Inhibition of human placental microsomal aromatase by thyroid hormone and iodothyronine derivatives

The initial velocity kinetics of the effect of thyroid hormone and analogs on human placental microsomal aromatase were studied. Thyroxine and its propionic analog 3,5,3',5'-tetraiodothyropropionic acid show a competitive inhibition with an apparent Ki of 1.4 microM and 8.5 microM, respectively, towards androstenedione aromatization. 3,5,3'-Triiodothyronine with a Ki of 1.8 microM shows a decreased affinity compared to thyroxine. Two analogs, 3,5,3'-triiodothyropropionic acid and 3,5,3'-triiodothyroacetic acid appear to have negligible competition.