Studies in this laboratory have identified three mammalian intramitochondrial dehydrogenases that, in mitochondrial extracts or after purification, are activated by CaZ+ with half-maximum effects (ko. 5 value) of Ca2+ being observed at close to lp~. These enzymes are pyruvate dehydrogenase, isocitrate dehydrogenase (NAD+)(EC 1.1. 1.4 1) and the 2-oxoglutarate dehydrogenase complex. The effects of Ca2+ on pyruvate dehydrogenase activity are brought about by changes in the proportion of the complex in its active non-phosphorylated form. Pyruvate dehydrogenase phosphate phosphatase is activated by Ca2+(Denton et al., 1972, 1975), whereas pyruvate dehydrogenase kinase, at least from heart muscle, may be inhibited by Ca2+(Cooper et al., 1974). The effects of CaZ+ on isocitrate dehydrogenase (NAD+) and oxoglutarate dehydrogenase are more direct. In both cases, the presence of Ca2+ results in a greatly decreased K,,, for their substrates (threo-D,-isocitrate and oxoglutarate respectively) with little or no change in V,,,=.(Denton et al., 1978; McCormack & Denton, 1979). the enzymes appear to have very similar properties in extracts of mitochondria of all mammalian tissues so far studied including those from pig heart and the following rat tissues: white and brown adipose tissue, heart and skeletal muscle, liver, kidney and brain. Moreover, in all cases the effects of Cs2+ are closely mimicked by Sr2+, but the k,, values are some ten times higher.

All three of the Ca2+-sensitive dehydrogenases are located exclusively in cells within the inner mitochondrial membrane and are generally considered to be important sites of regulation of intramitochondrial oxidative metabolism. In particular, the activity of all three dehydrogenases may be increased by elevations in the intramitochondrial ADP/ATP and NAD+/NADH concentration ratios (see McCormack & Denton, 1979). This can be viewed as the ‘intrinsic’means of regulation whereby the rate of NADH production in mitochondria is always matched to the requirements of the respiratory chain and thus ATP synthesis. We proposed that the activation of these same key dehydrogenases by Ca’+ ions may be an important means whereby ‘extrinsic’control of intramitochondrial oxidative metabolism by such factors as hormones and neurotransmitters could be superimposed on this ‘intrinsic’control. However, we realized the need to be cautious when ascribing regulatory importance to enzyme properties that have only been observed with separated enzymes. We have therefore sought more direct evidence by using intact mitochondria.