Human dihydropteridine reductase: a method for the measurement of activity in cultured cells, and its application to malignant hyperphenylalaninemia.

1. The kinetic, assay and culture variables involved in the determination of dihydropteridine reductase activity in human cultured cells have been investigated, and a modified assay is described which allows for accurate determination of activity. Unlike previously used assays, this assay utilizes saturating concentrations of substrates and its increased sensitivity allows for excellent replication on a smaller number of cells than was previously required. 2. The modified assay was used to investigate the levels of dihydropteridine reductase in patients with malignant hyperphenylalaninemia; results of assay for the enzyme in cultured cells from affected infants and parents are presented. These are compared with control data for dihydropteridine reductase activity in fibroblasts and continuous lymphoid cells from normal controls and phenylketonuric patients, in normal amniotic cells and in liver obtained at autopsy.