The mechanism of isocitrate oxidation by liver mitochondria has been studied by several investigators, in particular to determine whether substrate dehydrogenation occurs by way of the TPN-linked or the DPN-linked isocitrate dehydrogenases (for review see Plaut (1963)). Initial studies of DPN-linked isocitrate dehydrogenase from animal tissues demonstrated the activity in extracts of acetone powders of mitochondria from heart, pigeon breast muscle and kidney. The TPNand DPN-linked dehydrogenases were separated in the cases of heart, pigeon breast muscle (Plaut and Sung, 1954), and human placenta (Sung and Hsu, 1957). However, the reduction of DPN by isocitrate was very slow with preparations from liver mitochondria although relatively high activity for the corresponding TPN-linked enzyme was found. These experiments thus did not prove the occurrence of the DPN enzyme in liver. The finding that DPN-linked enzyme from heart is activated by ADP (Chen and Plaut, 1963) prompted reinvestigation of the levels of this enzyme in various tissues including liver (Goebell and Klingenberg, 1964; Stein et al., 1967). However, the measurement of the DPN enzyme at 340 rnp in the presence