Calcium-binding protein CaBP

Fig. 2. Homeostatic control of blood Ca " level where PTH is parathyroid hormone [9002-64-6], CC, cholecalciferol, ie, vitamin D HCC, hydroxycholecalciferol DHCC, dihydroxycholecalciferol CaBP, calcium-binding protein NAD PH, protonated nicotinarnide-adenine dinucleotide...

CaBP Calcium-binding protein FFA Free fatty acid... 

Calcium is absorbed from the intestine by facilitated diffusion and active transport. In the former, Ca " moves from the mucosal to the serosal compartments along a concentration gradient. The active transport system requires a cation pump. In both processes, a calcium-binding protein (CaBP) is thought to be required for the transport. Synthesis of CaBP is activated by 1,25-DHCC. In the active transport, release of Ca " from the mucosal cell into... 

S100A6 S100 calcium binding protein A6 Calcydin (CACY), 2A9, PRA, CABP 1q21 NM 014624... 

The next stage involves the synthesis of specific calcium-binding proteins, typified by the intestinal CaBP253 discussed in Section 62.1.3.4.5, which probably stimulates the transport of calcium. The role of the protein in vitamin D-dependent absorption of calcium is supported by the good correlation between the concentration of CaBP and the rate of calcium absorption. Under conditions of low calcium or phosphorus diets, chicks and other animals produce more intestinal CaBP to increase the efficiency of uptake of calcium. In general, adaptation to a low calcium diet involves increased synthesis of l,25-(OH)2D3 and the intestinal CaBP. Lowered requirements for calcium in old age are manifested by lower levels of both factors.449,450... 

PTH also increases intestinal calcium absorption by increasing 1,25 (OH) 2D. PTH is a major trophic factor for renal 25(OH)t>-la-hydroxylase. It increases the conversion of 25(0H)D to the active vitamin D metabolite, l,25(OH)2D. Calcium is absorbed principally in the duodenum, although it can also be absorbed by the distal small bowel and colon. About 30% of a daily calcium intake of 1 g (25 mmo ) is absorbed. Approximately 100 mg (2.5 mmol) of calcium is secreted into gut lumen by intestinal secretion therefore net calcium absorption is 200 mg (5.0 mmol)/day. Calcium is absorbed by passive diffusion and by an active transport system. It is estimated that passive diffusion accounts for absorption of about 10% of ingested calcium per day. Active calcium absorption in the duodenum is under the control of l,25(OH)2D. This vitamin D metabolite increases the intestinal cell synthesis of a calcium-binding protein (CaBP), which enhances the net absorption of ingested calcium. 

A developmentally regulated polypeptide (PEP-19), that is a presumptive neuron-specific calcium binding protein, was identified in adult and neonatal rat cerebellum and its amino acid sequence was determined (Ziai et al., 1986). PEP-19-like immunoreactivity is expressed by Purkinje cells and by the cartwheel cells of the dorsal cochlear nucleus of the mouse (Mugnaini et al., 1987). Berrebi et al. (1991) drew attention to the expression of PEP-19, CaBP and other Purkinje cell markers (cerebellin, L7 see below) by bipolar cells and other neurons of the retina. 

Corradino RA, Wasserman RH. 1970. Strontium inhibition of vitamin D3-induced calcium-binding protein (CaBP) and calcium absorption in chick intestine. Proc Soc Exp Biol Med 133(3) 960-963. 

Menczel J, Mor E. 1972. The effect of thyrocalcitonin (TCT) on calcium binding protein (CABP) and strontium binding protein (SRBP). In International Conference on Strontium Metabolism, ed. Second international conference on strontium metabolism, Glasgow and Strontian, 16-19 August, 1972. TID 4500 59th ed. Health and Safety Laboratory / U.S. Atomic Energy Commission, 273-276. 

Figure 6 A schematic representation of the mechanism of action of 1,25 OH)2D in various target cells resulting in a variety of biological responses. The free form of 1,25(OH)2D3 enters the target cell and interacts with its nuclear vitamin D receptor (VDR), which is phosphorylated (Pi). The 1,25(OH)2D-VDR complex combines with the retinoic acid X receptor (RXR) to form a heterodimer, which, in turn, interacts with the vitamin D responsive element (VDRE), causing an enhancement or inhibition of transcription of vitamin D-responsive genes including calcium-binding protein (CaBP), ECaC, 24-OHase, RANKL, alkaline phosphatase (alk Pase), prostate-specific antigen (PSA), and PTH.

Although only CaBP mRNA is known to increase in response to vitamin D, other vitamin D-dependent changes occur in the intestinal epithelium, including increases in activity of alkaline phosphatase, calcium ATPase, adenylate cyclase, and RNA polymerase. In response to vitamin D, several brush-border membrane proteins increase in concentration as does a calcium-binding complex. Non-cAMP-dependent phosphorylation of a brush-border membrane protein, increased synthesis and turnover of microvillar membrane phospholipids, and effects on mitochondria, Golgi membranes, and intracellular membrane vesicles are observed. Increased transport of Ca " " across the basolateral membrane may be produced by... 

A Kj value of 1.1 x 10 M was estimated for calcium binding from the fluorescence titration data. Twelve-kDa CaBP binds 1 mol of Ca +Zmol of protein at pH 7.2. Hence, the protein has a single high-affinity Ca + binding site. 

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