Citrate activation of NAD-specific isocitrate dehydrogenase from bovine heart

NAD-dependent isocitrate dehydrogenase from bovine heart was activated by lower concentrations of citrate in the presence of ADP (apparent S0.5 citrate = 0.033 mM) than in its absence (apparent S0.5 citrate = 2.64 mM) at low magnesium DL-isocitrate (0.18 mM) and free DL-isocitrate3- (0.45 mM) concentrations. Under these conditions, citrate (0.3 mM) lowered the apparent S0.5 for ADP from 0.24 to 0.05 mM. The binding of NAD+ was unaffected by citrate; however, saturating concentrations of citrate lowered the apparent S0.5 for magnesium isocitrate from 0.63 to 0.19 mM in the presence of ADP. Citrate does not appear to bind to the regulatory or catalytic magnesium isocitrate-binding sites, since the Hill coefficient for magnesium isocitrate was not lowered by citrate nor did inhibition occur at high citrate concentrations. The data suggest that magnesium citrate was the activating species. Citrate activation occurred from pH 6.5 to 8.0. As with magnesium citrate, calcium citrate lowered the S0.5 for magnesium isocitrate (apparent S0.5 for calcium citrate = 0.26 mM), and it did not appear to bind to the regulatory or catalytic magnesium isocitrate-binding sites. However, in contrast to the substantial facilitation of magnesium citrate activation by ADP, no activation by calcium citrate occurred in the presence of ADP. Differences in the mechanism of activation of the enzyme by magnesium citrate and calcium citrate were also indicated by the finding that, whereas a number of tricarboxylates could replace citrate as an activator with Mg2+ as the sole divalent cation activator, only citrate was effective for the further enhancement of velocity by added calcium.