Ca2+-ATPase activity

The biochemical basis of CAM-induced stimulation of Ca2+-ATPase activity in carrot cells was studied further by determining the parameters of the Ca2+-translocating reaction of the enzyme in the presence and absence of exogenous CAM, using EGTA-treated plasma membrane [45], The affinity of Ca2+-ATPase for Ca2+ was considerably increased by... 

Lindemann, J.P., Jones, L.R., Hathaway, D.R., et al., 1983, beta-Adrenergic stimulation of phospholamban phosphorylation and Ca2+-ATPase activity in guinea pig ventricles. J Biol Chem, 258(1), pp 464-71. 

A sphingosine derivative, symbioramide (308), was obtained from the laboratory-cultured dinofagellate Symbiodinium sp. as a Ca2+-ATPase activator in the sarcoplasmic reticulum (250). The complete stereostructure of 308 was established by total synthesis (251). The occurrence of eo>f/iro-docosasphinga-4,8-dienine, as an ester (309), was found in Ammonia sulcata (252). This is the first example of a sphinga-4,8-dienine ester. A... 

Fig. 6. Effect of in vivo aluminum exposure on the Ca2+ ATPase activity in synaptic plasma membranes. Values are mean SD of 8 animals/group. P < 0.001, statistically significant from control group. Data are from Julka and Gill (1996) [32]...

Aluminum in micromolar concentrations was found to inhibit calcium pumping in endoplasmic reticulum. The Ca2+ ATPase activity of rat brain and cerebellum was remarkably reduced and mitochondria showed increased Ca2+ release in the presence of exactly estimated 50 pmol L-1 Al3+ [67]. Aluminum was found to be an important disrupter of intracellular calcium homeostasis, interfering also with the mitochondrial Ca2+ pump, as well as activating an Na+-K+ ATPase - the antiport mechanism of ion exchange in the plasma membrane, which regulates the Ca2+-Na+ antiporter exchange [67]. 

The plasma membrane Ca2+-ATPase pump effects outward transport of Ca2+ against a large electrochemical gradient for Ca2+. The mechanism of the pump involves its phosphorylation by ATP and the formation of a high-energy intermediate. This basic mechanism is similar for both the plasma membrane and ER pumps however, the structures of these distinct gene products are substantially different. As discussed below, the ER pump, sometimes called a sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) pump, is inhibited potently by certain natural and synthetic toxins that do not affect the plasma membrane pump. The plasma membrane pump, but not the SERCA pump, is controlled in part by Ca2+ calmodulin, allowing for rapid activation when cytoplasmic Ca2+ rises. 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

In biological systems, therefore, the behavior of Li+ is predicted to be similar to that of Na+ and K+ in some cases, and to that of Mg2+ and Ca2+ in others [12]. Indeed, research has demonstrated numerous systems in which one or more of these cations is normally intrinsically involved, including ion transport pathways and enzyme activities, in which Li+ has mimicked the actions of these cations, sometimes producing inhibitory or stimulatory effects. For example, Li+ can replace Na+ in the ATP-dependent system which controls the transport of Na+ through the endoplasmic reticulum Li+ inhibits the activity of some Mg2+-dependent enzymes in vitro, such as pyruvate kinase and inositol monophosphate phosphatase Li+ affects the activity of some Ca2+-dependent enzymes— it increases the levels of activated Ca2+-ATPase in human erythrocyte membranes ex vivo and inhibits tryptophan hydroxylase. 

Burdyga I was involved in these experiments, and we were also working on the Na+/Ca2+ exchanger. When we used ouabain to block the Na+ pump, there was a limited rise of Na+ and then the Na+/Ca2+ exchanger drives Na+ out at the expense of Ca-ATPase activity. What I found later is that there is a massive load of the Ca2+... 

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