Calcium export from cells

In the case of prothrombin and related clotting factors, interruption of the vitamin K cycle leads to the production of nonfunctional, undercarboxylated proteins, which are duly exported from hepatocytes into blood (Thijssen 1995). They are nonfunctional because there is a requirement for the additional carboxyl residues in the clotting process. Ionized carboxyl groups can establish links with negatively charged sites on neighboring phospholipid molecules of cell surfaces via calcium bridges. 

Guerini, D., Coletto, L. and Carafoli, E. (2005) Exporting calcium from cells, Cell Calcium, 38, 281-289. 

The level of calcium in the cell is low, about 0.1 //M compared with approximately 1.3 mM outside the cell. The intracellular concentration is kept low by the activity of transport systems which transport calcium out of the cell and sequester it in the mitochondria and endoplasmic reticulum. The plasma membrane thus houses a Ca2+ transporting ATPase, as does the endoplasmic reticulum and nucleus. Calcium may also be stored in the mitochondria, and the intracellular protein calmodulin will bind calcium. Interference with any of these processes may be caused by toxic compounds and can alter calcium homeostasis. This can allow an influx of Ca2+, inhibition of export of Ca2+ out of the cell, or a release of Ca2+ from compartments within the cell. The result of each of these will be a rise of intracellular Ca2+ which can cause a variety of damaging events. 

Little is known about their biological activities, e. g., they participate in the export of potassium and calcium ions from the cell, also in the development of skin inflammations, and they promote the release of insulin from pancreatic islet cells. 

Interference with any of these processes may be caused by toxic compounds and can alter calcium homeostasis. This can allow an influx of Ca2+, inhibition of export of Ca2+ out of the cell, or a release of Ca2+ from compartments within the cell. The result of each of these will be a rise of intracellular Ca2+, which can cause a variety of damaging events. 

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