Towards enzyme replacement in GM2 gangliosidosis: organ disposition and induced central nervous system uptake of human beta-hexosaminidase in the cat.

The rapid plasma clearance of human placental beta-hexosaminidase in the cat is due mainly to a receptor-mediated mechanism recognizing terminal N-acetyl glucosaminyl and mannosyl residues on glycoproteins. Using a sensitive single radial immunodiffusion assay, specific for human beta-hexosaminidase, we have shown that, in normal cats, the liver is responsible for most of the clearance of human beta-hexosaminidase. Two hours after injection of approximately 6 X 10(6) U beta-hexosaminidase/kg bw, 70-90% of the enzyme was recovered in the liver. Spleen, kidney, lung, bone, bone, pancreas, adrenals, testes and ovaries, cardiac and skeletal muscle, lymph nodes, and placenta, however, also participated in the clearance, although specific uptake in most organs was < 5% of that of liver. Exogenous beta-hexosaminidase was also present in bile, indicating that the hepatocytes are involved in clearance. Injection of terminal mannose-rich S. cerevisiae mannans (50-150 mg/kg bw), prolonged the plasma half-life of the enzyme (t 1/2 up to 290 min). In these animals, beta-hexosaminidase uptake by liver was reduced to < 10% of controls but uptake by other organs was not proportionally or uniformly reduced, suggesting the existence of different uptake mechanisms in different tissues. Permeability of the blood-brain barrier was induced by exposing cats to 100% O2 at 2.5 ATA for 90 min. Injection of 6 X 10(6) U beta-hexosaminidase/kg bw during or immediately after exposure resulted in apparent uptake of enzyme by nervous tissue, qualitatively detectable by immunologic methods, but below the limits of sensitivity of the radial immunoassay(ie < 150 U/gr). When enzyme uptake by liver was inhibited by injection of ovomucoid or mannans, however, the hyperbaric oxygen-induced apparent uptake of beta-hexosaminidase by brain, cerebellum, and spinal cord was 200-500 U/gr of blood-free tissue, suggesting that the transport mechanism involved (presumably at the level of the nervous system vascular endothelium) is different from the carbohydrate-dependent hepatic uptake. The mechanism by which hyperbaric oxygenation induces permeability of the blood-brain barrier is not clear. The combination of this procedure (routinely used in human therapy) with specific inhibition of hepatic uptake, however, appears to be a promising approach for lysosomal enzyme targeting to the central nervous system.