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At least 271 records · Page 15Linked to original sources
PubMed1987

Word knowledge acquisition in retarded children: a longitudinal study of acquisition of a complex linguistic structure.

The purpose of this 1-year longitudinal study was to investigate the acquisition of word knowledge by 18 retarded children. The words examined were relevant to the correct interpretation of sentences of the type "John is eager/easy to please." The interest was in how retarded children acquire the subcategorization features of these words. The study revealed that retarded children, like intellectually normal children, show great inconsistencies in their interpretations of specific words during an intermediate stage of performance on this linguistic structure; but the errors by retarded children, in contrast to those by intellectually normal children, were highly correlated with word frequency. The results are interpreted in representations. Interpretations based on gradual acquisition of word knowledge are contrasted with a reorganization theory, and a synthesis of the two views is given. Possible explanations for the finding that the retarded children nevertheless differed from intellectually normal children in the relation of their errors to word frequency are also given.

Adolescent↗
PubMed1986

Natural populations of Trypanosoma cruzi, the agent of Chagas disease, have a complex multiclonal structure.

We have studied 15 gene loci coding for enzymes in 121 Trypanosoma cruzi stocks from a wide geographic range--from the United States and Mexico to Chile and southern Brazil. T. cruzi is diploid but reproduction is basically clonal, with very little if any sexuality remaining at present. We have identified 43 different clones by their genetic composition; the same genetic clone is often found in very distant places and in diverse hosts. There is much genetic heterogeneity among the different clones, and they cannot be readily classified into a few discrete groups that might represent natural taxa. These findings imply that the biological and medical characteristics need to be ascertained separately for each natural clone. The evidence indicates that clonal evolution is very ancient in T. cruzi. We propose two alternative hypotheses concerning the relationship between the biochemical diversity and the heterogeneity in other biological and medical characteristics of T. cruzi. One hypothesis is that the degree of diversity between strains simply reflects the time elapsed since their last common ancestor. The second hypothesis is that biological and medical heterogeneity is recent and reflects adaptation to different transmission cycles. A decision between the two hypotheses can be reached with appropriate studies, with important medical consequences.

Alleles↗
PubMed1986

Two-dimensional crystals of cholera toxin B-subunit-receptor complexes: projected structure at 17-A resolution.

The B subunit of cholera toxin forms two-dimensional crystals when bound to its membrane receptor, ganglioside GM1, in phospholipid layers. A rectangular crystal lattice gives diffraction extending to 15-A resolution in negative stain, and image-processing of electron micrographs reveals a ring of five protein densities. The diameter of the central hole and the outer diameter of the ring are about 20 and 60 A, respectively. These data are consistent with a pentameric, doughnut-shaped structure of the B subunit that lies flat on a membrane surface. A hexagonal crystal lattice is obtained as well, and results of image processing and chemical crosslinking allow two interpretations: the B subunit may exist in both pentameric and hexameric forms or, more likely, the hexagonal lattice may represent a disordered or liquid crystalline form, in which a pentamer undergoes rotational averaging about its 5-fold axis.

Cholera Toxin↗
PubMed1993

Stoichiometry and structure of complexes of DNA oligomers with microgonotropens and distamycin by 1H NMR spectroscopy and molecular modeling.

Dien-microgonotropen-c (5c), tren-microgonotropen-b (6b), and distamycin (Dm) bind the A.T-rich region of d(CGCAAATTTGCG)2 (oligo-12) and form 1:1 (5c and 6b) and 2:1 and 4:1 (Dm) complexes. At 1.75 mol ratio of Dm/oligo-12 the 4:1 complex starts to form and coexists with the 2:1 complex and the free double-stranded DNA. No 1:1 and 3:1 complexes were seen, implying a preferential dimeric binding mode of Dm to oligo-12. At 4:1 mol ratio of Dm/oligo-12 there is extensive exchange of the A.T imino protons with the solvent at the binding site. This is presumably due to the opening of the minor groove. Molecular modeling shows that four Dm molecules can fit in a tandem antiparallel way into the minor groove of oligo-12 by widening it to 16-17 A. On going from oligo-12 to the pseudosymmetrical hexadecamer oligo-16 [d(GGCGCAAATTTGGCGG).d(CCGCCAAATTTGCGCC)] the stoichiometry of binding of 5c changes from 1:1 to 2:1. Since oligo-12 and oligo-16 have the same A.T binding site this change in stoichiometry is due to the increase in the G.C terminal pairing. Hoechst 33258 displaces the two 5c molecules bound in the minor groove of oligo-16 at the A.T region. Marked exchange of A.T imino protons was seen in the case of (oligo-16).(Ht)2.

Base Composition↗
PubMed1986

A resonance Raman study on the structures of complexes of flavoprotein D-amino acid oxidase.

Resonance Raman (RR) spectra were obtained for the purple complexes of D-amino acid oxidase (DAO) with D-lysine or N-methylalanine. RR spectra of a complex of oxidized DAO with the oxidation product of D-lysine or D-proline were also measured. The isotope shifts of the observed bands of the purple complex with D-lysine upon 13C- or 15N-substitution of lysine indicate that the ligand is delta 1-piperideine-2-carboxylate. That the band at 1671 cm-1 for the purple intermediate with N-methylalanine shifts to 1666 cm-1 in D2O solution indicates that the imino acid, N-methyl-alpha-iminopropionate, has a protonated imino group. Many bands due to a ligand in the RR spectra of the complex of oxidized DAO with an oxidation product can be observed below 1000 cm-1, but no band for the purple complex is seen in this frequency region. The band associated with the CO2-symmetric stretching mode of the product, such as delta 1-piperideine-2-carboxylate or delta 1-pyrrolidine-2-carboxylate, complexed with the oxidized DAO shifts in D2O solution. This suggests that the product imino acid interacts with the enzyme through some proton(s).

Alanine↗
PubMed1977

The crystal structure of complexes between horse liver alcohol dehydrogenase and the coenzyme analogues 3-iodopyridine-adenine dinucleotide and pyridine-adenine dinucleotide.

We have studied the binding of the enzymatically active NAD+ analogue, 3-iodopyridine-adenine dinucleotide, and the inactive analogue, pyridine-adenine dinucleotide to the enzyme horse liver alcohol dehydrogenase using X-ray crystallographic methods. These studies were made under such conditions that crystals of the complexes were isomorphous to apoenzyme crystals. Both analogues bind in the same conformation. The binding of the adenosine moiety is very similar to that of ADP-ribose or NADH bound to the enzyme. The conformation and mode of binding of the remaining portions of the analogue molecules is, however, quite different. The pyridine ring is not situated in the active-site pocket as the nicotinamide group in the isomorphous enzyme-NADH-imidazole complex but lies at the surface of the crevice between the two domains of the subunit, approximately 1.5 nm away from the catalytically active zinc atom. Lys-228 which has been shown to be important for NADH dissociation is in this region of the molecule.

Alcohol Oxidoreductases↗
PubMed1984

A murine T cell receptor gene complex: isolation, structure and rearrangement.

We describe here our efforts to develop a systematic strategy for the enumeration and isolation of genes encoding T cell-specific, membrane-associated polypeptides. Of particular importance among the cDNA clones that we have isolated is one which encodes the beta chain of the murine T cell receptor for antigen. The gene product is strikingly similar to immunoglobulin in its variable (V), constant (C), joining (J), and diversity (D) - like elements and the ability of the latter three types of element to assort independently during differentiation. The genomic organization of the constant region locus which encodes this molecule consists of two tandemly arrayed units of the form JC/JC, where the J clusters consist of fourteen distinct elements, many of which may be functional. The two constant regions differ by only four amino acids and both are apparently expressed in T cells of the same phenotype, so they probably do not represent isotypes in the classical sense. The sequences flanking the V, D and J elements are also similar to the conserved heptamer and nonomer sequences of immunoglobulins thought to be important in the recombination of the specific gene segments. These similarities indicate both the common evolutionary origins of these loci and the strong selective pressure that must be operative. The T cell receptor locus differs somewhat from immunoglobulins in the large number of possible J region elements and to some extent in the hypervariable-framework regions of the V region. We and others have mapped this gene to chromosome 6 of the mouse, in the same portion of the chromosome but not closely linked the kappa locus. The significance of the predicted protein structure of the beta chain and its support of the one-receptor model of MHC-antigen recognition are discussed.

Amino Acid Sequence↗
PubMed1992

Cholesterol pools in rat adrenal mitochondria: use of cholesterol oxidase to infer a complex pool structure.

ACTH stimulates the side-chain cleavage of cholesterol in the adrenal cortex in a cycloheximide-inhibitable manner. Its mechanism involves mobilizing cholesterol to a "steroidogenic pool" where the sterol can be metabolized to pregnenolone. This pool has been proposed to be in the inner mitochondrial membrane where cytochrome P-450scc resides, and regulation may involve transport of cholesterol from the outer to the inner membrane. To investigate the structure of the mitochondrial cholesterol pools, cholesterol oxidase has been used as a membrane-impermeant probe which should have selective access to outer membrane cholesterol. At 37 C, almost all the cholesterol in mitochondria from ether-stressed rats was metabolized by cholesterol oxidase. Depletion of an intermembrane space but not a matrix marker enzyme indicated partial disruption of the outer membrane. However, at 16 C, mitochondria remained largely intact, and cholesterol oxidase identified a unique pool of cholesterol, which was about two-thirds of the total. In experiments using mitochondria from ether-stressed rats, the size of the 16 C cholesterol oxidase accessible and inaccessible pools was compared with that of the steroidogenic pool. The steroidogenic pool was enhanced by pretreatment of some animals with aminoglutethimide (a P-450scc inhibitor) or eliminated with cycloheximide, both of which increased the total mitochondrial cholesterol. This approach reveals that the steroidogenic pool is not equivalent to the cholesterol oxidase-inaccessible pool. Rather, it overlaps both the cholesterol oxidase accessible and inaccessible pools. These results are not consistent with a simple two pool model, but can be explained by assuming a minimum of three cholesterol pools.

Adrenal Cortex↗