Related Experiment Videos
NAD-specific isocitrate dehydrogenase from bovine heart. Interaction with Ca2+ chelators
Insights
Calcium ions (Ca2+) play a complex role in regulating NAD-specific isocitrate dehydrogenase activity. While not always essential, Ca2+ enhances activation by ADP and citrate, influencing enzyme regulation.
Area of Science:
- Biochemistry
- Enzyme kinetics
- Molecular biology
Background:
- NAD-specific isocitrate dehydrogenase (NAD-IDH) is a key enzyme in cellular metabolism.
- The role of divalent cations, particularly Ca2+, in modulating NAD-IDH activity is not fully understood.
- Adenosine diphosphate (ADP) is known to be a positive effector of NAD-IDH.
Purpose of the Study:
- To investigate the precise role of free Ca2+ in the activity of NAD-specific isocitrate dehydrogenase.
- To elucidate the mechanism by which Ca2+ chelators affect NAD-IDH activity in the presence and absence of ADP.
- To determine the influence of Ca2+ on ADP-mediated activation and reversal of inhibition.
Main Methods:
- Enzyme activity assays were performed using NAD-specific isocitrate dehydrogenase.
- Various Ca2+ chelators, including EDTA, EGTA, and carboxymethyltartronate, were employed to manipulate free Ca2+ concentrations.
- The effects of chelators were studied in the presence and absence of ADP, magnesium citrate, NADPH, and NADH.
Main Results:
- Nitrogen-containing polycarboxylate Ca2+ chelators inhibited NAD-IDH activity, an effect not solely due to Ca2+ removal.
- Carboxymethyltartronate showed differential inhibition depending on ADP presence, with activation decreasing as free Ca2+ was removed.
- Free Ca2+ enhanced ADP activation, potentiated citrate activation by ADP, and enhanced the reversal of NADPH inhibition by ADP, but was not absolutely required for these processes.
Conclusions:
- Ca2+ plays a modulatory role in NAD-IDH regulation, enhancing activation by ADP and citrate.
- The mechanism of inhibition by certain chelators is complex and not solely dependent on Ca2+ chelation.
- Free Ca2+ is important, but not absolutely essential, for several key regulatory events involving NAD-IDH and its effectors.
Abstract:
The activity of NAD-specific isocitrate dehydrogenase was inhibited by EDTA, EGTA and other nitrogen-containing polycarboxylate Ca2+ chelators in the absence and in the presence of ADP by a mechanism that could not be attributed solely to the removal of free Ca2+. Carboxymethyltartronate (2-oxapropane-1,1,3-tricarboxylate), an oxygen ether polycarboxylate chelator, did not inhibit when ADP was absent. The activation by ADP, a positive effector of the enzyme, decreased with increasing concentration of carboxymethyltartronate, paralleling the removal of free Ca2+ by this chelator. The following were found when free Ca2+ was decreased to negligible concentrations (5-50 nM) with carboxymethyltartronate. (1) Free Ca2+ enhanced, but was not absolutely required for, activation by ADP. (2) Activation of enzyme activity by magnesium citrate neither required nor was increased by Ca2+ when ADP was absent. However, the potentiation of citrate activation by ADP was facilitated by free Ca2+. (3) The reversal of NADPH inhibition of enzyme activity by ADP did not absolutely require Ca2+, but it was enhanced by free Ca2+. (4) The inhibition of enzyme activity by NADH was not reversed by ADP either with or without Ca2+.