COMPARISON OF THE EFFECTS OF CA2-NUCLEOTIDES AND PH ON THE KINETIC-PROPERTIES OF MITOCHONDRIAL NAD(+)-ISOCITRATE DEHYDROGENASE AND OXOGLUTARATE DEHYDROGENASE FROM THE YEAST SACCHAROMYCES-CEREVISIAE AND RAT-HEART(, ADENINE)

The regulatory properties of NAD(+)-isocitrate dehydrogenase and oxogl utarate dehydrogenase in extracts of yeast and rat heart mitochondria were studied under identical conditions. Yeast NAD(+)-isocitrate dehyd rogenase exhibits a low K-0.5 for isocitrate and is activated by AMP a nd ADP, but is insensitive to ATP and Ca2+. In contrast, the rat heart NAD(+)-isocitrate dehydrogenase was insensitive to AMP, but was activ ated by ADP and by Ca2+ in the presence of ADP or ATP. Both yeast and rat heart oxoglutarate dehydrogenase were stimulated by ADP, but only the heart enzyme was activated by Ca2+. All the enzymes studied were a ctivated by decreases in pH, but to differing extents. The effects of Ca2+, adenine nucleotides and pH were through K-0.5 for isocitrate of 2-oxoglutarate. These observations are discussed with reference to the deduced amino acid sequences of the constituent subunits of the enzym es, where they are available.