Calcium translocation

The phosphoryl group of the intermediate can enter two different reaction pathways leading to its decomposition. The phosphoryl group can either be transferred to water or to ADP. The hydrolytic pathway leading to the liberation of phosphate must be coupled to calcium translocation as it infers from the fixed coupling between calcium accumulation and phosphate liberation. 

In addition to the organs responsible for the development and maintenance of the fetus and newborn, l,25(OH)2D3 receptors have also been localized in several organs from the reproductive apparatus such as the uterus [71], ovary [72] and testis [73]. Since these tissues are not directly associated with calcium translocations, the presence of l,25(OH)2D3 receptors may be related to a role of the hormone in cellular proliferation, differentiation and/or maturation. Accordingly, the levels of testicular l,25(OH)2D3 receptors have been found to correlate with the meiotic and mitotic development of the spermatogonia [73], Clearly, more studies are needed in this area to clarify the role of the vitamin D hormone in these tissues. However,... 

Connolly, J. H. Jellison, J. (1995). Calcium translocation, calcium oxalate accumulation, and hyphal sheath morphology in the white-rot fungus Resinicium bicolor. Canadian Journal of Botany, 73, 927-36. 

Pumiliotoxin B has both cardiotonic and myotonic activity in isolated atrial or rat phrenic nerve diaphragm preparations (97). The cardiotonic activity is markedly dependent on the structure of the pumiliotoxin (95). Subsequent studies on the activity of pumiliotoxin B in neuromuscular preparations were interpreted as due to an apparent facilitation of calcium translocation from internal storage sites (99 see review in Ref. 5). Inhibitory effects on the calcium-dependent ATPase of sarcoplasmic reticulum were shown to be due not to pumiliotoxin B, but to phenolic impurities, namely, fcis(2-hydroxy-3-terf-butyl-5-methylphenyl)methane, 3,5-di-/ert-butyl-4-hydroxytoluene (BHT), and nonylphenols (100). 

In summary, pumiliotoxin B and congeners now appear to represent another class of alkaloids that modulate the function of voltage-dependent sodium channels. They are valuable research tools, and perhaps models, for the development of new myotonic or cardiotonic agents. Direct effects of pumiliotoxin-A class alkaloids on calcium translocation, as originally proposed (99), still remain a possibility for further pharmacological study. 

Gill, D.L., Mullaney, J.M. and Ghosh, T.K. (1988). Intracellular calcium translocation mechanism of activation by guanine nucleotides and inositol phosphates. J. Exp. Biol. 139, 105-133. 

In contrast to the well established existence of external high-affinity calciumbinding sites the permanent presence of low-affinity sites in the molecule seems ambiguous. This uncertainty is inherent in the scheme used to describe the reaction sequence of phosphate transfer and calcium translocation (cf. [3,4]). In this scheme, low-affinity sites are assumed to result from the phosphorylation of the transport protein. The low-affinity calcium binding sites in the unphosphorylated protein are... 

These comments apply to all enzyme mechanisms to varying degrees and particularly also to a problem closely related to the calcium pump, that of the myosin ATPase coupled to muscle contraction. The methods used to study the latter enzyme are discussed in some detail below and are also the ones which helped to elucidate the steps involved in the coupled process of ATP hydrolysis and calcium translocation. 

Calcium translocation is the second mechanism frequently suggested from experiments using skeletal muscles. However, high concentrations of caffeine are also necessary to modify intracellular calcium ion storage. 

There is abundant evidence for the view that calcium translocation, as well as ATP hydrolysis, is mediated by a Ca " -ATPase situated in the sarcoplasmic reticulum membranes (MacLennan and Holland, 1975). Lipid dependency of both of these functions has also been known for some time. Treatment of membrane preparations or of the Ca -ATPase with phospholipases results in parallel loss of both functions, whereas the restoration of both follows phospholipid addition. In early studies, lecithin alone has been effective in restoring ATPase activity of the delipidated enzyme while, for calcium translocation, phosphatidyl-ethanolamine was required as well (MacLennan and Holland, 1976). 

Further evidence for facilitation by pumiliotoxin B of calcium translocation across membranes derives from crayfish skeletal muscle (24). Pumiliotoxin B enhanced calcium-dependent action potentials in crayfish muscle with a half maximal effect at about 25 pM. Pumiliotoxin B also markedly potentiated directly evoked contractions of crayfish muscle. 

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