1. α-L-Iduronidase activity was assayed by incubation of iduronosyl anhydro[1-3H]mannitol 6-sulphate with homogenates of cultured skin fibroblasts, amniotic cells and leucocytes derived from normal individuals, patients affected with α-L-iduronidase deficiency disorder (mucopolysaccharidosis type I: Hurler, Scheie and Hurler-Scheie compound) and parents of such patients. 2. The assay for α-L-iduronidase, described for use with these cell types, clearly distinguished affected homozygotes from heterozygotes and normal controls. 3. The mean specific activity of α-L-iduronidase in homogenates prepared from cultured skin fibroblasts and leucocytes from more than seven obligate heterozygotes for mucopolysaccharidosis type I was found to be about one-half of the mean of more than 40 normal controls. A number of heterozygotes had α-L-iduronidase activity that identified them as carriers; others had values clearly within the normal range. Thus heterozygote detection of mucopolysaccharidosis type I is not certain. 4. With iduronosyl anhydro[1-3H]mannitol 6-sulphate used as substrate, cell types from five pregnancies at risk for α-iduronidase deficiency were examined; each foetus was predicted to be unaffected. For one foetus it was not possible from measurments of total enzyme activity alone to distinguish between the heterozygous carrier state and affected homozygote. The difficulty was resolved by comparing Michaelis-Menten kinetics of this enzyme in fibroblasts derived from the propositus, mother and normal controls with the kinetics of the enzyme in amniotic cells isolated from the foetus at risk and from normal controls.
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