Abstract
The thermogenic spadix of homeothermic skunk cabbage, Symplocarpus foetidus, shows an inverse relationship between the levels of respiration and changes in the ambient air temperature. To clarify the molecular mechanism underlying respiration control in the spadix, mitochondrial respiratory activities mediated by alternative oxidase (AOX) and uncoupling proteins (UCPs) were analyzed. Pecoll-based centrifugation with crude mitochondria derived from the thermogenic spadix gave a distinct and respiratory active fraction that contains the cytochrome c oxidase, which suggested the successful purification of undamaged mitochondria. Interestingly, the purified mitochondria showed significant AOX and UCP activities, which were characterized by cyanide-resistant and SHAM-sensitive respirations, and FCCP-insensitive respirations, respectively. Because the major UCPs detected in the purified mitochondria were found to be a novel UCP isoform, termed UCPb, which lacks the conserved fifth transmembrane domain, two different mitochondrial reactions induced by AOX and UCPb seem to play an important role in the thermoregulation of the spadix.
