A FACILITATIVE ROLE FOR CARBONIC-ANHYDRASE ACTIVITY IN MATRIX VESICLEMINERALIZATION

Carbonic anhydrase (CA) which catalyzes the reversible hydrolysis of c arbon dioxide is known to be important in osteoclastic bone resorption , however, suggested roles in calcium phosphate mineral formation have not been previously demonstrated. Biochemical evidence is provided fo r the presence of CA in growth plate matrix vesicles (MV) and the leve l of activity determined by enzyme assay. Inhibition of CA activity wi th the specific inhibitor acetazolamide resulted in reduced rates of M V mineralization. Other inhibitor studies showed that MV mineralizatio n was also impaired by 4,4-diisothiocyanatostilbene-2,2-disulfonic aci d (DIDS), a blocker of membrane bicarbonate channels. No evidence was found for the presence of any proton pumps or channels. When acetazola mide and DIDS were combined, their inhibitory effects on MV mineraliza tion were additive. These findings suggest that MV posess a pH regulat ion system composed of carbonic anhydrase and a putative bicarbonate c hannel. This system may function in the MV by providing intraluminal b uffering capacity. The control of intravesicular pH is important for t he stabilization of the acid-labile nucleational core