1. Rapid choline oxidation and the onset of Pi-induced swelling by liver mitochondria, incubated in a sucrose medium at or above pH7.0, required the addition of both Pi and an uncoupling agent. Below pH7.0, Pi alone was required for rapid choline oxidation and swelling. 2. Choline oxidation was inhibited by each of several reagents that also inhibited Pi-induced swelling under similar conditions of incubation, including EGTA, mersalyl, Mg2+, the Ca2+-ionophore A23187, rotenone and nupercaine. None of these reagents had any significant effect on the rate of choline oxidation by sonicated mitochondria. There was therefore a close correlation between the conditions required for rapid choline oxidation and for Pi-induced swelling to occur, suggesting that in the absence of mitochondrial swelling the rate of choline oxidation is regulated by the rate of choline transport across the mitochondrial membrane. 3. Respiratory-chain inhibitors, uncoupling agents (at pH6.5) and ionophore A23187 caused a loss of endogenous Ca2+ from mitochondria, whereas nupercaine and Mg2+ had no significant effect on the Ca2+ content. Inhibition of choline oxidation and mitochondrial swelling by ionophore A23187 was reversed by adding Ca2+, but not by Mg2+. It is concluded that added Pi promotes the Ca2+-dependent activation of mitochondrial membrane phospholipase activity in respiring mitochondria, causing an increase in the permeability of the mitochondrial inner membrane to choline and therefore enabling rapid choline oxidation to occur. Nupercaine and Mg2+ appear to block choline oxidation and swelling by inhibiting phospholipase activity. 4. Choline was oxidized slowly by tightly coupled mitochondria largely depleted of their endogenous adenine nucleotides, suggesting that these compounds are not directly concerned in the regulation of choline oxidation. 5. The results are discussed in relation to the possible mechanism of choline transport across the mitochondrial membrane in vivo and the influence of this process on the pathways of choline metabolism in the liver.

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