Electron microscopic localization of hydrolytic enzymes in osteoclasts

Abstract

Synopsis.Acid glycerophosphatase activity (pH optimum, 5.o) has been found within osteoclasts by numerous workers but relatively few studies have been concerned with the neutral hydrolytic enzymes that have pH optima around 7.2. Evidence is presented in this paper to show that neutral enzyme activity can be demonstrated with p-nitrophenyl phosphate, ATP and p-chloranilidophosphonate as substrates. Activity against /3-glycerophosphate, inorganic trimetaphosphate or p-nitrocatachol sulphate as substrates was found to be limited to an acid pH range.Electron microscopic evidence indicated that many of the hydrolytic enzymes were present within single membrane-bound bodies, vacuoles, Golgi elements, and the agranular endoplasmic reticulum of the osteoclast. This reticulum was dilated to form large lysosomes. Such activity was distingly different from the function of the Golgi membrane in its formation of primary lysosomes.The relationships between lysosomes, cytoplasmic vacuoles, and extracellular release of enzyme through the specialized 'ruffled' border are shown and discussed.

Cytoplasmic pH regulation and chloride/bicarbonate exchange in avian osteoclasts.

“…It remains to be seen whether deficiency of carbonic anhydrase type II limits osteoclast function under some circumstances; our results thus indicate that it does not affect the acidification of the osteoclast's cytoplasm by Cl-/HCO-exchange. The functional polarization of the osteoclast by this acidic bone resorption compartment at the cell-bone interface, and our observation of active Cl-/HCO-exchange by these same cells, is reminiscent of the mechanism for acid secretion observed in renal epithelial cells (7). We therefore propose that osteoclasts neutralize the alkaline equivalents produced during acidification of the bone resorbing compartment by Cl-/ HCO-exchange.…”

Recent advances in the ultrastructural assessment of osteoclastic resorptive functions

“…Osteoclasts do, in fact, contain lysosomes which harbor various hydrolytic enzymes, functioning optimally at an acidic pH, responsible for intracellular digestion of incorporated materials (Baronet al, 1986(Baronet al, ,1988Doty and Schofield, 1972;Goto et al, 1993;Lucht, 1971;Ueno-Matsuda, 1992, 1993;Sasaki et al, 1989). H+-ATPase localized in lysosomal membranes may function to provide an acidic internal pH for these membrane-bound structures (Sasaki et al, 1994).…”

“…Thus, after removal of bone crystals, osteoclasts seem to degrade bone collagen at the extracellular site by means of released collagenolytic enzymes. Previous enzyme cytochemical studies have localized various hydrolytic enzymes within osteoclasts and along subosteoclastic resorptive bone surfaces (Baron et al, 1986;Doty and Schofield, 1972;Lucht, 1971;Shimizu and Sasaki, 1991). Recently, cysteine-proteinases such as cathepsin B and L, which exhibit collagenolytic activity under acidic conditions, have been considered to be the principal osteoclastic agents of bone collagen degradation (Delaisse et al, 1984(Delaisse et al, , 1987Everts et al, 1988Everts et al, , 1992Goto et al, 1993;Rifkin et al, 1991;UenoMatsuda, 1992, 1993).…”

Spatial Distribution of Osteocyte Lacunae in Equine Radii and Third Metacarpals: Considerations for Cellular Communication, Microdamage Detection and Metabolism

“…Nutrient/metabolite availability and transport between cells and/or lacunocanalicular spaces may impose important constraints on Ot.Lc.N/B.Ar [Doty and Schofi eld, 1972;Kelly and Bronk, 1990;Vashishth et al, 2000;Mishra and Knothe Tate, 2003]. In elderly human femoral neck cortices, Power et al [2002] showed that Ot.Lc.N/B.Ar decreased with increasing distance from Haversian canals.…”