As a first step toward the use of cats with Gm2 gangliosidosis as models for enzyme replacement therapy, we studied plasma clearance, organ disposition, and subcellular localization of human ß-hexosaminidase in normal cats. Plasma half-life of placental ß-hexosaminidase at low doses was 3–4 min; both Hex A and Hex B were cleared at approximately the same rate. The half-life of human plasma ß-Miexosaminidase in contrast, was >60 min. Clearance curves at higher doses approached zero order kinetics, suggesting the existence of a saturable clearance mechanism. Injection of periodate-treated placental ß-Miexosaminidase resulted in a plasma half-life of ∼50 min, strongly suggesting that rapid clearance of both Hex A and Hex B was mediated by carbohydrate-specific mechanisms. Circulatory bypass of liver resulted in plasma half-life of the enzyme of ∼60 min, indicating that the liver was the main clearing organ. As both main feline ß-Miexosaminidase isozymes did not crossreact with antihuman ß-Miexosaminidase immune sera, independent evidence of preferential hepatic uptake was obtained by immunofixation clectrophoresis; immunoelectrophoresis, and immunotitration. The human enzyme detected in liver accounted for ∼80% of the injected dose; small amounts of exogenous enzyme were detected in spleen and kidney. Subcellular fractionation of liver showed that human Hex A and Hex B had entered the lysosomal-vacuolar apparatus of hepatic cells.
Speculation Therapeutic applications of lysosomal enzyme replacement in patients with storage diseases are fraught with difficulties. Normal cats infused with human ß-Miexosaminidase can be used to develop enzyme replacement methodologies to be tested in cats with genetic Gm2 gangliosidosis. This unique animal model makes it possible to explore in vivo rational approaches to therapeutic intervention in human patients.