873 α-L-IDURONIDASE IN LEUKOCTYES AND PLASMA: ISOZYMES OF INTEREST IN BONE MARROW TRANSPLANTATION FOR HURLER SYNDROME

Abstract

Three mechanisms are proposed to account for metabolic correction in Hurler syndrome after bone marrow transplantation. Of these, endogenous enzyme replacement is predicated upon the availability and catalytic activity of iduronidase (Idu) in marrow-derived cells and plasma. To characterize enzyme available after transplantation, we studied Idu from normal individuals utilizing the 4-methylumbelliferyl-α-L-iduronide substrate. Idu specific activities were determined for leukocyte subpopulations (mean, n=18): granulocytes 54.7 nmoles/mg-hr, lymphocytes 61.2, and monocytes 47.5. Of further interest mere physicochemical characteristics of Idu. In vitro, leukocyte and plasma Idu activities, with optima at pH 3.0-3.4, had less than 1% of maximal activity at pH 7.3. We also quantitated Idu isozymes after separation by DEAE chromatography since enzyme affinity for DEAE has been related to cellular binding and endocytosis. In leukocytes, high-affinity Idu B was the predominant form comprising approximately 90% of total activity. In plasma, Idu B was likewise the major component contributing 64-76% total activity. We conclude that Idu is available in transplantable leukocytes; however, lack of catalytic activity at neutral pH suggests that glycosaminoglycan (GAG) degradation in plasma is insignificant in vivo. Since only lysosomal GAG catabolism is likely to be physiologically important, cellular uptake of enzyme may be crucial. Leukocytes and plasma contain high-affinity Idu B available for endocytosis by deficient cells.