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 to 18 mg/ml. This contrasts to a recent report in which human TPO produced in insect cells was found to be insoluble at high concentrations. Our concentrated 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 a 6 A resolution and the diffraction data from the crystals have been analysed giving unit cell dimensions. A potential molecular replacement solution has been identified using myeloperoxidase (MPO) as a phasing model.

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