Biomedical subjects

H Levkovitz

Publications and source records attributed to H Levkovitz.

22 records · Page 2Linked to original sources

PubMed1977

The regulatory GTPase cycle of turkey erythrocyte adenylate cyclase.

It has recently been suggested that adenylate cyclase activity is controlled by a regulatory cycle consisting of two reactions: a hormone induced formation of the active adenylate cyclase-GTP complex, and a subsequent turn-off reaction in which hydrolysis of the bound nucleotide reverts the system to the inactive state. To test this model each of the two reactions was measured separately and their rate constants were used to estimate the steady state adenylate cyclase and GTPase activities. The first order rate constants were kon = 3 min-1 for the activation reaction and koff = 15 min-1 for the turn-off reaction. Substitution of these rate constants in the steady state equation of the regulatory cycle gave values of hormone stimulated adenylate cyclase and GTPase activities similar to those determined by direct measurements. Treatment of the adenylate cyclase with cholera toxin caused a decrease of 96% in the rate constant of the turn-off reaction. In this case too the activities calculated from the steady state equation were in good agreement with those determined directly.

Adenylyl Cyclases↗

PubMed1976

Conjugation in Tetrahymena pyriformis. The effect of polylysine, concanavalin A, and bivalent metals on the conjugation process.

The polycation polylysine, at different degrees of polymerization, was found to cause a marked inhibition of the conjugation process. Inhibition of conjugation by polylysine was highly dependent on the molecular weight of the polymer. When polylysine of a mol wt of 1,250 (degree of polymerization=6) was used, a concentration of 1.6 X 10(-5) M was required for a complete inhibition of conjugation, while only 2 X 10(-7) M of polylysine of a mol wt of 71,000 (degree of polymerization=340) was needed for the same effect. Polyaspartic acid prevented the inhibition of conjugation by polylysein. Chelators of bivalent metals such as O-phenanthroline (10(-3) M), EDTA (10(-3) M), and EGTA (5 X 10(-3) M) strongly inhibit the conjugation process in Tetrahymena pyriformis. The inhibition was partially prevented when bivalent metals such as Zn++, Fe++, and Ca++ were added together with the chelators. The lectin concanavalin A (25 mug/ml) completely prevented the conjugation process, while other lectins, such as phytohemagglutinin (500 mug/ml), soybean agglutinin (75 mug/ml) and wheat germ agglutinin (250 mug/ml) had no effect. Inhibition of conjugation by concanavalin A is completely reversible by 40 mM of alpha-methyl-D-mannoside.

Animals↗

PubMedDate not supplied

Magnesium fortification of drinking water suppresses atherogenesis in male LDL-receptor-deficient mice.

Magnesium, an important cofactor of more than 300 enzymes, has previously been found to modulate blood lipid levels, atherogenesis and atherosclerosis in rabbits, when added to their diet. The aim of this study was to examine whether magnesium fortification of drinking water, without a change in diet content, can affect atherogenesis. The study included six groups of LDL-receptor-deficient mice. The mice received either distilled water or water containing 50 g of magnesium sulfate per liter. In the first (12 weeks) and second (6 weeks) stages of the experiment, the mice received low- and high-cholesterol diets, respectively. At the end of each stage, blood was drawn for the determination of plasma magnesium, calcium and lipid levels. In addition, the extent of atherosclerosis was determined at the aortic sinus. In both males and females, magnesium fortification was associated with higher levels of plasma magnesium (50 and 37% increase, respectively), without any differences in plasma calcium content. The extent of atherosclerosis at the aortic sinus in the male mice that received high levels of magnesium was a third of that of the male mice that received distilled water. However, these differences were not found in the female groups. Surprisingly, the female mice that received water fortified with magnesium had higher levels of cholesterol after stage 2, whereas no differences regarding plasma lipid levels were found among the male mice. These results confirm that magnesium fortification of drinking water is capable of inhibiting atherogenesis in male LDL-receptor-deficient mice. The mechanisms of action are yet to be discovered, and are probably not related to diminished lipid excretion, but possibly to the prevention of calcium influx into vascular smooth muscle cells, elevated antioxidative capacity, or other yet undetermined mechanisms.

Animals↗

PubMedDate not supplied

Suppression of atherogenesis in female low-density lipoprotein receptor knockout mice following magnesium fortification of drinking water: the importance of diet.

OBJECTIVE: Magnesium (Mg) has previously been found to modulate blood lipid levels, atherogenesis and atherosclerosis in rabbits when used as a dietary supplement. In addition, we have reported that Mg fortification of drinking water can attenuate atherogenesis in male low-density lipoprotein (LDL)-receptor-deficient mice, but had a mild and nonsignificant effect on female mice fed a high-cholesterol diet supplemented with cholic acid. The aim of this study was to examine whether Mg has an antiatherogenic effect in female mice fed a high-cholesterol diet without cholic acid. METHODS: Two groups of female LDL-receptor-deficient mice were included. The mice received either distilled water or water with 50 g of Mg sulfate per liter. In the first (12 weeks) and second (6 weeks) stages of the experiment, the mice received low- and high-cholesterol diets, respectively, both without cholic acid. At the end of each stage of the experiment, blood was drawn for the determination of plasma Mg, calcium and lipid levels. In addition, the extent of atherosclerosis was determined at the aortic sinus level. RESULTS: Mg fortification was associated with higher levels of plasma Mg while the mice were on a high-cholesterol diet, and the extent of atherosclerosis at the aortic sinus was significantly decreased in the female mice that received high levels of Mg compared with the female mice that received distilled water. The female mice that received water fortified with Mg had lower levels of triglycerides after stage 2, whereas no differences regarding cholesterol levels were found. CONCLUSION: These results confirm that Mg fortification of drinking water is capable of inhibiting atherogenesis also in female LDL-receptor-deficient mice fed a high-cholesterol diet, and demonstrate the importance of the nutritional composition of diet in this experimental model.

Animals↗