A fluorometric assay using 4-methylumbelliferyl α-l-iduronide for the estimation of α-l-iduronidase activity and the detection of Hurler and Scheie syndromes

Incubation of 4-methylumbelliferyl α-L-iduronide with whole cell homogenates prepared from cultured skin fibroblasts and amniotic cells, and peripheral blood leukocytes gave 4-methylumbelliferone which was easily measured fluorometrically. This reaction, presumably due to the action of α-l-iduronidase, has a maximum hydrolytic activity at pH 3.25. The apparent K_M value of α-l-iduronidase in leukocyte whole cell homogenates for this substrate was 179 μmol/l compared to 353, 41 and 116 μmol/l for other α-l-iduronidase substrates phenyl α-L-iduronide, iduronosyl anhydro [l-³H]mannitol 6-sulfate and iduronosyl anhydro [l-³H] mannitol respectively; the corresponding V_max values were 617, 394, 158 and 10 pmol/min/mg protein respectively. Incubation of the 4-methylumbelliferyl α-L-iduronide with whole cell homogenates prepared from cultured skin fibroblasts and leukocytes from a Hurler patient gave 4-methylumbelliferone at a rate more than 20 times less than found for control normal preparations. 4-Methylumbelliferyl α-L-iduronide is a sensitive, convenient and superior substrate to phenyl α-L-iduronide for the assay of α-L-iduronidase activity, but is not a suitable replacement for the radiolabelled substrate iduronosyl anhydro [1-su3H] mannitol 6-sulfate.