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H M Levy

Publications and source records attributed to H M Levy.

34 records · Page 2Linked to original sources
PubMed1977

Mechanism of oxygen exchange in actin-activated hydrolysis of adenosine triphosphate by myosin subfragment 1.

The gamma-phosphoryl groups of two intermediates (M-ATP and M-ADP-P1) in the pathway of MgATP hydrolysis by myosin undergo extensive oxygen exchange with water. Actin activates the overall rate of hydrolysis at a rate-limiting step which follows these exchange reactions. Thus, actin, by decreasing the turnover time of hydrolysis, would be expected to proportionately decrease the time available for oxygen exchange. Using subfragment 1 of myosin, the turnover time of hydrolysis can be varied over a wide range by changing the concentration of actin. An estimate for the rate constant of exchange can then be obtained by relating these turnover times to measured values for oxygen exchange (incorporation of 18O from H218O into the inorganic phosphate (Pi) released by hydrolysis). The results of such an experiment, with turnover times between 0.2 and 25 s, indicate that, for each gamma-phosphoryl group, one oxygen from the medium is added rapidly (to cleave the phosphoryl group or form a pentacoordinate phosphroyl complex); two more oxygens exchange with a rate constant, kc, of about 1 s-1; and a fourth oxygen exchanges slowly with ke about 0.2 s-1. The higher value is about 18 times smaller than the rate constant, 5-3, for the reverse cleavage step of the myosin pathway, which is postulated to be responsible for oxygen exchange. The data, then, indicate that the rate-limiting step for oxygen exchange is not k-3, but may be the rate of rotation of oxygens around the phosphorus atom, with one oxygen severely restricted by its binding to the active site. The finding that kc differs for the four oxygens in each phosphate group is related to past observations on myosin-catalyzed oxygen exchange.

Actins↗
PubMed1971

Amnesia produced by spreading depression and ECS: evidence for time-dependent memory trace localization.

Rats were given electroconvulsive shock and bilateral cortical spreading depression, either alone or in combination, at various times after a single passive avoidance training trial. Assessment of retention deficits, 24 hours after training, revealed a U-shaped amnesic function for cortical spreading depression as compared with the short linear function consistently obtained with electroconvulsive shock in this situation. Induction of cortical spreading depression immediately after training resulted in an extension of the amnesic gradient produced by electroconvulsive shock, presumably by disruption of the subcortically confined memory trace. In addition to indicating a stibcortical locus of action for the amnesic effects of electroconvulsive shock, these results are interpreted as favoring a hypothesis of time-dependent memory trace localization in short-term memory processing, which involves an initial subcortical localization of the trace followed by a phase involving either direct or indlirect cortical participation in a mulltistage memory fixation process.

Amnesia↗
PubMed1967

Heat inactivation of the relaxing site of actomyosin: prevention and reversal with dithiothreitol.

Adenosine triphosphate and magnesium (MgATP) inhibit contraction by binding to a specific relaxing site on natural actomyosin gel. This inhibitory control site is distinct from the active sites where MgATP causes contraction. In high concentrations of MgATP, calcium triggers contraction by releasing the protein from substrate inhibition, allowing the contractile reactions to occur. Heating the protein for 5 minutes at 43 degrees C selectively inactivates the relaxing site. After this treatment, actomyosin with MgATP contracts as well without calcium as with it. That this effect of heat is prevented and reversed by dithiothreitol (an agent that reduces disulfide bonds) indicates that the structure of the relaxing site depends on certain labile sulfhydryl groups, which may be those of tropomyosin. When these are oxidized to disulfide bonds, the site loses its activity; when the disulfide bonds are reduced, the site regains its activity.

Adenosine Triphosphate↗
PubMed1967

The contractile and control sites of natural actomyosin.

The various contractile and control sites of natural actomyosin gel were studied by comparing the kinetics of ATP hydrolysis with those of gel contraction, measured as an increase in turbidity. Contraction of actomyosin gel seems to require the cooperative reaction of ATP (with Mg) at two different sites. One of these sites catalyzes the hydrolysis of ATP and most probably contributes the driving force for contraction; the binding of ATP to the other site appears to break certain links that retard movement of the gel components. At limiting concentrations of ATP, the rate of contraction seems to depend on the rate of breaking these links as well as on the rate of ATP hydrolysis. But when both sites are saturated, the rate of contraction appears to be limited only by the rate of ATP hydrolysis. In addition to these two contractile sites, there are also two different control sites. At one, the relaxing site, the binding of ATP with Mg inhibits ATP hydrolysis and gel contraction. At the other, the binding of calcium activates contraction by overcoming the inhibitory action of Mg and ATP at the relaxing site. This control system-inhibition by substrate and disinhibition by calcium-can be selectively inactivated by heat and reactivated by dithiothreitol, a disulfide-reducing agent. These observations on the isolated contractile system are discussed in relation to the contraction and relaxation of muscle.

Adenosine Triphosphate↗
PubMedDate not supplied

Pneumoperitoneum resembling air in the biliary tree: CT features.

The various CT appearances of free abdominal air have been described. We present a hitherto undescribed CT manifestation of this condition. When subhepatic free air tracks along the porta hepatis and suspensory ligaments of the liver, it may resemble biliary air.

Aged↗
PubMedDate not supplied

Dental considerations for the patient receiving dialysis for renal failure.

A review of the literature describing the dental management of patients receiving hemodialysis because of renal failure is presented. A description of the renal failure process is given. Pretreatment and day of treatment precautions are considered. A pertinent dental case report of a dialysis patient is also presented.

Dental Care for Persons with Disabilities↗