Analysis of proton exchange kinetics with time-dependent exchange rate

Mass spectrometry is used to probe the kinetics of hydrogen–deuterium exchange in lysozyme in pH 5, 6 and 7.4. An analysis based on a Verhulst growth model is proposed and effectively applied to the kinetics of the hydrogen exchange. The data are described by a power-like function which is based on a time-dependence of the exchange rate. Experimental data ranging over many time scales is considered and accurate fits of a power-like function are obtained. Results of fittings show correlation between faster hydrogen–deuterium exchange and increase of pH. Furthermore a model is presented that discriminates between easily exchangeable hydrogens (located in close proximity to the protein surface) and those protected from the exchange (located in the protein interior). A possible interpretation of the model and its biological significance are discussed.     

Is Na+-dependent exchange diffusion a true exchange?

Trans-stimulation of glycine uptake by cellular glycine in Ehrlich cells is a Na+-dependent phenomenon. In contrast trans-stimulated methionine or leucine uptake is Na+-independent. Trans-stimulated uptake of glycine does not show any characteristics of an ex change process but rather appears to be due to changes in membrane potential which occur as a result of a net Na+-dependent loss of cellular amino acids. Trans-stimulated influx of glycine occurs during the time of net loss of cellular glycine and is absent when the cellular amino acid level is at steady or when the cell is depolarized. Exchange of leucine or methionine occurs when the amino acid level is at steady state and it is not directly affected by depolarizing agents such as gramicidin.     

Africa Climate exchange

Golden Nightjar Caprimulgus eximius is an apparently sedentary sub-Saharan species with a breeding range extending from Senegal and Mauritania to Sudan. Although genetic studies of nightjars and related Caprimulgiformes have been published previously, none has included Golden Nightjar. In this study, mitochondrial and nuclear DNA of a Golden Nightjar found dead in Western Sahara in April 2016 was sequenced and compared with other species in the genus Caprimulgus. It was concluded with strong support that Golden Nightjar is closely related to Egyptian Nightjar C. aegyptius. It is hypothesised that Golden and Egyptian Nightjars may have arisen by splitting of a single ancestral species into migratory and sedentary populations.     

Antibody exchange immunochemistry

Antibodies are found to transfer rapidly between antigen samples attached to separate solid supports. The half-times for this antibody exchange at 20 and 37 degrees C were 7 and 1 h, respectively. Antibody exchange was exhibited by all 17 polyclonal and monoclonal antibodies tested and was readily detected by enzyme-linked immunosorbent assay, immunoblotting, or immunocytochemical assays. Taking advantage of this phenomenon, antibodies can be affinity-purified using less than a picomole of antigen and small amounts of antibody.     

Sodium-calcium exchange.

During the past year, significant advances have been made in the investigation of molecular, kinetic and electrophysiological aspects of Na(+)-Ca2+ exchange. The cardiac and retinal exchangers have been cloned and structure-function studies have begun.     

Positional isotope exchange

The detection of intermediates in enzyme-catalyzed reactions can be accomplished by several techniques. For those intermediates which do not have easily observed electronic spectra, use can be made of isotope exchange phenomena if the chemistry of the reaction is appropriate. Recently, the technique of positional isotope exchange (intramolecular isotopic scrambling) has been used to study several reactions which have been thought to involve high-energy intermediates in their mechanisms. A review of some of these reactions and the limitations of the method are presented in this article.     

Thrombotic thrombocytopenic purpura treated with plasma exchange or exchange transfusions.

Of 40 patients with thrombotic thrombocytopenic purpura, 17 were treated with plasma exchange, 15 with exchange transfusions, and 6 with both types of therapy. One patient died before being treated and another patient was seen but not treated. Plasma exchange was performed daily for a mean of seven exchanges per patient. The replacement fluid during plasma exchange was fresh frozen plasma in all cases. The complete response rates for each type of treatment were as follows: 88% for plasma exchange (15 patients), 47% for exchange transfusions (7 patients), and 67% for exchange transfusions and plasma exchange (4 patients). Clinical and laboratory factors were examined for any statistically significant association with therapy response. Treatment with plasma exchange was statistically the initial factor most strongly associated with prognosis. Paresis, paresthesias, seizures, mental status change, and coma showed no association with response to treatment. Some of the laboratory factors that did not show significant association with treatment response were the initial creatinine, hemoglobin, platelet count, lactate dehydrogenase, and total bilirubin. This study supports the hypothesis that plasma exchange has significantly improved the prognosis of patients with thrombotic thrombocytopenic purpura. These patients should be treated aggressively regardless of the severity of their symptoms.     

Sodium-calcium exchange and calcium-calcium exchange in internally dialyzed squid giant axons.

The influx and efflux of calcium (as 45Ca) and influx of sodium (as 24Na) were studied in internally dialyzed squid giant axons. The axons were poisoned with cyanide and ATP was omitted from the dialysis fluid. The internal ionized Ca2+ concentration ([Ca2+]i) was controlled with Ca-EGTA buffers. With [Ca2+]i greater than 0.5 muM, 45Ca efflux was largely dependent upon external Na and Ca. The Nao-dependent Ca efflux into Ca-free media appeared to saturate as [Ca2+]i was increased to 160 muM; the half-saturation concentration was about 8 muM Ca2+. In two experiments 24Na influx was measured; when [Ca2+]i was decreased from 160 muM to less than 0.5 muM, Na influx declined by about 5 pmoles/cm2 sec. The Nao-dependent Ca efflux averaged 1.6 pmoles/cm2 sec in axons with a [Ca2+]i of 160 muM, and was negligible in axons with a [Ca2+]i of less than 0.5 muM. Taken together, the Na influx and Ca efflux data may indicate that the fluxes are coupled with a stoichiometry of about 3 Na+-to-1 Ca2+. Ca efflux into Na-free media required the presence of both Ca and an alkali metal ion (but not Cs) in the external medium. Ca influx from Li-containing media was greatly reduced when [Ca2+]i was decreased from 160 to 0.23 muM, or when external Li was replaced by choline. These data provide evidence for a Ca-Ca exchange mechanism which is activated by certain alkali metal ions. The observations are consistent with a mobile carrier mechanism which can exchange Ca2+ ions from the axoplasm for either 3 Na+ ions, or one Ca2+ and an alkali metal ion (but not Cs) from the external medium. This mechanism may utilize energy from the Na electrochemical gradient to help extrude Ca against an electrochemical gradient.