Recombinant human thyroid peroxidase expressed in insect cells is soluble at high concentrations and forms diffracting crystals

Human thyroid peroxidase (TPO), the key enzyme in thyroid hormone synthesis , can be produced in active form in the High Five insect cell line and when purified from the cell culture medium is soluble at concentrations of up t o 18 mg/ml. This contrasts to a recent report in which human TPO produced i n insect cells was found to be insoluble at high concentrations. Our concen trated TPO grows trigonal trapezohedral crystals Of UP to 0.5 mm in length in a vapour diffusion apparatus using polyethelene glycol as a precipitant. The crystals diffract X-rays to 6 Angstrom resolution and the diffraction data from the crystals have been analysed giving unit cell, dimensions. A p otential molecular replacement solution has been identified using myelopero xidase (MPO) as a phasing model.