sentence: Enzymatic activities have been characterized for approximately one half of the selenoproteome , including the glutathione peroxidases ( Gpx1 – Gpx6 ) , thioredoxin reductases ( Trxr1 – Trxr2 ) , iodothyronine deiodinases ( Dio1 – Dio3 ) , methionine sulfoxide reductase ( MsrB1 or SelX ) and selenophosphate synthetase ( Sps2 ) .