Calbindin-D9k is present in mammalian intestinal epithelial cells (enterocytes). Calbindin-D9k can also be found in the kidney and uterus in some mammalian species. It in encoded in humans by the S100G gene which has also been termed CALB3.
Calbindin-D9k is a member of the S100 family of calcium-binding proteins. It has two EF-hands sequences which bind Ca2+ with high affinity.
Calbindin-D9k 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. 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, like that of calbindin-D28k, is 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.
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