Mitochondria from normal rat liver and heart, and also Ehrlich tumor cells, respiring on succinate as energy source in the presence of rotenone (to prevent net electron flow to oxygen from the endogenous pyridine nucleotides), rapidly take up Ca2+ and retain it so long as the pyridine nucleotides are kept in the reduced state. When acetoacetate is added to brin the yridine nucleotides into a more oxidized state, Ca2+ is released to the medium. A subsequent addition of a reductant of the pyridine nucleotides such as P-hydroxybutyrate, glutamate, or isocitrate causes reuptake of the released Ca2+. Successive cycles of Ca2+ release and uptake can be induced by shifting the redox state of the pyridine nucleotides to more oxidized and more reduced states, respectively. Similar observations were made when succinate oxidation was replaced as energy source by ascorbate oxidation or by the hydrolysis of ATP. These and other observations form the basis of a hypothesis for feedback regulation of Ca2+-dependent substrate-or energy-mobilizing enzymatic reactions by the uptake or release of mitochondrial Ca2+, mediated by the cytosolic phosphate potential and the ATP-dependent reduction of mitochondrial pyridine nucleotides by reversal of electron transport.Mitochondria of animal tissues are very active in respirationcoupled inward transport of Ca2+ (reviewed in refs. 1-4). However, relatively little is known regarding the biochemical factors controlling retention and release of Ca2+ from respiring mitochondria into the cytosol, presumably important in regulation of many Ca2+-dependent cell activities. Increasing evidence indicates that release of Ca2+ takes place by a membrane transport pathway different from that followed during energy-dependent Ca2+ uptake (5-9). In addition, several substances have been found to promote release of Ca2+ from respiring mitochondria in vitro, including phosphate (10), various prostaglandins (11), phosphoenolpyruvate (12), and Na+ (13) EXPERIMENTALMitochondria were prepared from rat liver homogenates in 0.25 M sucrose by a standard method and from rat heart and Ehrlich ascites cells by a method to be described elsewhere (A. Vercesi, B. Reynafarje, and A. L. Lehninger, unpublished data). The mitochondrial fractions were washed two or three times with cold 0.25 M sucrose and stored as stock suspensions in 0.25 M sucrose at 0°, in a concentration of 50 mg of mitochondrial protein per ml. The mitochondria were generally used within 3-4 hr of preparation. Oxygen consumption was monitored with a Teflon-coated oxygen electrode and Ca2+ movements with a calibrated Ca2+-selective electrode coupled to a dualchannel strip-chart recorder. Ultraviolet absorption changes accompanying oxidation and reduction of mitochondrial pyridine nucleotides were monitored in a double-beam spectrophotometer with the wavelength pair 340 and 370 nm. The mitochondrial incubations were carried out at 25°. RESULTSRetention, Release, and Reuptake of Ca2+ by Mitochondria Respiring on Succinate. Although all three ener...

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1. The metabolic oxidation of debrisoquine has been studied in a group of 123 Nigerian volunteers. 2. All subjects excreted unchanged drug together with five oxidation products, namely, 4-, 5-, 6-, 7- and 8-hydroxy-debrisoquine. 3. The 4-hydroxylation reaction exhibits polymorphism; ten subjects were defective in their ability to effect this reaction. 4. The incidence (q) of the allele governing impaired 4-hydroxylation (DL) among Nigerians was calculated as being 0.28 (95% confidence limit of 0.20-0.37). 5. An association was demonstrated between the ability to effect 4-hydroxylation and 6- and 7-hydroxylation of debrisoquine, suggesting that the alleles controlling alicyclic oxidation also influence aromatic hydroxylation.

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