Some peculiarities of the steady-state kinetics of electron transfer in submitochondrial particles. A kinetic model based on the idea of activation on the respiratory chain induced by electron transfer

Cardiac contractile function, oxygen consumption rate and cytosolic phosphates during inhibition of electron flux by amytal--a 31P-NMR study

In order to investigate the potential role of cytosolic phosphates ([ATP], [ADP] and [Pi]) in the integration of mitochondrial respiration and mechanical function in the perfused heart, inhibition of the substrate end of the respiratory chain by amytal has been employed. A stepwise increase in amytal concentration (from 0.2 to 1.2 mM) resulted in the progressive abolition of the cardiac oxygen consumption, rate (VO2) in hearts oxidizing pyruvate (5 mM). The inhibition curve for VO2 was S-shaped, with K0.5 = 1.1 mM, and independent of the initial VO2 values varied by coronary flow and isoproterenol (Iso) addition. ADP-stimulated respiration of isolated mitochondria (malate + pyruvate) was twice as sensitive to amytal inhibition, whereas state 2 respiration (before ADP addition) had the same sensitivity as cardiac VO2. Decrease in VO2 was followed by a decline in phosphocreatine (PCr) content and augmentation of Pi at nearly constant ATP level and intracellular pH as assessed by the 31P-NMR method. These changes were associated with an elevation of cytosolic free [ADP] and a reduction of the [ATP]/[ADP] ratio and ATP affinity calculated from creatine kinase equilibrium. Concomitantly, pressure-rate product (PRP), maximal rates of contraction and relaxation fell down and the end diastolic pressure (EDP) rose at all initial loads. Amytal-inhibited hearts retained the capability to respond to Iso stimulation (0.1 microM, about 50% enhancement of PRP) even at 1 mM amytal, but their response to elevation of coronary flow was greatly diminished. Alterations in the PRP value induced by the inhibitor at a fixed coronary flow correlated negatively with cytosolic [ADP] and [Pi], and positively with [ATP]/[ADP] and A(ATP). In contrast, EDP correlated with all these parameters in the opposite manner. However, when PRP was varied by coronary flow in the absence of the inhibitor or at its fixed concentrations, such correlations were absent. These data imply that cytosolic phosphates can serve as a feedback between energy production and utilization when the control point(s) is (are) at the mitochondria. In contrast, other regulatory mechanisms should be involved when control is distributed among different steps located both in energy producing and utilizing systems.

Steady-state kinetics of electron transfer through cytochrome chain of uncoupled submitochondrial particles. I. General kinetic analysis

Steady-state kinetics of electron transfer through the cytochrome chain of uncoupled ultrasonic submitochondrial particles at different pH values has been studied. Rate constants calculated from the Pring equation (ki' = V/PirPi+1ox) increased with the increase of the rate of the process. As in the previous work (Saks, V. A., Kupriyanov, V. V. and Luzikov, V. N. (1972) Biochim. Biophys. Acta 283, 42-53) this dependence was linear, but only at comparatively low rates of electron transfer. To explain the experimental data several kinetic models, based on the assumption that respiratory chains are activated when functioning, have been proposed and analysed. The best agreement with the experimental data was obtained for the model suggesting that the rate of activation of the carriers is directly proportional to the overall rate of electron transfer and to the proportion of non-activated respiratory chains in the system. Hence it appeared that electron transfer through already activated chains entailed activation of adjacent non-activated chains. This model allowed rate constants for non-activated (ki) and activated (ki) states of the carriers, as well as the life-time of the activated carriers (tau) to be determined.