Role of ionic interactions in ligand binding and catalysis of R67 dihydrofolate reductase

R67 dihydrofolate reductase (DHFR), which catalyzes the NADPH dependent reduction of dihydrofolate to tetrahydrofolate, belongs to a type II family of R-plasmid encoded DHFRs that confer resistance to the antibacterial drug trimethoprim. Crystal structure data reveals this enzyme is a homotetramer that possesses a single active site pore. Only two charged residues in each monomer are located near the pore, K32 and K33. Site-directed mutants were constructed to probe the role of these residues in ligand binding and/or catalysis. As a result of the 222 symmetry of this enzyme, mutagenesis of one residue results in modification at four related sites. All mutants at K32 affected the quaternary structure, producing an inactive dimer. The K33M mutant shows only a 2-4-fold effect on K(m) values. Salt effects on ligand binding and catalysis for K33M and wildtype R67 DHFRs were investigated to determine if these lysines are involved in forming ionic interactions with the negatively charged substrates, dihydrofolate (overall charge of -2) and NADPH (overall charge of -3). Binding studies indicate that two ionic interactions occur between NADPH and R67 DHFR. In contrast, the binding of folate, a poor substrate, to R67 DHFR.NADPH appears weak as a titration in enthalpy is lost at low ionic strength. Steady-state kinetic studies for both wild type (wt) and K33M R67 DHFRs also support a strong electrostatic interaction between NADPH and the enzyme. Interestingly, quantitation of the observed salt effects by measuring the slopes of the log of ionic strength versus the log of k(cat)/K(m) plots indicates that only one ionic interaction is involved in forming the transition state. These data support a model where two ionic interactions are formed between NADPH and symmetry related K32 residues in the ground state. To reach the transition state, an ionic interaction between K32 and the pyrophosphate bridge is broken. This unusual scenario likely arises from the constraints imposed by the 222 symmetry of the enzyme.

Amyloid-like features of polyglutamine aggregates and their assembly kinetics

The repeat length-dependent tendency of the polyglutamine sequences of certain proteins to form aggregates may underlie the cytotoxicity of these sequences in expanded CAG repeat diseases such as Huntington's disease. We report here a number of features of various polyglutamine (polyGln) aggregates and their assembly pathways that bear a resemblance to generally recognized defining features of amyloid fibrils. PolyGln aggregation kinetics displays concentration and length dependence and a lag phase that can be abbreviated by seeding. PolyGln aggregates exhibit classical beta-sheet-rich circular dichroism spectra consistent with an amyloid-like substructure. The fundamental structural unit of all the in vitro aggregates described here is a filament about 3 nm in width, resembling the protofibrillar intermediates in amyloid fibril assembly. We observed these filamentous structures either as isolated threads, as components of ribbonlike sheets, or, rarely, in amyloid-like twisted fibrils. All of the polyGln aggregates described here bind thioflavin T and shift its fluorescence spectrum. Although all polyGln aggregates tested bind the dye Congo red, only aggregates of a relatively long polyGln peptide exhibit Congo red birefringence, and this birefringence is only observed in a small portion of these aggregates. Remarkably, a monoclonal antibody with high selectivity for a generic amyloid fibril conformational epitope is capable of binding polyGln aggregates. Thus, polyGln aggregates exhibit most of the characteristic features of amyloid, but the twisted fibril structure with Congo red birefringence is not the predominant form in the polyGln repeat length range studied here. We also find that polyGln peptides exhibit an unusual freezing-dependent aggregation that appears to be caused by the freeze concentration of peptide and/or buffer components. This is of both fundamental and practical significance. PolyGln aggregation is revealed to be a highly specific process consistent with a significant degree of order in the molecular structure of the product. This ordered structure, or the assembly process leading to it, may be responsible for the cell-specific neuronal degeneration observed in Huntington's and other expanded CAG repeat diseases.

A microtiter plate assay for polyglutamine aggregate extension

Polyglutamine (polyGln) aggregates are neuropathological markers of expanded CAG repeat disorders, and may also play a critical role in the development of these diseases. We have established a highly sensitive, fast, reproducible, and specific assay capable of monitoring aggregate-dependent deposition of polyglutamine peptides. This assay allows detailed studies on various aspects of aggregation kinetics, and also makes possible the detection and quantitation of low levels of "extension-competent" aggregates. In the simplest form of this assay, polyGln aggregates are made from chemically synthesized peptides and immobilized onto microplate wells. These wells are incubated for different times with low concentrations of a soluble biotinylated polyGln peptide. Europium-streptavidin complexation of the immobilized biotin, followed by time-resolved fluorescence detection of the deposited europium, allows us to calculate the rate (fmol/h) of incorporation of polyGln peptides into polyGln aggregates. This assay will make possible basic studies on the assembly mechanism of polyGln aggregates and on critical features of the reaction, such as polyGln length dependence. The assay also will be a valuable tool for screening and characterizing anti-aggregation inhibitors. It will also be useful for detection and quantitation of aggregation-competent polyGln aggregates in biological materials, which may prove to be of critical importance in understanding the disease mechanism.

Human immunodeficiency virus and the orthopaedic surgeon

Human immunodeficiency virus is a devastating disease that ends in death. The fear that it engenders in healthworkers and patients may be out of proportion to the risks involved but an emotional response to the dangers is understandable. However, a careful study of the risks, the adoption of high standards of practice, and control of the clinical environment probably can eliminate most of the risk. There is a clear conflict of interest between the maintenance of confidentiality for the infected person and the right to information of those with whom they come into contact. Mandatory clinical testing for human immunodeficiency virus is likely to become the most contentious medicolegal issue of the next decade.

Role of fine catheter peritoneal cytology and laparoscopy in the management of acute abdominal pain

Laparoscopy and fine catheter peritoneal cytology (FCPC) have been advocated as aids in the assessment of acute abdominal pain. In all, 411 patients admitted to a district general hospital during a 10-month period were managed using a standard protocol incorporating these techniques. After initial assessment by a surgical registrar, 151 patients were excluded from further progress through the protocol (age less than 16 years, definite diagnosis made or contraindication to FCPC. The remaining 260 patients were placed in one of four management groups: (A) urgent operation (23 patients); (B) 'look and see' (40 patients); (C) 'wait and see' (59 patients); (D) urgent operation not indicated (138 patients). Eighty-eight of 99 patients (88 per cent) in groups B and C, where the need for operation was uncertain, underwent successful FCPC and 39 patients (39 per cent) underwent laparoscopy. In these patients the initial registrar management decision proved to be incorrect in 33 cases (33 per cent), but by following the protocol the number of management errors actually made was reduced to 13 (13 per cent, P less than 0.001). This would have been reduced to 8 per cent if the protocol had not been violated in five patients. This study demonstrates the effectiveness of a protocol using FCPC and laparoscopy to improve the management of patients with acute abdominal pain.

Strain differences in target organ weight changes among mice treated with androgens

When raised together in a controlled environment, 26 inbred strains of mice differed considerably in weights of seminal vesicles (SV), submandibular glands (SG), and tests in healthy males 100 days of age. In representative strains (high, medium, and low weight SV) castrate mature males administered various regimens of identical doses of exogenous androgen gave SV and SG weights that increased with increasing androgen and maintained weights in similar rank order to those in untreated animals. The relative stability of these differences suggests the usefulness of these animals for models to study the biochemical and cellular mechanisms of target organ sensitivity to androgen and perhaps to suggest genetic influences or susceptibilities to hormonally induced or dependent pathology in animals and man.

Effect of lithium on parathyroid hormone-induced calcium release from bone rudiments

Lithium treatment of humans and animals has been associated with adverse effects on bone and mineral metabolism. In order to determine whether lithium was altering the skeletal response to parathyroid hormone, we incubated bone rudiments for 5 days in the presence or absence of the drugs. Lithium had no effect on either parathyroid hormone-induced cyclic AMP generation or 45Ca release from the bone rudiments. The data are consistent with the hypothesis that the skeletal effects of lithium are not mediated via inhibition of the parathyroid hormone-adenyl cyclase-cyclic AMP system.

Differences in survival among germfree mice following transfer to a conventional colony

Sex and strain differences in survival were studied in 7-9 month old germfree mice following transfer to a conventional colony. One outbred and three inbred strains were observed. All outbred CD-1 mice survived transfer and in 4 months increased their weight by 50%. The majority of inbred mice survived 7 months after transfer. Sex differences in survival were evident throughout the experimental period and were most marked 7 months after transfer. An unexpected new finding was the viability of the male sex in germfree mice after transfer. Possible explanations are considered.

Testicular lipids in mice with testicular feminization

Morphological, histochemical and biochemical studies of the testis of mice with testicular feminization (tfm/y) reveal a large accumulation of lipids in Leydig cells and in Sertoli cells. In Leydig cells of tfm/y mice, lipid droplets do not exhibit the special relationship with smooth endoplasmic reticulum that exists in normal adult Leydig cells. Compared to the surgically-cryptorchid control, the tfm/y testis contains more lipid in Leydig cells but less in Sertoli cells. There are also quantitative differences in testicular lipids in tmf/y and normal testes but no significant differences were noted between tfm/y and surgically-cryptorchid testes. The testes of both the genetically defective and surgically-cryptochid animals contain increased amounts of total lipids and phospholipids, and of free and esterified cholesterols. Exogenous testosterone has no effect on lipids or other characteristics of these cells. The present results suggest that the increased lipids in tfm/y mice result from a genetic disorder that asserts itself (1) in Leydig cells where it is associated with, and is probably a result of, impaired lipid metabolism and steroidogenesis, and (2) in Sertoli cells where it is perhaps attributable to arrested spermatogenesis and impaired steroidogenesis.

Number of germ cells in known male and known female genotypes of vertebrate embryos (Oryzias latipes)

The number of primordial germ cells in embryos of known genotypic sex was the same in XY males and XX females until the gonad became recognizable as testis or ovary. It has been claimed that the heterogametic sex chromosome causes the gonad to differentiate as a testis in mammals and as an ovary in birds as a result of earlier and more mitoses. This claim was not supported in the present study where a sex difference in numbers of germ cells was first noted during differentiation of oocytes in the XX embryos.

Seat-belt injuries

The drivers and passengers of two cars which collided head-on all wore lap and diagonal seat-belts. Three of the four suffered ruptured viscera and two incurred flexion-compression fractures of the neck.A victim of a traffic accident who was wearing a seat-belt and who has superficial bruising or pain presents a difficult diagnostic problem. Visceral injury should be suspected in such cases.