Demonstration of acid hydrolase activity in primary cultures of bovine brain microvessel endothelium.

The existence of lysosomes and acid hydrolase activity was demonstrated in an in vitro blood-brain barrier (BBB) model comprising primary cultures of bovine brain microvessel endothelial cell (BMEC) monolayers. BMEC lysosomes were observed by the uptake of acridine orange and fluorophore-labeled acetylated low-density lipoprotein by fluorescence microscopy. Cytochemical localization of the acid hydrolase, sulfatase, and acid phosphatase (AcP) activities with light microscopy also revealed hydrolase-positive vacuoles or lysosomes that varied in number from cell to cell. BMEC monolayers were fractionated and biochemical assays of the sulfatase, AcP, and beta-galactosidase were performed. Significant activities of the acid hydrolases were found to be associated with lysosome and microsome fractions (69-77%). The majority of beta-galactosidase (approximately 48%) and total sulfatase (approximately 58%) activity was associated with the lysosome fraction of the BMECs. In contrast, approximately 52% of AcP activity was associated with the microsome fraction of the cells. The results of this study are consistent with the demonstration in vivo of acid hydrolases as potential factors in the endocytic pathway for transport of proteins through the BBB and as contributors to the BBB's enzymatic barrier function.