Abstract
The effects of Ca2+ and Zn2+ on 5''-nucleotidase activity in rat liver plasma membranes were investigated. Addition of Ca2+ (1.0-100 .mu.M) caused a significant decrease of the enzyme activity. The decrease of 5''-nucleotidase activity induced by Ca2+ (25 .mu.M) was completely restored by the presence of a calcium-binding protein (CaBP 5.0 .mu.M). Of the various metals tested, 5''-nucleotidase activity was significantly decreased by the presence of Zn2+ and Cd2+, while Cu2+, Co2+, Ni2+ and Mn2+ did not inhibit the enzyme. In particular, the effect of Zn2+ was remarkable (decrease of about 60%). A significant decrease of the enzyme activity was also seen at 5.0 .mu.M Zn2+, but not at 1.0 .mu.M Zn2+. The inhibitory effect of 25 .mu.M Zn2+ on 5''-nucleotidase was not appreciably blocked by the presence of CaBP (2.5 and 5.0 .mu.M). The present data suggest that CaBP uniquely regulates the Ca2+ effect on 5''-nucleotidase activity in the hepatic plasma membranes. It is proposed that CaBP, which may regulate the Ca2+ effects on liver cell function, should be called regucalcin.