1. Biotin in chicken egg yolk is non-covalently bound to a specific protein that comprises 0.03% of the total yolk protein (0.8 mg/yolk). This biotin-binding protein is not detectable by the normal avidin assay owing to the biotin being tightly bound. Exchange of [14C]biotin for bound biotin at 65 degrees C is the basis of an assay for this protein. 2. Biotin-binding protein from egg yolk is distinguishable from egg-white avidin on Sephadex G-100 gel filtration, although the sizes of the two proteins appear quite similar. 3. Biotin-binding protein is denatured at a lower temperature and freely exchanges biotin at lower temperatures than does avidin. 4. The biotin-binding protein in egg yolk is postulated to be responsible for the deposition of biotin in egg yolk. D-[carboxyl-14C]Biotin injected into laying hens rapidly appears in the egg bound to yolk biotin-binding protein and avidin. Over 60% of the radioactivity is eventually deposited in eggs. The kinetics of biotin deposition in the egg suggests a 25 day half-life for an intracellular biotinyl-coenzyme pool in the laying hen.

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