion of an isoleucine residue (881). The crystal structure of the C-terminal domain of the Escherichia coli MS predicts that the Pro to Leu mutation could disrupt activation since it is embedded in a sequence that makes direct contacts with the bound AdoMet. Deletion of isoleucine in the B12-binding domain would result in shortening of a β-sheet. Our data provide the first evidence for mutations in the methionine synthase gene being culpable for the cblG phenotype. In addition, they suggest directly that mutations in methionine synthase can lead to elevated homocysteine, implicated both in neural tube defects and in cardiovascular disease

Analysis of Solution Properties of Polystyrene in 2-Butanone in the Framework of the Hard-Sphere Model

The hydrodynamic properties of near-monodisperse polystyrene (PS) in 2-butanone have been studied by photon-correlation spectroscopy and viscometry. The derived translational diffusion coefficients D0 and intrinsic viscosities [η], along with literature data for the second virial coefficient A2, have been used to calculate the corresponding equivalent sphere radii: Rh, Rv, and Rt (hydrodynamic, viscometric, and thermodynamic radii, respectively). Various ratios of these radii and ratios of the equivalent sphere radii with the radius of gyration Rg have been used to test theoretical predictions of dilute-solution behavior for linear chains. Of particular note is the observed strong variation of the ratio Rv/RH with the specific nature of the polymer/solvent system. We tentatively attribute this behavior to crossover effects and, possibly, to changes in the average shape of the chain (from more spherical to more elliptical) brought about by specific polymer-solvent interactions. © 1991, American Chemical Society. All rights reserved

Synthesis and characterization of poly(methyl methacrylate) star polymers

Star‐branched poly(methyl methacrylate)s (PMMA) were synthesized by linking ‘living’ arms (produced by anionic polymerzation) with ethylene glycol dimethacrylate. Stars having arm molecular weights of 10000 and 40000 and between 4.9 and 18.7 branches were produced. The polymers were characterized using light scattering, size exclusion chromatography, and viscometry. It was found that well‐defined PMMA stars were obtained only at the higher (40000) arm molecular weight. The stars prepared using the lower molecular weight (c. 10000) arms contained very high molecular weight gel components. © 1994 Society of Chemical Industr