Effect of concanavalin A on the activity of membrane-bound and detergent-solubilized Mg2+ -ATPase

Divalent cation-activated ecto-ATPase activity of rat glomerular mesangial cells

Evidence is presented of a Mg2+ and Ca(2+)-activated ATPase at the surface of cultured rat glomerular mesangial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. The apparent Km values were 0.46 and 0.32 mM for Mg(2+)-ATPase and Ca(2+)-ATPase, respectively. Inhibition studies have revealed that this enzyme is different from the (Na+ + K+)-ATPase and from the divalent cation activated mitochondrial ATPases. Culture in the serum-free medium markedly decreased ecto-ATPase activity. Con A treatment reduced at 50 micrograms/ml by 15% the Ca(2+)-ATPase activity. Endocytosis of serum-treated zymosan (STZ) did not significantly affect ecto-ATPase activity. In contrast, endocytosis of STZ by macrophages was accompanied by a biphasic response, an increase in Mg(2+)-ATPase and Ca(2+)-ATPase activities after ingestion of smaller amounts of STZ and a marked decrease after loading doses of STZ.

Characterization of Mg2+-ATPase from slow-twitch muscle membranes

SR vesicles from rabbit slow-twitch muscle reveal high activity (0.7-0.9 mumol/mg X min) of "basic" or Mg2+-ATPase. This enzyme differs in its biochemical properties from the well characterized Ca2+ pump ATPase. It is active in millimolar concentration of magnesium or calcium. The activity is inhibited by various detergents except for digitonin. This enzyme seems to be an integral membrane protein since it remains in the membrane after removal of peripheral proteins with EDTA. It can be partially solubilized from the membrane using digitonin without a decrease in specific activity. Ion exchange chromatography on DEAE-Sephacel of the post digitonin supernatant allows us to obtain a 5-fold increase in Mg2+-ATPase specific activity concomitantly with the enrichment in two proteins of Mr = 30,000 and 150,000.

Partial purification and characterization of rabbit-kidney brush-border (Ca2+ or Mg2+)-dependent adenosine triphosphatase

The ATPase activity of rabbit-kidney brush border can be activated almost equally well by Ca2+ and Mg2+ and, therefore, should be called (Ca2+ or Mg2+)-ATPase. This enzyme was solubilized and enriched 14-fold by the following steps: pretreatment with papain removed 69% of alkaline phosphatase without attacking a significant portion of the ATPase activity. Addition of 1% cholate removed 65% of the protein but no ATPase activity. The combination of cholate (0.5%) and deoxycholate (0.4%) solubilized most of the ATPase activity and most of the remaining protein. A column chromatography of the extract on Sepharose CL-2B resulted in an 6.5-fold increase of specific ATPase activity. A precipitation by ammonium sulfate (40% saturation) produced an additional 1.9-fold increase. The yield of this partial purification was 16%. Towards the nucleotides UTP and GTP the enzyme showed an activity slightly higher, and towards ITP and CTP an activity slightly lower than that with ATP. ADP was split about half as fast as ATP. AMP was not accepted by the enzyme. Replacing MgCl2 by CaCl2 resulted in an ATPase activity of 92% of that with MgCl2. Using calcium- and magnesium-ATP as substrates, apparent Km values of 0.22 and 0.33 mM, respectively, were obtained. The gel electrophoresis revealed the enrichment of a protein with an apparent Mr of 95000 and also that of microvillus actin.

Partial deglycosylation of chloroplast coupling factor 1 (CF1) prevents the reconstitution of photophosphorylation

Treatment of spinach chloroplast coupling factor 1 (CF1) with a mixture of glycosidases resulted in the removal of approximately 75% of the carbohydrate associated with the enzyme. The ATPase of CF1 was not activated by this treatment nor were its heat-activated or methanol-dependent ATPase activities inhibited. The deglycosylated enzyme, however, was unable to catalyze photophosphorylation when recoupled with CF1-depleted thylakoids. The glycosidase-treated protein competed with native CF1 for specific binding sites on the depleted membranes and was able to reconstitute proton uptake in uncoupled thylakoids. The inhibition of photophosphorylation could not be overcome with added delta subunit. We conclude that deglycosylated CF1 was unaffected in its ability to bind to the membrane sector of the chloroplast proton-pumping ATPase (CF0) but was altered in some property essential for photophosphorylation but not ATPase activity.