Effects of Polyamines on the Oxidation of Exogenous NADH by Jerusalem Artichoke (Helianthus tuberosus) Mitochondria

The effect of polyamines (putrescine, spermine, and spermidine) on the oxidation of exogenous NADH by Jerusalem artichoke (Helianthus tuberosus L. cv. OB1) mitochondria, have been studied. Addition of spermine and/or spermidine to a suspension of mitochondria in a low-cation medium (2 millimolar-K⁺) caused a decrease in the apparent K_m and an increase in the apparent V_max for the oxidation of exogenous NADH. These polycations released by screening effect the mitochondrially induced quenching of 9-aminoacridine fluorescence, their efficiency being dependent on the valency of the cation (C⁴⁺ > C³⁺). Conversely, putrescine only slightly affected both kinetic parameters of exogenous NADH oxidation and the number of fixed charges on the membranes. Spermine and spermidine, but not putrescine, decreased the apparent K_m for Ca²⁺ from about 1 to about 0.2 micromolar, required to activate external NADH oxidation in a high-cation medium, containing physiological concentrations of Pi, Mg²⁺ and K⁺. The results are interpreted as evidence for a role of spermine and spermidine in the modulation of exogenous NADH oxidation by plant mitochondria in vivo.