Abstract
In a preceding report, we described a series of experimental evidences indicating that an intravenous administration of calcitonin to rats produces in the kidney in situ an oxidation shift of the oxidation-reduction state of pyridine nucleotide and a rise in the energy charge of adenine nucleotides on the one hand, and a fall in Ca++ in the cells (most likely in cytosol) on the other. The former events are suggested to represent mitochondrial reactions that occur as a consequence of the latter hormone effect. Thus, a mechanism is expected to exist by which the regula- tion by cytosol Ca++ of mitochondrial energy metabolism comes into effect. We propose that the mechanism involves the succinoxidase system of the inner mitochondrial membrane, and the data in favor of these contentions are presented.
1. The activity in isolated renal tubules of succinate oxidation was very low, in sharp contrast to the very high activity in isolated mitochondria prepared from the tubules. Because a sufficient succinate penetration into the cell appears to take place, the succinoxidase system is suggested to be functionally in a controlled state in in situ mitochondria.
2. A mitochondrial preparation, which was devoid of calcium contarriination, and rendered to have a restricted activity of succinate oxidation by a previous treatment with AMP and DNP, exhibited a definite requirement for both ATP and Ca++ in its full activation. Succinate dehydrogenase was the step that limited the overall reaction.
3. In the presence of Ca++, ATP at 0.2mM caused full activation. With regard to Ca++ in the presence of ATP, the activation curve exhibited a wide dose-response relationship, in which 0.5-0.7μmoles Ca++ per mg mitochondrial protein caused a half of the maximal activation. Because the Ca++ effect was demonstrable at a suitable concentration (-10-6M), Ca++rather than ATP is the candidate for the possible physiological regulator of this reaction.
4. The Ca++ effect required the integrity of the inner membrane and was preven-ted not by La+++ or Pr+++, but by ruthenium red or high concentrations of KCl. Thus the low affinity sites for Ca++ binding on the inner membrane are suggested as the locus which detects any changes in cytosol Ca++, and leads to activation or deactivation of succinate dehydrogenase. This may regulate mitochondrial energy metabolism by switching off or on the mitochondrial oxidation of DPNH.
5. Similar reactions were demonstrated in mitochondria isolated either from kidney, liver, or from brain. Thus they appear to be a universal mechanism by which alterations in cytosol Ca++ is reflected by changes in mitochondrial metabolism.
