Mucopolysaccharidosis type I subtypes. Presence of immunologically cross-reactive material and in vitro enhancement of the residual α-L-iduronidase activities

Abstract

The enzymatic and immunologic properties of the defective residual α-L-iduronidase activities were investigated in fibroblast extracts from the three subtypes of mucopolysaccharidosis type I, Hurler (MPS IH), Scheie (MPS IS), and Hurler-Scheie (MPS IH-S) diseases. Using 4-methylumbelliferyl-α-L-iduronide (4MU-α-Id), the activities in fibroblast extracts from all three subtypes were less than 0.1% of normal. Rocket immunoelectrophoresis with monospecific rabbit anti-human α-L-iduronidase polyclonal antibodies, as well as immunoblots using a monoclonal antibody, revealed the presence of cross-reactive immunologic material (CRIM) in extracts prepared from each subtype. When the samples were equalized for β-hexosaminidase A activity, 38-105% of normal enzyme protein was detected. The sequential addition of cystamine, MgCl2 and pyridoxal phosphate increased the residual 4MU-α-Id activities in subtype extracts up to about 35% of normal mean fibroblast activity. Cystamine, MgCl2 or pyridoxal phosphate alone enhanced the residual activities two- to fourfold, whereas the sequential addition of all three compounds was required for maximal effect. Of the six B6 vitamers evaluated, only the negatively charged forms, pyridoxamine (PLN), pyridoxamine phosphate (PNP), and pyridoxal phosphate (PLP), stimulated the residual activities. The addition of dermatan sulfate or heparan sulfate to the subtype extracts, followed by treatment with the effector compounds, similarly inhibited both the normal and enhanced MPS I activities. Heat inactivation experiments confirmed the fact that the mutant iduronidase activity was reconstituted and that the observed increase in enzymatic activity was not an artifact of the fluorogenic assay. These results suggest that the presence of certain thiol reducing agents, divalent cations and negatively charged B6 vitamers can alter the conformation of the mutant α-L-iduronidase in vitro such that the hydrolysis of 4MU-α-Id is enhanced into the heterozygote range.