Mucopolysaccharidosis type I (MPS I; McKusick 25280; Hurler syndrome, Hurler-Scheie syndrome and Scheie syndrome) is caused by a deficiency in the lysosomal hydrolase, alpha-L-iduronidase (EC 3.2.1.76). MPS I patients present within a clinical spectrum bounded by the extremes of Hurler and Scheie syndromes. The alpha-L-iduronidase missense mutations R89Q and R89W were investigated and altered an important arginine residue proposed to be a nucleophile activator in the catalytic mechanism of alpha-L-iduronidase. The R89Q alpha-L-iduronidase mutation was shown to result in a reduced level of alpha-L-iduronidase protein (< or =10% of normal control) compared to a normal control level of alpha-L-iduronidase protein that was detected for the R89W alpha-L-iduronidase mutation. When taking into account alpha-L-iduronidase specific activity, the R89W mutation had a greater effect on alpha-L-iduronidase activity than the R89Q mutation. However, overall the R89W mutation produced more residual alpha-L-iduronidase activity than the R89Q mutation. This was consistent with MPS I patients, with an R89W allele, having a less severe clinical presentation compared to MPS I patients with either a double or single allelic R89Q mutation. The effects of the R89Q and R89W mutations on enzyme activity supported the proposed role of R89 as a nucleophile activator in the catalytic mechanism of alpha-L-iduronidase.