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Biomedical subjects

D A Bergman

Publications and source records attributed to D A Bergman.

12 recordsLinked to original sources
PubMed1990

Thyroid physiology and immunology.

The thyroid gland, like the paintings of Chagall, has forced us to look at our traditional understanding of things in a new way and to gain fresh insight into ancient concepts of human physiology.

Humans↗
PubMed1989

Thermodynamic nonideality in enzyme catalysis. Effect of albumin on the reduction of pyruvate by lactate dehydrogenase.

The enhanced catalytic reduction of pyruvate by rabbit muscle lactate dehydrogenase that results from the addition of serum albumin [Nichol, L. W., Sculley, M. J., Ward, L. D. & Winzor, D. J. (1983) Arch. Biochem. Biophys. 222, 574-581] is shown to emanate solely from an increase in maximal velocity, there being no discernible effect of this inert space-filling macromolecular solute on the Michaelis constant for either pyruvate or NADH. As part of the search for a mechanistic explanation of this kinetic phenomenon, the space-filling effects of albumin have been used to eliminate the possibility that the increase in sedimentation coefficient of lactate dehydrogenase effected by inclusion of oxamate with enzyme-NADH complex reflects preferential binding of this pyruvate analog to a more compact isomeric state of the binary complex. The enzyme kinetic results are therefore considered in terms of a reaction scheme entailing gross conformational changes during the formation of ternary enzyme-NADH-pyruvate complex and its isomerization to an activated transition state. The experimentally observed insensitivity of the Michaelis constant for pyruvate to albumin concentration is in keeping with theoretical prediction, but incorporation of the measured extent of maximal velocity enhancement into the kinetic model leads to a predicted volume for the fully saturated transition-state complex that is far too small to be experimentally feasible. A more complex mechanistic model involving additional isomerizations of enzyme-substrate species is thus required to achieve quantitative description of the albumin effect solely in terms of thermodynamic nonideality.

Kinetics↗
PubMed1989

Space-filling effects of inert solutes as probes for the detection and study of substrate-mediated conformational changes by enzyme kinetics: theoretical considerations.

From expressions derived for the space-filling effects of small inert solutes on kinetic parameters for univalent enzymes undergoing isomerizations that are substrate-induced and pre-existing, it is concluded that experimental observation of an enhanced maximal velocity in the presence of inert solute can only reflect the existence of the former type of conformational change; and that the isomerization must be governed by a relatively small equilibrium constant. Similar conclusions apply to multivalent enzymes exhibiting Michaelis-Menten kinetics. Extension of the theory to provide quantitative expressions for multivalent enzymes has made possible the numerical simulation of thermodynamic non-ideality effects on systems conforming with the Monod and Koshland models of allostery. In that regard the simulated Scatchard plots for the two models differ sufficiently in form to suggest that detailed examination of the space-filling effects of small solutes on the kinetics of an allosteric enzyme may, under favourable circumstances, allow identification of the appropriate allosteric mechanism. Finally, these considerations of thermodynamic non-ideality in relation to the kinetics of allosteric enzymes have revealed formal similarities between the consequences of space-filling by inert solutes and the specific effects of allosteric activators or inhibitors. Attention is drawn to the possible implications of this observation in relation to the functioning of allosteric enzymes in vivo, where catalytic performance may be modified by factors no more specific than the ability of unrelated solutes to occupy space in the highly concentrated cellular environment.

Chemical Phenomena↗
PubMed1989

Effects of thermodynamic nonideality on the kinetics of ester hydrolysis by alpha-chymotrypsin: a model system with preexistence of the isomerization equilibrium.

The effects of a small inert solute, sucrose, on the kinetics of hydrolysis of N-acetyl-tryptophan ethyl ester by bovine alpha-chymotrypsin have been investigated. In studies at pH 7 and 20 degrees C the presence of 0.5 M sucrose in assay mixtures caused no discernible change in kinetic parameters, a result consistent with existence of the enzyme in a single conformational state under those conditions. However, at pH 3.5 and 50 degrees C, conditions under which the enzyme comprises an equilibrium mixture of compact and expanded isomeric states, inclusion of the inert solute led to a considerable decrease in Michaelis constant (0.84 to 0.61 mM) but no significant change in maximal velocity. These results were shown to be amenable to quantitative interpretation in terms of thermodynamic nonideality effects on catalysis by an enzyme undergoing reversible isomerization in the absence of substrate. For that analysis, which required experimental estimates of the equilibrium constant for preexisting isomerization of enzyme and the activity coefficient of substrate, the magnitude of the former (0.3) was obtained by difference spectroscopy: liquid-liquid partition studies with bromobenzene as organic phase were used to determine the effect of sucrose on the activity coefficient of N-acetyltryptophan ethyl ester. Such agreement between experimental kinetic findings and theoretical predictions based on considerations of excluded volume points to the possible use of the space-filling effects of small solutes for delineating the gross extent of conformational changes associated with reversible isomerization of proteins, and hence to the potential of thermodynamic nonideality as a probe for studying protein denaturation mechanisms as well as substrate-mediated changes associated with enzyme reaction mechanisms.

Chymotrypsin↗
PubMed1987

Allowance for "analyte valence" in the retention model of ion-exchange chromatography. Studies of adenosine 5'-phosphates on DEAE cellulose.

Quantitative expressions are derived for various retention models of the ion-exchange behaviour of multivalent analytes. Their effectiveness as descriptions of experimental results is then tested by application to partition equilibrium studies of the effect of phosphate concentration on the interaction between the three adenosine 5'-phosphates and diethylaminoethylcellulose at pH 4.4. A completely general multi-state model has pointed to quantitative deficiencies in the currently accepted cooperative two-state model, which is, however, superior to a multi-state model devoid of cooperative effects. A similar situation pertains to the ion-exchange behaviour of cytochrome c on carboxymethylcellulose at pH 7.0.

Adenosine Diphosphate↗
PubMed1986

Quantitative affinity chromatography: increased versatility of the technique for studies of ligand binding.

The potential of affinity chromatography for the characterization of strong solute-ligand interactions is explored by studying the NADH-dependent elution of rabbit muscle lactate dehydrogenase from a column of trinitrophenyl-Sepharose in 0.067 M phosphate, pH 7.2. An interesting development is the simplification of the general affinity chromatography theory that emanates from the use of affinity matrices with a high concentration of immobilized reactant groups. The resultant expression allows evaluation of the intrinsic association constant for solute-ligand interactions from a single series of either zonal or frontal affinity chromatographic experiments conducted in the presence of a range of free ligand concentrations. Thus, contrary to previous belief, an affinity matrix designed for solute purification work should prove to be an asset for, rather than an impediment to, the study of solute-ligand interactions by quantitative affinity chromatography.

Adsorption↗
PubMed1986

The impact of reading a clinical study on treatment decisions of physicians and residents.

The study presented here reports on the impact of newly published clinical research on physicians' decisions. Eighty-three pediatricians, pediatric residents, and family practitioners were presented with a common, potentially serious problem, an infant with a high fever, and were required to estimate the probabilities of bacteremia and of acquiring meningitis and to choose from management options. The participants then read a published scientific report addressing the risks of meningitis in febrile infants and were asked afterward to answer again the questions relating to the clinical problem. After reading the study, the participants significantly increased their probability estimates of the patient acquiring meningitis. Only 14 percent of the participants would have hospitalized the patient before reading the article, but 47 percent would have done so after reading the article. Pediatricians were more likely than family practitioners to use antibiotics after reading the article. These decisions were not based not based on logical processing of information, as there was no correlation between the physicians' estimate of the risk of meningitis and the underlying risk of bacteremia and no correlation between the participants' decisions to hospitalize or use antibiotics and their estimated risk of the patient developing meningitis. Physicians appear to have considerable difficulty in using probability data and appear to base estimates of serious disease and subsequent management on intuition rather than calculation.

California↗
PubMed1986

Using structured medical information to improve students' problem-solving performance.

In the study reported here, the authors assessed the use of efficient organization of knowledge and of problem-solving strategies to enhance medical students' clinical problem-solving skills. Thirty-five preclinical medical students were randomly assigned to a experimental or control group and given a knowledge base containing information on eight congenital heart diseases to learn. Information for the experimental group emphasized disease groupings (based on their similar clinical presentation), symptom-disease associations, and clinical problem-solving heuristics. The same information for the control group was presented in a textbook format that emphasized the pathophysiology of the diseases. The students then diagnosed three computerized diagnostic problems of varying difficulty while verbalizing their problem-solving strategies. The results showed that the experimental group acquired a higher ratio of diagnostic to nondiagnostic cues, mentioned the correct diagnosis sooner in their workups, and correctly diagnosed the most difficult case more often than the control group. These results provide support for revisions in the organization and presentation of information that are aimed at improving clinical problem-solving skills.

Clinical Competence↗