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Human β-GIucuronidase. II. Fate of Infused Human Placental β-Glucuronidase in the Rat

Summary: Human placental β-glucuronidase could be identified in rat organs by beating organ extracts to 65° which selectively inactivated endogenous rat enzyme. Infused enzyme was rapidly cleared from rat plasma (t0.5 of 3.5 min). From 50–60% of the infused dose was accounted for in rat liver and spleen 24 hr after infusion. Removal of abdominal viscera, including the spleen, and interruption of the portal circulation before infusion slowed the plasma disappearance (t0.5 of 60 min) and allowed significant uptake by bone and other organs. Subcellular fractionation of liver 18 hr postinfusion localized the human enzyme in the mitochondrial-lysosomal fraction. The half-disappearance times of infused human enzyme were 2.6 days in rat liver and 5.8 days in rat spleen. Periodate treatment of human placental β-glucuronidase destroyed 90% of its binding to concanavalin A-Sepharose, reduced its heat stability, and abolished its rapid clearance from rat plasma after infusion.Speculation: Experiments with the rat model for evaluation of the uptake, distribution, intracellular localization, and metabolic fate of infused human β-glucuronidase can provide a number of answers about enzyme replacement therapy that could not be obtained easily by human experimentation. The experiments presented suggest that placental β-glucuronidase is cleared predominantly by organs with significant reticuloendothelial components. The clearance mechanism appears to recognize the carbohydrate structure of the enzyme, but may recognize carbohydrate components different from those responsible for enzyme uptake by fibroblasts. Delineation of the cell-specific receptors responsible for organ-specific localization of native and modified human enzymes such as β-glucuronidase may be important to developing a rational basis for enzyme replacement thereapy in lysosomal storage disease.