Circulating binding protein

The placenta keeps the maternal and fetal circulation systems separate, nourishes the fetus, eliminates fetal wastes, and produces hormones vital to pregnancy. It is composed of large collections of fetal vessels called villi, which are surrounded by intervillous spaces in which maternal blood flows. For substances to move from maternal circulation to fetal circulation, they must cross through the trophoblasts and several membranes. The transfer of any substance depends largely on the concentration gradient between the maternal and fetal circulatory systems, the presence or absence of circulating binding proteins, the hpid solubility of the substance, and the presence of facilitated transport, such as ion pumps or receptor-mediated endocytosis (Box 54-1). The placenta is an effective barrier to the movement... 

Finally, to participate in a nutritionally relevant process for the organism as a whole, a mineral must be transported away from the intestine. The concentration of circulating binding proteins and the degree of saturation of their metallic binding sites may influence the rate and magnitude of transport of recently absorbed minerals (Rosenberg and Solomons, 1984). 

Protein S. Protein S is a single-chain molecule of approximately 78,000 daltons that contains 10 y-carboxy glutamic acid residues in the NH -terminal portion of the molecule. Protein S is a regulatory vitamin K-dependent protein. In plasma 40% of this protein circulates free and 60% circulates bound to C4b binding protein. Free Protein S functions as a nonenzymatic cofactor that promotes the binding of Protein C to membrane surfaces (22—25). 

These binding proteins also provide a circulating reservoir of hormone, and because of the relatively large... 

Vrieling, A., D. W. Voskuil, J. M. Bonfrer et al. 2007. Lycopene supplementation elevates circulating insulin-like growth factor binding protein-1 and -2 concentrations in persons at greater risk of colorectal cancer. Am J Clin Nutr 86(5) 1456-1462. 

VreugdenhU, A.C., Snoek, A.M., van t Veer, C., Gieve, J.W., and Buurman, W.A., 2001, EPS-binding protein circulates in association with apoB-containing hpoproteins and enhances endotoxin-LDIWLDL interaction, J. Clin. Invest.Wl 225-234. 

Much discussion of vitamin D focuses on bone health, though this is by no means the only focus on vitamin D action. One result of l,25(OH)2D action is the upregulation of the synthesis of a calcium-binding protein whose function is to transport dietary calcium across the intestinal mucosa and into the systemic circulation. Phosphate accompanies the calcium. This has the effect of increasing the fraction of dietary calcium that is actually absorbed and is, therefore, potentially useful for bone formation. In addition, l,25(OH2)D has the effect of mobilizing calcium from bone. Both actions tend to raise the extracellular level of calcium. 

Pharmacokinetics The parietal cells of the stomach secrete intrinsic factor, which regulates the amount of vitamin B-12 absorbed in the terminal ileum. Bioavailability of oral preparations is approximately 25%. Vitamin B12 is primarily stored in the liver. Enterohepatic circulation plays a key role in recycling vitamin B-12 from mainly bile. If plasma-binding proteins are saturated, excess free vitamin B- 2 will be excreted in the kidney. 

Figure 8.13. Growth hormone found in the circulation is generally bound to GH-binding proteins. Binding to the cell surface receptor promotes receptor dimerization and phosphorylation and hence activation. This leads to the phosphorylation of various cystolic protein substrates, which mediate intracellular effects of the hormone...

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