Effects of the thyroid status on the sarcoplasmic reticulum in slow skeletal muscle of the rat

The effects of the thyroid status on the Ca++-transporting capabilities of rat slow skeletal muscle (m.soleus) were studied. The oxalate supported Ca++-uptake activity and Ca++-loading capacity of muscle homogenates from hyperthyroid rats showed an approximate 4.2 and 2.5 fold increase, respectively, as compared to values found in the hypothyroid group. Muscle homogenates of euthyroid rats gave intermediate values. The specific activity of oxalate supported Ca++ uptake, but not the Ca++-loading capacity, of membrane preparations enriched with respect to sarcoplasmic reticulum (SR) increased in proportion to the thyroid status. This was paralleled by a 3.5 fold increase in the amount of active Ca++ pumps in the SR preparations in the transition from hypothyroidism to hyperthyroidism as determined by measurement of Ca++-dependent 32P incorporation. These observations are not explained by differences in degree of purification of the examined SR preparations. Protein profiles of the membrane preparations obtained by gel electrophoresis indicated a thyroid-hormone dependent increase in Ca++-pump content relative to other SR proteins. The results suggest that thyroid hormone stimulates the proliferation of the SR and possibly also increases the Ca++-pump density in the SR membrane.     

The effect of temperature on the kinetics of ethanol production by a thermotolerant strain of Kluveromyces marxianus

Summary The batch fermentation kinetics of a novel thermotolerant strain of the yeast Kluveromyces marxianus were evaluated between 30°C and 48°C. The most significant effects of elevated temperature were reductions in overall biomass and ethanol yields. Decreases in the concentration of ethanol attained, and the presence of unutilized substrate suggested increased ethanol inhibition at the higher temperatures studied.     

Internal motional averaging and three-dimensional structure determination by nuclear magnetic resonance.

Dynamic averaging effects from internal motions on interproton distances estimated from nuclear Overhauser effects (NOE) are determined by using a molecular dynamics simulation of lysozyme. Generalized order parameters measuring angular averaging and radial averaging parameters are calculated. The product of these two parameters describes the full averaging effects on cross-relaxation. Analysis of 2778 non-methyl NOE interactions from the protein interior and surface indicates that distances estimated by assuming a rigid molecule have less than 10% error for 89% of the NOE interactions. However, analysis of 1854 methyl interactions found that only 68% of the distances estimated from cross-relaxation rates would have less than 10% error. Qualitative evaluation of distances according to strong, medium and weak NOE intensities, when used to define only the upper bound for interproton separation, would misassign less than 1% of the distance constraints because of motional averaging. Internal motions do not obscure the identification of secondary structure, although some instances of significant averaging effects were found for interactions in alpha-helical regions. Interresidue NOEs for amino acids more than three residues apart in the primary sequence are more extensively averaged than intraresidue or short-range interresidue NOEs. Intraresidue interactions exhibit a greater degree of angular averaging than those involving interresidue proton pairs. An internal motion does not equally affect all NOE interactions for a particular proton. Thus, incorporation of averaging parameters in nuclear magnetic resonance structure determination procedures must be made on a proton-pair-wise basis. On the basis of the motional averaging results, particular fixed-distance proton pairs in proteins are suggested for use as distance references. A small percentage of NOE pairs localized to three regions of the protein exhibit extreme averaging effects from internal motions. The regions and types of motions involved are described.     

Conformation of the glucotriose unit in the lipid-linked oligosaccharide precursor for protein glycosylation.

The conformation of the glucotriose unit of the protein glycosylation precursor Glc3Man9GlcNAc2 was assessed by deuterium exchange studies on the model tetrasaccharide alpha Glc----2 alpha Glc----3 alpha Glc----3 alpha Man----OCH2CH2CH3 dissolved in deuterated dimethyl sulfoxide. The hydroxyl proton on C-2 of the nonreducing end glucose and on C-4 of the glucose attached to mannose both show dramatic isotope shifts indicative of a strong hydrogen bond between these two hydroxyl groups. Such a hydrogen bond requires a fixed conformation of the glucotriose unit that brings these hydroxyl groups within 3 A of each other, a conformation that is supported by molecular modeling based on hard-sphere exo-anomeric (HSEA) calculations. The temperature dependence of the hydroxyl proton chemical shifts supports the postulated hydrogen bond, and the torsional angles between the three glucose units derived from the HSEA calculations are consistent with results from related studies on other saccharides. The results support a model for biochemical function in which the glucotriose unit could modulate the activity of the oligosaccharyltransferase by binding in a fixed conformation to a specific effector site in the enzyme.     

The reaction of arsenite-complexed xanthine oxidase with oxygen. Evidence for an oxygen-reactive molybdenum center

The effects of arsenite on the reaction of reduced xanthine oxidase with oxygen are determined. The kinetics of the reaction monitoring the return of enzyme absorbance are investigated as are the kinetics and stoichiometries of peroxide and superoxide formation. Although some of the effects of arsenite are qualitatively consistent with expectations based on the known perturbation of the molybdenum midpoint potentials by arsenite, several results cannot be so easily explained. Specifically, arsenite introduces a very rapid phase (kobs = 110 s-1 at 125 microM oxygen) to the oxidative half-reaction which is not observed with the native enzyme. Arsenite also diminishes the amount of superoxide produced and eliminates one-electron reduced enzyme as a detectable kinetic intermediate in the reoxidation pathway. These differences appear to result from the ability of arsenite to greatly enhance the oxygen- and/or superoxide-reactivity of the reduced molybdenum center. This is reflected in the observation that reduced forms of arsenite-complexed xanthine oxidase lacking functional FAD (iodoacetamide-alkylated enzyme and deflavo enzyme) react relatively rapidly with oxygen whereas these reactions are quite slow in the absence of arsenite.     

Cascade of autophosphorylation in the beta-subunit of the insulin receptor

Insulin stimulated autophosphorylation of the beta-subunit of the insulin receptor purified from Fao hepatoma cells or purified from Chinese hamster ovary (CHO/HIRC) or Swiss 3T3 (3T3/HIRC) cells transfected with the wild-type human insulin receptor cDNA. Autophosphorylation of the purified receptor occurred in at least two regions of the beta-subunit: the regulatory region containing Tyr-1146, Tyr-1150, and Tyr-1151, and the C-terminus containing Tyr-1316 and Tyr-1322. In the presence of antiphosphotyrosine antibody (alpha-PY), autophosphorylation of the purified receptor was inhibited nearly 80% during insulin stimulation. Tryptic peptide mapping showed that alpha-PY inhibited autophosphorylation of both tyrosyl residues in the C-terminus and one tyrosyl residue in the regulatory region, either Tyr-1150 or Tyr-1151. Thus, a bis-phosphorylated form of the regulatory region accumulated in the presence of alpha-PY, which contained Tyr(P)-1146 and either Tyr(P)-1150 or 1151. In intact Fao, CHO/HIRC, and 3T3/HIRC cells, insulin stimulated tyrosyl phosphorylation of the beta-subunit of the insulin receptor. Tryptic peptide mapping indicated that the regulatory region of the beta-subunit was mainly (greater than 80%) bis-phosphorylated; however, all three tyrosyl residues of the regulatory region were phosphorylated in about 20% of the receptors. As the phosphotransferase was activated by tris-phosphorylation but not bis-phosphorylation of the regulatory region of the beta-subunit (White et al.: Journal of Biological Chemistry 263:2969-2980, 1988), the extent of autophosphorylation in the regulatory region may play an important regulatory role during signal transmission in the intact cell.     

Genetic Variability in Taro, Colocasia esculenta (L.) Schott (Araceae)

Tablés 1 to 3 were omitted from the final printing ofthis article. They are reprinted here.     

A resonance Raman study of Ambystoma tigrinum hemoglobins: evidence for intraspecies hemepocket variations

Using resonance Raman difference spectroscopy, the Raman-active vibrational modes of hemoglobins from adult, neotenic, and larval forms of the salamander, Ambystoma tigrinum have been compared to each other and to human hemoglobin. The local heme environment of the adult and neotenic proteins were identical and differed from that of the larval protein. Differences were observed in modes sensitive to porphyrin pi electron density and axial ligation. Systematic differences were also observed between human and adult salamander hemoglobins particularly in modes sensitive to the heme vinyl environment. The relationship between these environmental differences, oxygen binding affinity, and the effects of allosteric modulators are discussed.