Sulphate Assimilation in Higher Plants: A Thioredoxin-Dependent PAPS-Reductase from Spinach Leaves

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Abstract
Higher plant leaf protein was investigated for the enzyme activity catalyzing a thioredoxin-dependent reduction of 3'-phosphoadenylylsulphate (PAPS) to sulphite. The enzyme became detectable when heterologous thioredoxin from Escherichia coli was used substituting for the hitherto unidentified plant thioredoxin. The enzyme's cross-reactivity with heterologous thioredo­ xin enabled the partial purification and brief characterization. The molecular weight of the en­ zyme as estimated by HPLC size exclusion and gel filtration was 68-72 k. The protein reduced PAPS only when thioredoxin was present as cosubstrate. The function of this enzyme in the assimilation of inorganic sulphate by higher plants is discussed in comparison to the function of the respective enzymes from Escherichia coli and Saccharom yces cerevisiae.

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