Redox state of respiratory chain enzymes and potassium transport in silkworm mid-gut

The midgut of Hyalophora cecropia actively transports potassium from hemolymph to lumen and the energy for this process appears to be intimately linked to oxidative metabolism. In the present investigation, we monitored concurrently the rate of active transport and the redox levels of the components of the respiratory chain in the intact tissue under a variety of experimental conditions. Approximately equal concentrations of cytochromes a₃, a, c and b-557 were found. Other investigators (Pappenheimer, Jr, A. M. and Williams, C. M. (1954) J. Biol. Chem. 209, 915, Shappirio, D. G. and Williams, C. M. (1957) Proc. R. Soc. Lond. Ser. B 147, 233 and Chance, B. and Pappenheimer, Jr, A. M. (1954) J. Biol. Chem. 209, 931) have identified cytochrome b-557 with b₅ and found that it exists primarily in an extramitochondrial location.Steady-state experiments demonstrated that all these cytochromes were approximately 50% reduced while active transport proceeded at a high rate in regular cecropia Ringer containing 32 mM KCl. When the potassium concentration was reduced, the active transport decreased and all the cytochromes became more oxidized. Addition of 1 mM cyanide inhibited active transport by 90% and caused a 100% reduction of all cytochromes. Redox state and short circuit current (I_sc) kinetics measured as the tissue was made anoxic showed that all the respiratory enzymes, except cytochrome b-557, became fully reduced at a faster rate than the rate of inhibition of the I_sc. The rate of cytochrome b-557 reduction followed kinetically the I_sc.These observations are interpreted in a scheme where cytochrome b-557 (possibly b₅) branches off cytochrome c from the conventional respiratory chain, utilizing cytochrome a₃ as the terminal oxidase for both branches. Cytochrome b-557 may be involved in providing a direct link between oxidative metabolism and active transport in the midgut of the silkworm.