Iduronidase activity

See section 4.1.3.4, subheading Calculation. As iduronidase activity is rather low in leukocytes, results should be expressed as nmol/h-mg protein. 

For the measurement of a-iduronidase activity, a novel substrate was developed that was detected by mass spectrometry. It was possible to combine this method with similar assays for Niemann-Pick type A/ , Krabbe, Gaucher, Pompe, and Fabry disease. However, separate incubation is necessary, and some additional work-up procedures are usually required to purify the sample before mass spectrophotomet-ric analysis [68]. 

MPS I dogs were used to evaluate the biodistribution of rh-iduronidase, pharmacodynamic reduction of tissue GAGs and in vivo efficacy. Pharmacokinetics and the effects of various doses and regimens of rh-iduronidase were also evaluated in this model. Heterozygote dogs from the MPS I colony, referred to as normal carriers, were used to derive normal tissue levels of a-l-iduronidase activity and GAG storage. 

Several endeavors are being directed towards identifying biomarkers that can serve as a surrogate indicator of disease severity, in terms of either overall disease burden or involvement of a particular organ/system in patients with an LSD. In mucopolysaccharidosis type I, the analysis of the levels of oligosaccharides derived from GAGs in cultured fibroblasts (as measured by electrospray ionization tandem mass spectrometry) combined with the residual a-L-iduronidase activity have been shown to distinguish patients with and without CNS involvement (Fuller et al.,... 

H3. Hall, C. W., and Neufeld, E. F., a-i.-Iduronidase activity in cultured skin fibroblasts and amniotic fluid cells. Arch. Biochem. Biophys. 158, 817-821... 

A fluorimetric assay, which employs 4-methylumbelliferyl a-L-idopyrano-syluronic acid for the estimation of oc-L-iduronidase activity has been reported for the detection of Hurler and Scheie syndromes. 

A simple assay for a-L-iduronidase activity in leucocytes has been reported for the detection of Hurler syndrome and its carriers. The method avoids time-consuming stages and tedious extraction procedures, but is based on phenyl cK-L-idopyranosyluronic acid as substrate. 

Skin fibroblasts obtained from patients with Maroteaux-Lamy syndrome exhibited normal levels of a-L-iduronidase activity. ... 

Canine MPS I was discovered in a Plott hound that presented with corneal clouding [10]. Studies by Shull and Neufeld showed that the dogs were deficient in a-l-iduronidase [11], Being null, MPS I dogs are genetically similar to the most severe form of MPS I in humans, but clinically they more closely resemble moderately affected patients. These animals provide a valuable bio-chemical/clinical model for MPS I disease. Since they have no confounding residual enzyme activity, they accumulate GAGs in relevant tissues, and their clinical phenotype closely resembles the human disease. 

Recently, specific treatment for mucopolysaccharidosis, type I, using replacement therapy of the missing enzyme activity, a-L-iduronidase, with recombinant human enzyme has been successful in a one-year clinical trial involving ten patients [8]. The results are promising in that there was a reduction in size of some of the somatic tissue, i.e., liver and spleen. A reduction in the stiffness of joints was observed. Also, there was a decrease in excretion of urinary glycosaminoglycans by 63% [8]. 

The screening of heterozygotes for Hurler s syndrome is now possible, since the specific activity of the leucocyte-derived a-L-iduronidase in carriers is reduced by roughly 50%. The molecular size of dermatan sulphate excreted in the urine of patients with Hurler s syndrome can be monitored by gel-filtration chromatography on Sephadex G-75. The in vitro correction of the defect in Hurler s fibroblasts with bovine testicular hyaluronidase has produced encouraging results, and such treatment is also effective with Hunter s cells. A sulphated tetrasaccharide, which was obtained on digestion of chondroitin 6-sulphate with testicular hyaluronidase, has been used as the substrate in the determination of 2-acetamido-2-deoxy-D-galactose 6-sulphate sulphatase, an enzyme that is absent in patients with Morquio s disease. ... 

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