Biotin plasma protein binding

As a result of this resorption and the protein binding of plasma biotin, which reduces filtration at the glomerulus, renal clearance of biotin is only 40% of that of creatinine. This efficient conservation of biotin, together with the recycling of biocytin released from the catabolism of biotin-containing enzymes, may be as important as intestinal bacterial synthesis of the vitamin in explaining the rarity of deficiency. 

Egg Yolk and Chicken Plasma Biotin Binding Proteins. Apart from avidin, streptavidin, and biotin-dependent enzymes, a few proteins bind biotin bio-tinidase, HCS (already discussed), and two proteins found in the egg yolk of chicken (59). The role of these proteins is to transport and store biotin for the developing embryo. 

Biotinidase is also the major plasma binding protein for biotin. The pH optimum of the enzyme is 4.5 to 5.5, and its is in the micromolar range, compared with the nanomolar concentrations of biocytin, so it will have little enzymic activity in plasma. Rather, it functions as a transport protein for biotin, preventing its urinary excretion children with biotinidase deficiency (Section 11.2.3.1) excrete large amounts of both biocytin and free biotin. Biotin is covalently bound to biotinidase in plasma, as a thioester to a cysteine residue in the active site of the enzyme (see Figure 11.1). This thioester is formed only from biocytin, not free biotin, and is presumably the (normally transient)... 

The biotinylated OP called FP-biotin, when given to mice at a dose that caused no cholinergic signs of toxicity, labeled at least 12 proteins in mouse plasma. Four of the.se proteins have been identified as AChE, BuChE, carboxylcstcrasc, and albumin (Peeples et al., 2005). Thi.s experiment demonstrates that this particular OP binds to at least 12 proteins. 

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