Search PubMed⌕ Search

Biomedical subjects

M L Johnson

Publications and source records attributed to M L Johnson.

At least 361 records · Page 20Linked to original sources
PubMed1977

Effect of corticosteroids on collagen synthesis.

Contraction and epithelization are two phenomena of "wound healing" retarded by corticosteroids. It is unclear how these agents affect collagen synthesis and wound remodeling. The methods used by several authors to conclude that steroids inhibit collagen synthesis are questioned. Therefore, collagen synthesis was measured in cultured steroid-treated chick embryo calvaria, 5 day open wounds in treated rats, and intralesionally injected human keloids. Collagen synthesis was suppressed only by long-term administration of massive systemic doses of a sustained release form of methylprednisolone (Depo-Medrol). Large, intermittent doses of corticosteroids (triamcinolone and Solu-Medrol) did not alter collagen synthesis. Because human keloids become softer and smaller following intralesional administration of triamcinolone without lowering the normally high rate of collagen synthesis, we hypothesize that corticosteroids enhance collagen degradation.

Animals↗
PubMed1976

Oxygenation-linked subunit interactions in human hemoglobin: experimental studies on the concentration dependence of oxygenation curves.

An experimental study on the concentration dependence of oxygenation curves for human hemoglobin has been carried out between 4 X 10(-8) M heme and 5 X 10(-4) M heme in 0.1 M tris(hydroxymehtyl)aminomethane hydrochloride, 0.1 M NaCl, 1 mM disodium ethylenediaminetetraacetic acid, pH 7.4, 21.5 degrees C. With decreasing hemoglobin concentration the curves show pronounced shifts in position and shape, consistent with dissociation of tetrameric hemoglobin into dimeric species of high affinity and low cooperativity. Combination of these data with independently determined values of dissociation constants for unliganded and fully liganded hemoglobin permits a resolution of the seven parameters necessary to define the linked binding and subunit association processes. The total oxygenation-linked subunit dissociation energy (6.34 kcal) was resolved into intersubunit contact energy changes between alphabeta dimers in tetrameric hemoglobin which accompany binding of the first, middle two, and last oxygen molecules. The resolution is accurate to within approximately +/-0.3 kcal. To within this limit the isolated dimers are found to bind oxygen noncooperatively and with the same affinity as isolated alpha and beta chains. Equally good fits to the data are obtained when dimers are slightly anticooperative. At least three major energetic states are apparently assumed by hemoglobin tetramers, with respect to the alpha1beta2 contact region, corresponding to (a) unliganded, (b) singly liganded, (c) triply and quadruply liganded species. The results do not establish whether these states may be assumed by a single molecule, or whether they arise as averages over a distrubution of conformational states. They do provide unequivocal evidence against a concerted transition at any particular binding step in a system with only two energetic states of tetramer (i.e., an all or none switchover between T and R states at a particular binding step).

Computers↗
PubMed1976

Oxygenation-linked subunit interactions in human hemoglobin: analysis of linkage functions for constituent energy terms.

Resolution of the linkage functions between oxygenation and subunit association-dissociation equilibria in human hemoglobin into the constituent microscopic terms has been explored by numerical simulation and least-squares analysis. The correlation properties between parameters has been studied using several choices of parameter sets in order to optimize resolution. It is found that, with currently available levels of experimental precision and ranges of variables, neither linkage function can provide sufficient resolution of all the desired energy terms. The most difficult quantities to resolve always include the dimer-tetramer association constant for unliganded hemoglobin and the oxygen binding constants to alphabeta dimers. A feasible experimental strategy for overcoming these difficulties lies in independent determination of the dimer-tetramer association constants for unliganded and fully oxygenated hemoglobin. These constants, in combination with the median lignad concentration, provide an estimate of the energy for total oxygenation of tetramers which is essentially independent of the other constituent energies. It is shown that if these separately determinable parameters are fixed, the remaining terms may be estimated to good accuracy using data which represents either linkage function. In general it is desirable to combine information from both types of experimental quantities. A previous paper (Mills, F.C., Johnson, M.L., and Ackers, G.K. (1976), Biochemestry, 15, the preceding paper in this issue) describes the experimental implementation of this strategy.

Hemoglobins↗
PubMed1976

Kinetics of deoxyhemoglobin subunit dissociation determined by haptoglobin binding: estimation of the equilibrium constant from forward and reverse rates.

Deoxyhemoglobin tetramers dissociate into dimers very slowly, with half-times on the order of several hours. It is demonstrated that absorbance changes in the Soret region which accompany this dissociation and persist upon binding of haptoglobin 1-1 to the dissociated dimers can be used for accurate kinetic determinations over the necessarily long periods required for study. This method of study for the slow reactions depends upon long-term spectral integrity of the reaction mixtures and upon accurate measurement. The variation in rate constants determined by this procedure has been correlated with variations in structural constraints at the dimer-dimer contact region. In the presence of 2,3-diphosphoglycerate the rate constant is decreased, consistent with the role of this effector in binding to both beta chains and stabilizing the constrained deoxy tetramer against dissociation into alphabeta dimers. With hemoglobin specifically modified (des-Arg-141alpha) to eliminate half the constraining salt links within the dimer-dimer contact region, the dissociation rate is increased by approximately three orders of magnitude. In hemoglobin S where the amino acid substitution is not directly in the intersubunit contact region of interest, the dissociation rate is found to be approximately the same as that for hemoglobin A. Combination of the dissociation rate constants determined by haptoglobin binding with stopped-flow determinations of the rate constant for reassociation of dissociated dimers provides an estimate of the equilibrium constant, 0K2, for the deoxyhemoglobin dimer-tetramer equilibrium. This estimate is independent of any assumptions regarding other energetic quantities, and yields a value of 2.54 +/- 0.7 X 10(10)M-1 (heme) in 0.1 M Tris-HCl, 0.1 M NaCl, and 1 mM EDTA, pH 7.4, 21.5 degrees C. Thus the intersubunit contact energy is -14.0 +/- 0.2 kcal/mol of heme. The stabilization energy between deoxy and oxy tetramers is found to be approximately 6.4 kcal/mol, under these conditions.

Binding Sites↗
PubMed1976

Numerical study of the Johnston-Ogston effect in two-component systems.

Numerical solutions of the Lamm equation are presented for systems exhibiting the Johnston--Ogston effect. From these solutions it is apparent that the movement of the maxima of the concentration gradient curves reflects the sedimentation velocity of the slow or fast components in their appropriate plateaus. A simple generalization of the Johnston--Ogston analysis is presented, valid for all centrifugation times in a radial field and sector shaped cell provided only that there exist both a plateau of the slow component by itself and the mixed plateau with both slow and fast components present.

Mathematics↗
PubMed1975

The linkage between oxygenation and subunit dissociation in human hemoglobin. Consequences for the analysis of oxygenation curves.

For human hemoglobin, a pronounced dependence of oxygenation curves upon protein concentration can be demonstrated experimentally in the range between 10(-4) and 2 X 10(-6) M heme. The effects of such protein concentration dependence upon analysis of saturation curves have been explored using a model-independent linkage analysis which incorporates the dissociation of tetramers to dimers. We have carried out stimulations of oxygenation curves representing a variety of energy distributions designed to cover a wide range of values which are relevant to known hemoglobin systems and experimental conditions. The resulting simulated oxygenation curves were analyzed by least-squares minimization procedures in terms of the tetramer binding isotherm to yield the four apparent Adair constants. These derived constants were compared with the originally assumed values used in the simulation in order to assess the extent to which their values may be altered by the presence of dimer. For each energy distribution the analysis has been carried out over a wide range of protein concentration. We have found that the presence of even small amounts of dimer that are necessarily present at the low protein concentrations commonly employed may have a devastating effect upon the reliability of Adair constant determinations. In addition to these simulated cases, we have analyzed two sets of highly precise experimental data from the literature in order to assess the degree to which constants obtained may have been influenced by the presence of dimer.

Binding Sites↗
PubMed1975

Antimicrobial therapy of experimental endocarditis caused by Staphylococcus aureus.

The rate at which various antimicrobial agents eradicated Staphylococcus aureus from cardiac vegetations in a rabbit model of endocarditis was studied. The rate at which various drugs and combinations killed high titers of bacteria in broth correlated with the relative effectiveness of the agents in vivo. Gentamicin plus penicillin proved to be synergistic in vitro and more effective in eradicating bacteria from cardiac vegetations in vivo than was penicillin alone. Vancomycin killed bacteria at a rate similar to that for the combination of penicillin and gentamicin, and the rate for cefazolin was similar to that for penicillin alone. Clindamycin was less effective in vivo and in vitro than penicillin. Therapy with rifampin led to the emergence of resistant organisms, and, when penicillin, this drug was less effective in vitro and in vivo than was penicillin alone. This model appears to offer an effective method for evaluation of antimicrobial treatment of staphylococcal endocarditis.

Animals↗
PubMed1975

Effect of a potent interferon inducer on acute and chronic lactic dehydrogenase virus viremia.

The effect of a single and multiple doses of polyinosinic-polycytidylic acid (poly (I). poly(C) on lactic dehydrogenase virus viremia has been studied. A single injection of 200 mug of poly (I)-poly(C) given 6 h before infection with lactic dehydrogenase virus caused a temporary decrease in viremia of about 2.5 log10. Repeated injections of 100 mug of poly(I)-poly(C) every 24 h beginning at -24 h caused a temporary decrease in viremia of about 4.0 log 10. Therapeutic treatment with optimal dosage schedules (100 mug every 24 h) caused a reduction in viremia of 1.0 to 2.0 log10. This decrease lasted for at least 24 h after the treatment was stopped. Some possible reasons for the limited effectiveness of poly (I)-poly(C) in this system are discussed.

Animals↗
PubMed1975

Diastolic vibration of the interventricular septum in aortic insufficiency.

The echocardiograms of 46 patients with aortic insufficiency which manifested typical high frequency vibrations of the anterior mitral valve leaflet were reviewed. All patients had overt clinical evidence of aortic insufficiency which was confirmed by angiography in 17 cases. In 17 cases (eight of whom underwent catheterization and angiography), high frequency diastolic vibrations of the interventricular septum of similar frequency to those on the anterior mitral leaflet were observed. This finding was not seen in 100 control echocardiograms from patients without clinical evidence of aortic insufficiency, and represents a previously undescribed echocardiographic manifestation of this lesion.

Adolescent↗
PubMed1975

A reassessment of the echocardiogram in mitral stenosis.

Echocardiographic and cardiac catheterization findings were compared in 61 patients with mitral stenosis without other significant lesions in an attempt to determine the clinical usefulness of echocardiography in the assessment of such patients. There was a poor correlation between the E-F slope on the echocardiogram and the calculated mitral valve area (r = 0.51). A review of reported data relating the E-F slope to mitral valve area indicated that echocardiographic assessment of mitral valve area had low sensitivity and specificity. The amplitude of excursion of the anterior leaflet did not differ significantly in patients undergoing valvotomy and those undergoind valve replacement. The data obtained suggest that while the echocardiogram is a reliable method of diagnosing mitral stenosis, the E-F slope is an unreliable index of the severity of the lesion.

Adult↗