2 . 4 . 1 Albumin and other carrier proteins

Soluble proteins make up 6.4–8.3% of plasma by weight [1], a value comparable to their concentration in cell water [2]. Many plasma proteins are synthesized by the liver. Their concentrations may decrease in the setting of liver disease, malnutrition or protein-losing states such as nephrotic syndrome, severe burns, bowel inflammation, intestinal lymphangiectasia or major bleeding episodes. The most abundant plasma proteins have little or no enzymatic activity. Instead, their function is typically binding to other molecules, generically referred to as ligands. Some plasma binding proteins that have been characterized are listed in Table 1. Most of these proteins bind to only one (or a few related) molecules with high affinity. For example, haemopexin binds haem, retinol-binding protein binds retinol, and transferrin binds iron. In contrast, other binding proteins such as albumin and alpha-fetoprotein are far less selective. Albumin is the most important serum binding protein in terms of concentration (about 4.2%) [1,3] and number of different ligands bound. The latter include fatty acids [4], unconjugated bilirubin [5], many divalent cations such as copper [6,7], hydrophobic bile acids [6] and a variety of drugs and toxins [6,8–11] (Table 2). Albumin’s broad specificity reflects not only 2.4 Synthetic function