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...

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... 

The UOP method uses CABP which, for all practical purposes, is the same as VABP as shown in Appendix 2. The factor is more popular than because the VABP data are readily available. The use of MeABP in the Watson method generally results in a lower K value than that of UOP. Example 2-1 illustrates steps to calculate the and factors. 

Calculate MeABP and CABP by adding corrections from Appendix 2 to VABP. 

Step 3 From Appendix 2, corrections to VABP are approximately -34 F for MeABP and -10°F for CABP. 

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

Johnson E.W., Eller P. and Jafek B. (1995). Distribution of OMP, PGP 9.5- and CaBP-like immunoreactive chemoreceptor neurons in the developing human olfactory epithelium. Anat Embryol 191, 311-317. 

It has been observed that the IP3-R can be activated in the absence of its ligand, I(1,4,5)P3> by a group of proteins termed Ca2+-binding proteins (CaBPs), a subfamily of E-F hand-containing proteins known as neuronal Ca2+ sensors [17]. These proteins are exclusively expressed in neurons and are particularly enriched in synaptic/dendritic fields. 

The addition of CaBPs can directly activate the IP3-R in a Ca2+-dependent manner, via interaction with the N-terminus region of the receptor. This raises the possibility that CaBPs can act on IP3-Rs to potentiate the Ca2+ signals that arise from other sources, e.g. ligand-gated ion channels. 

CaBP (Calcyclin), 46 454-456 Cadherins, calcium binding in, 46 483-484 Cadmium... 

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... 

Fig. 8. The EF hand represents a widespread Ca2+ binding motif that was first characterized in calmodulin. Generally, EF hands are observed in pairs. Each consists of two short stretches of o-helix connected by a loop. Two possible sets of ligands have been identified through which calcium cations bind to the residues comprising the loop (see text). This figure illustrates the structural forms observed that relate to different Ca2+ binding states (PDB closed, 1GFC open, 1GLL semi-open, 1WDC). Figure from http //structbio.vanderbilt.edu.chazin/cabp database/seq.

CaBPs Neurite extension Chemotactic activity Thymic hormone activity Extracellular matrix component... 

A potent analog. 2-Carboxyarabinitol 1,5-bisphosphate (CABP) has been useful in studies of rubisco. 

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. 

Insulin Decreased bone mass in insulin-dependent diabetes in rats, decreased insulin lowers intestinal Ca " absorption, intestinal CaBP, and serum l,25(OH)2D. Probably increases conversion of 25-(OH)D to 1,25-(0H)2D. 

In bone, a CaBP called osteocalcin contains 49 amino acids (M.W. 5500-6000). Its synthesis is stimulated by 1,25-(0H)2D. Osteocalcin contains four residues of y-carboxyglutamic acid, which require vitamin K for their synthesis and are important as binding sites for calcium (Chapter 36). Although vitamin K deficiency reduces the osteocalcin content of bone, it does not cause functional bone defects. For this reason, osteocalcin may function in calcium mobilization rather than deposition. Alternatively, as an effective inhibitor of hydroxyapatite formation, it may prevent overmineralization of bone. 1,25-(OH)2D increases y-glutamyl carboxylase activity in the renal cortex. The relationship between vitamin D and vitamin K needs clarification. 

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... 

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