Participation of Cytochromes in the Respiration of the Aroid Spadix.

Abstract

The rapid spadix respiration of Peltandra yirginica and Philodendron grandifolium, like that of Arum maculatum (New Phytol. 49: 353, 1950), is not inhibited by 10-3 [image] HCN or 19/1 CO/O2 mixtures. Provided the rate of O2 supply is adequate, the partial pressure of O2 which supports half the maximal respiratory rate was 0.002 atmospheres. This value indicates an O2 affinity much greater than that of flavo-protein or copper oxidases and approaches that of tissues respiring via cytochromes. Isolated spadix mitochondria exhibit an active cytochrome c oxidase, which shows the typical high affinity for HCN and CO. The intact spadix was examined spectrophotometrically, and difference spectra (N2 minus O2) reveal maxima at wavelengths corresponding to cytochromes a + a3, b, and c; molar concentrations of each is of the order of 10~6. Difference spectra in cyanide show suppresion of the rate of oxidation of cytochromes c and a + a3, but not of b. These observations and others are consistent with the following respiratory chain, which includes an alternative pathway from cytochrome b to O2:.