Studies on the Mechanism of Ascorbate-induced Swelling and LY sis of Isolated Liver Mitochondria*

Earlier work (2) demonstrated that low, but not high, concentrations of ascorbate produce a swelling or lytic change in dilute suspensions of isolated liver mit.ochondria. This structural change shows both similarities to and differences from the phosphate-induced, electron transport-dependent swelling which has been rather extensively investigated (3-6). Both phenomena require O2 and can be prevented by ethylenediaminetetraacetate, cyanide, 2-hydroxy-3-(2.methyloctyl)-1,4-naphthoquinone, and ant.imycin A (2, 3). Inhibition by the last three substances would seem to indicate participation of the electron transport chain, particularly cytochromes, in the reactions. However, the concentration of cyanide or antimycin A required to block ascorbate-induced swelling and lysis is higher than that for inhibition of electron transport. Therefore, it cannot be stated conclusively that they block ascorbate-induced swelling and lysis by blocking electron transport. Phosphate-induced swelling may show a lag period, but the characteristic curve for ascorbate-induced DbZO changes shows a longer lag followed by a rapid fall to very low values, far below those seen with phosphate. Considerable additional evidence suggests that ascorbateinduced lysis and phosphate-induced swelling are different phenomena. Added substrates are not needed with ascorbate. Ascorbate-induced lysis is inhibited by high concentrations of mitochondrial protein, high concentrations of ascorbate, certain reducing agents, and a number of chelators which do not inhibit phosphate-induced swelling. In addition, ascorbate-induced lysis occurs after phosphate-induced swelling has reached a plateau. It is not inhibited by a number of things which can block phosphate-induced swelling, such as azide, dmytal, malonate, and uncouplers like 2,4-dinitrophenol. This paper presents further studies on the nature of the phenomenon, the action of accelerators which shorten or eliminate the lag period, the action of inhibitors, and the possible role of lipid peroxide formation and other reactions in the mecha-