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G L Alonso

Publications and source records attributed to G L Alonso.

34 records · Page 2Linked to original sources
PubMed1985

Effect of pH on calcium ion dependence of dog cardiac sarcoplasmic reticulum adenosine triphosphatase activity.

The effect of [Ca2+] on (Ca2+ + Mg2+)-ATPase activity from dog heart sarcoplasmic reticulum fragments was evaluated under three different pH values: 6.0, 6.8 and 7.6. Considering pH 6.8 as reference, under pH 7.6 an increase of the activity at saturating [Ca2+], an increase of pCa needed for half-maximal activation, and a decrease of the Hill coefficient, were observed. Opposite results were obtained under pH 6.0. The positively cooperative effect of Ca2+ was only observed at the lowest pH. The results are in line with previous suggestions regarding the existence of two enzymatic states with different Ca2+ affinities; this paper suggests as well, that equilibrium between these states is affected by pH. The results are compared with data obtained with skeletal sarcoplasmic reticulum fragments under the same experimental conditions.

Adenosine Triphosphatases↗
PubMed1983

Steady-state kinetics of immobilized enzymes at very low substrate concentrations studied using the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase.

A method for determining initial velocities of enzymatic reactions at very low substrate concentrations is presented. It is based on the continuous perfusion of substrate-containing media through the enzyme, previously deposited as a thin layer on a solid support. An analytical rationalization of the dependence of the enzymatic activity upon the substrate supply and the flow rate was developed (substrate supply (mumol/min) = flow rate (ml/min) x inflowing substrate concentration (mumol/ml). This paper shows that a straight line should be expected from a double-reciprocal plot of the velocity of the enzymatic reaction and flow rate. The reciprocal of the ordinate at the origin is the strict initial velocity for a given, constant, and very low substrate concentration, since substrate consumption and product accumulation tend to zero. Results obtained with two different sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase preparations agree with the theoretical predictions. The method enabled the use of ATP concentrations in the range of 10(-8) M; it required neither an ATP-regenerating system nor the dilution of the enzyme protein, and it presented no limitations for the reaction time. Both ATPase preparations showed two apparent Km values for the substrate in the submicromolar and micromolar ranges: 0.25-12.0 microM for the purified ATPase, and 0.17-1.65 microM for the microsomal ATPase.

Animals↗
PubMed1976

The interaction of magnesium ions with the calcium pump of sarcoplasmic reticulum.

1. In the presence of Ca2+, ATP phosphorylates the Ca2+ pump of sarcoplasmic reticulum at the same site and to the same extent regardless of whether Mg2+ is added or not to the incubation media, the main effect of added Mg2+ being to increase the rate of phosphorylation. 2. When phosphoenzyme is made in Mg2+-containing media it dephosphorylates about 30-times faster than when it is made in the absence of added Mg2+. Addition of Mg2+ after phosphorylation is uneffective in accelerating the hydrolysis of phosphoenzyme even in solubilized enzyme, suggesting that phosphorylation of the Ca2+ pump results in occlusion of the site at which Mg2+ combines to accelerate the release of phosphate. 3. Occlusion of the site for Mg2+ can be partially reversed by trans-1,2-diaminocyclohexonetetraacetic acid (CDTA). Use was made of this property to demonstrate that for the rapid release of phosphate to occur Mg2+ has to be bound to the enzyme. 4. Results seem to indicate that Mg2+ combines with the Ca2+ pump prior to phosphorylation.

Acetates↗
PubMed1971

Adenosine triphosphate--dependent calcium uptake by rat submaxillary gland microsomes.

Microsomes from rat submaxillary glands are able to take up calcium from the suspension media. Calcium uptake is greatly increased by the presence of ATP. This effect of ATP is not detected at 0 degrees C. ADP cannot replace ATP to potentiate calcium uptake. ATP-dependent calcium uptake is not observed in the absence of magnesium. ATP-dependent calcium uptake is enhanced by oxalate and, to a lesser degree, by inorganic phosphate. Total calcium per milligram of microsomal protein observed when tests were performed without oxalate closely parallels the amounts for skeletal and cardiac muscles reported by several authors. Calcium uptake in salivary gland microsomes is slower than in muscle microsomes. Speculations are considered about the role of ATP-dependent calcium uptake. It is suggested that a decrease in intracellular free calcium levels returns these cells to the resting state after secretion.

Adenosine Diphosphate↗
PubMedDate not supplied

Kinetics of calcium metabolism in rat dentin and enamel.

The time course of the specific activity of 45Ca in the dentin and enamel of rat incisors and molars was followed after an intravenous injection of the radiotracer. It was then correlated with the specific activity of 45Ca in plasma to obtain the accretion rates on the dentin of molars and incisors. The exchangeable calcium pool sizes in the dentin and the enamel of the molar were also calculated.

Animals↗