Effects of zinc and divalent cation chelators on ATP hydrolysis and Ca deposition by rachitic rat matrix vesicles

Abstract

Elsewhere it has been shown that zinc is highly concentrated in the hypertrophic zone of epiphyseal cartilage. It has also been shown that zinc deficiency can result in abnormal bone development, suggesting a direct or indirect role for zinc in calcification. Because matrix vesicles have been implicated in the initiation of calcification, we tested the effect of zinc and its chelators, such as EGTA and phenanthroline, on ATP-dependent Ca uptake by rat matrix vesicles. EGTA pretreatment of matrix vesicles inhibited ATP-dependent Ca uptake by 50%. To see if zinc depletion by EGTA pretreatment is responsible for decreased levels of ATP-dependent Ca uptake, ZnCl2 concentrations, ranging from 5 to 100 mumol/L, were tested for their ability to restore Ca deposition. Zinc exerted a striking enhancing effect on ATP-dependent Ca uptake of both untreated and EGTA-pretreated matrix vesicles in a dose-dependent manner. A 50% activation occurs at about 16 mumol/L Zn2+. At 63 mumol/L Zn2+, there was a fourfold increase in Ca-depositing activity. Addition of an excess amount of phenanthroline relative to Zn2+ concentration to the reaction mixture failed to abolish activation of Ca uptake by Zn2+, indicating that the putative chelator-Zn2+ complex formation did not prevent activation. The observed partial inhibition of ATPase and the activation of ATP-dependent Ca uptake of Zn2+ suggest that, in addition to ATPase, some other Ca and/or Pi uptake activators responsive to Zn2+ treatment are present in mammalian matrix vesicles.