Characteristics of leukocyte and plasma methylumbelliferyl-alpha-L-iduronide iduronidase

Some characteristics of the human leukocyte and plasma alpha-L-iduronidase are described. The enzyme from both sources is sufficiently stable and linear in time to allow accurate determinations. The leukocyte and plasma enzyme have a low acid pH optimum at 3.5 and 4, respectively, which is in agreement with the lysosomal origin of the enzyme in the cell. Both enzymes are inhibited by phenyl-alpha-L-iduronide, heparin, and heparitin sulfate although other mucopolysaccharides also inhibit the leukocyte enzyme. When kept frozen at -20 degrees C, the leukocyte as well as the enzyme in acidified plasma are very stable. We studied the plasma enzyme in more detail. If the plasma is acidified, iduronidase is very stable between 0 and 37 degrees C. CuCl2 and Na2SO4 were very potent inhibitors at concentrations of 10 and 100 mM, respectively. The determination of iduronidase in leukocyte homogenates of patients suspected of Hurler disease together with plasma activities is useful for confirming or corroborating the diagnosis of genetic iduronidase deficiency. Further investigation is needed to determine if the plasma enzyme test would be useful in the biochemical diagnosis of Scheie disease and the Hurler-Scheie compound, two diseases which are also caused by a deficiency of alpha-L-iduronidase.