Biotin-binding protein from chicken egg yolk. Assay and relationship to egg-white avidin

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.     

Ectopic pregnancies associated with low dose progestagen-releasing IUDs

Dimethyl-polysiloxane capsules containing pure progestagens were attached to Tatum's T IUDs and tested in 594 fertile women for contraceptive performance. The control group was represented by 71 women who received identical devices containing barium sulphate instead of steroid and 100 women who received a Copper T-200. The progestagens and the doses tested were megestrol acetate (4.8, 19.2, 26 and 32 micrograms/day; levonorgestrel (2.1, 3.4 and 8.5 micrograms/day); norethindrone (18 micrograms/day); R2323 (28.6 and 45 micrograms/day); and norgestrienone (26 micrograms/day). Twelve pregnancies were diagnosed during 5201 woman-months of exposure accumulated within the first year of use among users of the steroid-bearing IUDs. Five of these were ectopic gestations. Ten pregnancies, all uterine, were detected during 1701 woman-months of exposure in the control group. Intrauterine delivery of progestagens by means of a carrier IUD is effective in decreasing the pregnancy rate but it might effect postovulatory events in a way which increases the rate of tubal implantation. Because of this property, progestagen-releasing IUDs should be limited to doses that assure maximal effectiveness to avoid increasing the risk of ectopic pregnancy.