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 aa3. 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.