Effect of ionic strength on the regulatory properties of 2-oxoglutarate dehydrogenase complex

Abstract

The effects of changing ionic strength on the activity of the 2-oxoglutarate dehydrogenase complex from pig kidney cortex were explored. This enzyme complex is found to be influenced in many ways by the ionic strength of the reaction medium. The enzyme shows an optimum activity at 0.1 M ionic strength. Increase in ionic strength from 0.1 M to 0.2 M resulted in a decrease of S 0.5 for 2-oxoglutarate, and in an increase of S 0.5 for NAD. Changes in ionic strength over the range of 0.05–0.2 M have little, if any, effect on S 0.5 for CoA. The Hill coeffient for 2-oxoglutarate and NAD at 0.2 M ionic strength was 1.0, whereas at 0.05 M ionic strength it was 0.85 and 1.2 for 2-oxoglutarate and NAD, respectively At 0.05 M ionic strength the pH optimum of the enzyme ranges between 7.4–7.6, but at 0.15 M ionic strength the pH optimum shifts to 7.8. The magnitude of inhibition of enzyme activity by ATP is not influenced by changes in ionic strength in the absence of calcium. However, in the presence of Ca 2+, increases in ionic strength lower the inhibitory effects of ATP. The S 0.5 i for ATP in both presence and absence of Ca 2+ was not affected by changes in ionic strength in the range of 0.1–0.2 M. In contrast, the a 0.5 for ADP in the absence of Ca 2+ decreases as ionic strength increases. In the presence of calcium and 0.2 M ionic strength ADP has no effect on 2-oxoglutarate dehydrogenase complex activity.