Calbindin

Calbindin describes calcium binding proteins first described as the vitamin D-dependent calcium binding proteins in intestine and kidney.

Calbindin-D9k
Calbindin-D9k is present in mammalian intestinal epithelial cells (enterocytes) and mediates the transport of calcium across the enterocytes from the apical side, where entry is regulated by the calcium channel TRPV6, to the basolateral side, where calcium pumps such as PMCA1 utilize intracellular adenosine triphosphate to pump calcium into the blood. Calbindin-D9k can also be found in the kidney and uterus in some mammalian species.

The transport of calcium across the enterocyte cytoplasm appears to be rate-limiting for calcium absorption in the intestine; the presence of calbindin increases the amount of calcium crossing the cell without raising the free concentration. Calbindin-D9k may also stimulate the basolateral calcium-pumping ATPases. Expression of calbindin-D9k and calbindin-D28k are both stimulated by the active vitamin D metabolite, calcitriol although the precise mechanisms are still controversial. In mice in which the receptor for vitamin D is not expressed, calbindin-D9k is reduced, but not absent.

Calbindin-D28k
Calbindin-D28k performs a similar role in the intestine in birds and in the mammalian kidney; it is also found in a number of other neuroendocrine cells, particularly in the cerebellum. There is no homology between calbindin-D28k and calbindin-D9k, except Apart from their calcium binding domains (EF-hands): calbindin-D9k has two EF-hands, and calbindin-D28k has six.