Does the low respiratory rate in unfertilized eggs result mainly from depression of the redox reaction catalyzed by flavoproteins? Analysis of the respiratory system by light-induced release of CO-mediated inhibition.

In unfertilized eggs of the sea urchin, the quite low respiratory rate is enhanced by tetramethyl-p-phenylenediamine (TMPD), phenazine methosulfate (PMS) and sperm and this augmentation is completely inhibited by carbon monoxide (CO). Exposure to light releases eggs from this CO-mediated inhibition. The action spectra for photoreactivation of CO-inhibited cytochrome c oxidase in isolated mitochondria and CO-blocked respiration in TMPD-treated eggs were found to be similar to the absorption spectrum of CO-bound cytochrome aa . In PMS-treated eggs and fertilized eggs, the maximum photoreactivation of CO-inhibited respiration occurred at a light fluence rate higher than that for maximum photoreactivation of CO-inhibited respiration in TMPD-treated eggs, with peaks at the same wavelengths as those in the absorption spectrum of reduced cytochrome b. A similar phenomenon was seen for NADH cytochrome c reductase in mitochondria. Thus, cytochrome c oxidase and NADH cytochrome c reductase, whose activities are not altered by fertilization, seem to be functional, even in unfertilized eggs. In unfertilized eggs, difference spectra indicated that PMS and sperm augmented cytochrome b reduction and that TMPD accelerated cytochrome c reduction without cytochrome b reduction. Therefore, it is likely that depression of electron transport to cytochrome b, which is augmented by PMS and sperm, is responsible for the low respiratory rate in unfertilized eggs.