Inhibition of the bioenergetic functions of isolated rat liver mitochondria by polyamines.

The abilities of the naturally occurring polyamines, putrescine, spermidine and spermine, to affect variables related to the bioenergetic functions of isolated rat liver mitochondria were studied. At concentrations comparable to those present intracellularly, the polyamines inhibited state 4 respiration, but they had much less effect on state 3 or uncoupled respiration. The concentrations required to produce 25% inhibition (I25) of state 4 respiration varied according to the polyamine, with putrescine being least effective (I25, 20 mM) and spermidine and spermine being more effective and comparable (I25, 7.5 and 7.0 mM respectively). This inhibition was antagonized by 15 mM potassium and enhanced by valinomycin and 4 mM magnesium. Inhibition of monoamine oxidase, an enzyme of outer mitochondrial membrane, was also observed to occur. Addition of polyamines to mitochondrial suspensions caused an increase in the optical density and protected against the swelling effects of sublytic concentrations of Triton X-100. By electron microscopy, polyamines were found to cause the outer mitochondrial compartment to collapse bringing the inner and outer membranes into apparent contact with one another. The electrophoretic mobility of mitochondria toward the anode was markedly slowed by polyamines (i.e. 50% by 1.25 mM spermine), indicating surface binding and neutralization of the negative surface charge. In almost all of the above mitochondrial effects, spermine and spermidine were similar in effectiveness and putrescine was less effective. It is suggested that polyamines may be capable of modulating respiration of isolated mitochondria by binding to non-specific anionic sites at the surface of the inner mitochondrial membrane. Neutralization of the net negative surface potential may interfere with cation fluxes across the membrane, particularly those of potassium.