Modification of the blood-brain barrier: increased concentration and fate of enzymes entering the brain.

The blood-brain barrier of rats was opened reversibly by infusing a hyperosmotic solution of arabinose into the external carotid artery. Permeability was increased maximally in the first 15 min and remained slightly elevated at 1 hr. Osmotic barrier opening significantly increased brain uptake of intravenously injected alpha-mannosidase (alpha-D-mannoside mannohydrolase, EC 3.2.1.2.4) (derived from human placenta) and horseradish peroxidase (donor:hydrogen-peroxide oxidoreductase, EC 1.11.1.7). By injection of 4 X 10(5) units of alpha-mannosidase into an animal, brain activity rose to about twice the normal control activity of the enzyme. After 30 min, activity of administered enzyme in the extracellular space of the brain was calculated to be 30% of the serum concentration. Biochemical and histological studies with horseradish peroxidase showed that exogenously administered enzyme entered the brain extracellular space immediately after barrier opening and was incorporated within neuronal lysosomal packets during the next 24 hr. Measurable peroxidase activity was found in brain as much as 72 hr after osmotic treatment. The results demonstrate that the blood-brain barrier can be reversibly opened to enzymes, that a glycoprotein enzyme is incorporated into neuronal lysosomes, and that the brain may now be considered a potential target for enzyme replacement therapy in heritable metabolic disorders.