Cytochemical study on uptake of exogenous peroxidase by Vx2 carcinoma cells transplanted into the rabbit.

When horse-radish peroxidase (HPO) was administered iv to Vx2 carcinoma-bearing rabbits, an HPO reaction product was detected in the lumina of blood vessels and the extracellular spaces between tumor cells in the first few minutes after administration. HPO was also seen in vesicles in tumor cells. Fifteen minutes to 1 hour after administration, the HPO reaction product was found mainly in the large membrane-bound vacuoles. Within 6--12 hours, the HPO activity gradually diminished in large membrane-bound vacuoles (lysosomes). In conclusion, exogenous HPO was rapidly incorporated into Vx2 carcinoma cells by pinocytosis, and then pinocytotic vesicles were fused with lysosomes.