The Electron Transfer System of Skunk Cabbage Mitochondria.

Abstract

Respiratory components of mitochondria isolated from the spadix of Symplocarpus foetidus are studied with the double and split-beam spectrophotometers, vibrating platinum microelectrode, rapid flow apparatus, and other techniques to determine their reactions to antimycin A and various inhibitors of the mammalian system as well as the effects of these inhibitors upon oxygen and diphosphopyridine nucleotide (DPNH) utilization. Spectroscopic studies reveal absorption bands for cyto-chromes of types a3 (445 m[mu]) and a (603 m[mu]), flavoprotein (460 m[mu]), a c component (551 m[mu]) unlike that of the mammalian system, and an atypical b component (558 m[mu]) designated b7. Dithionite treatment reveals a cytochrome-like material at 557.5 m[mu] that does not participate in electron transfer. Whereas the a and c components are reduced in the presence of cyanide or azide and oxidized in the presence of 2-n-heptyl-4-hydroxyquinoline N-oxide (HOQNO), b7 shows a reverse effect and is also sensitive to antimycin A inhibition. In the steady state, however, spectroscopic studies show that the extent of inhibition depends on the extent of activation by substrate, in this case, DPNH. Although each of the cytochromes is shown to be sensitive to cyanide, azide, and HOQNO in the steady state, sufficient amounts of the oxidized components of the c-a-a3 system remain to make consideration of an alternate pathway for electron transfer premature. HOQNO permits a more detailed study of the possible role of b7 in electron transfer. Its oxidation depends upon electron transfer through the c-a-a3 system: cytochrome c is an oxidant for b7 and b7 is a reductant for c. Kinetic studies show that the rate of reduction of the b7 component by DPNH and the kinetics of its oxidation in the uninhibited system are consistent with its role in the c-a-a3 system. Only a discrepancy in the rates of reduction of the cytochromes in the aerobic-anaerobic transition raises any question of the function of b7 as a component of a single cytochrome chain of the mitochondria of Symplocarpus foetidus.