Effect of Natural Oestrogens on Coagulation

with oral conitraceptives. Without being able categorically to deny this possibility I think it unlikely for the following reasons. Retinol in human blood is predominantly bound to a specific transport-protedin known as the retinol-binding protein (RBP). Under physiological conditions RBP ibinds to prealbumin.4 The amount of free retinol is very small, indeed. Using a fluorescence assay5 measurements have been made of the amoun-t of RBP,in plasma from 10 women before starting to take a combined oral contraceptive (Minovlar: 1 mg nocethisterone acetate and 50 ,ug ethynylestradiol) and again after three months' treatment. The mean pretreatment level of RBP was 42±8 (S.D.) ,ug/ml, but this rose to 75 ± 10 ug/ml during treatmenrt. This finding indicates that the rise in iseruum vitamin A level during the taking of oral contraceptives is due to an increased concentrtion of RBP. The amount of free retinod is unchanged. Hypervitaminosis A and the teratogeic effects of the vitamnm in pregnant animls are seen with doses which far exceed the binding capacty of RBP. I,t seems unlikely that an increase in protein-bound retinol is hazadous. A very analogous isiituation occurs with coricosterodds, where plasma levels are very high in women taking oral cotraceptives owng to an increase in,transcartin.6 The increase in plasma RBP seen in women taking oral contraceptives is probably due to the oestirogen component, for plasma retinol is not increased by progestogens alone.2 Serum vitamin A is also raised in volunteers exposed to DDT,7 for this compound yields oestrogenic mexabolites.-I am, etc., M. H. BRIGGS Biochemistry Department, Alfred Hospital, Melbourne