La fixation des hormones thyroidiennes par Escherichia coli et son mécanisme

Abstract

Resting cells of Escherichia coli rapidly bind 3,5,3′-triiodo- l-thyronine (T 3) and l-thyroxine (T 4) to a considerable degree. Neither iodotyrosines nor iodides are concentrated by the bacterial cells under the same conditions. Concentration of thyroid hormones by the bacteria is due to the interaction of iodothyronines with endocellular proteins (bTBP, or bacterial thyroxine-binding proteins). Fixation of T 3 is greater, and proceeds at a higher speed, than does fixation of T 4; bTBP shows a greater affinity for T 3 than for T 4. Electrophoretic analysis on a cellulose column of the acellular extracts obtained by ultrasonic breaking of bacteria and incubated with T 3 or T 4, shows that each of these hormones is bound to a particular protein system. The binding of T 3 or T 4 to E. coli is inhibited by the addition of human serum or of bacterial extract. A reversible equilibrium is established between the hormone and the intracellular and extracellular binding proteins. Compared to human serum thyroxine-binding globulin, the binding capacity of bTBP for T 3 or T 4 is high and its affinity is relatively low. The E. coli system could serve as a model for the partition of thyroid hormones between extracellular and intracellular binding proteins.