A difference in the enzyme contents of resorption lacunae and secondary lysosomes of osteoclasts.

The distributions of tartrate sensitive lysosomal acid phosphatase (LAP) and cathepsin L in osteoclasts and the effect of parathyroid hormone (PTH) stimulation on them were investigated by using the protein A-gold method on ultracryosections of rat trabecular bone. LAP was located in association with the ruffled border membrane, in the resorption lacuna, on the mineral phase surrounding the lacuna, and in some primary lysosomes. After PTH treatment, the extracellular and ruffled border membrane associated LAP apparently increased. Heavy gold labelling for cathepsin L was confined exclusively to secondary lysosomes. No labelling was seen in the extracellular resorption lacuna or at the ruffled border. Acceleration of bone resorption by PTH-treatment did not change detectably the distribution or intensity of labelling. This study shows that the enzyme contents of secondary lysosomes and resorption lacunae are different and suggests that LAP is directly involved in extracellular bone degradation whereas the role of cathepsin L is restricted to lysosomes.