Nitrite reductase activities of myoglobins from five whale species with distinct diving capacities

Abstract

The O2 carrier myoglobin (Mb) is expressed at high concentrations in the skeletal muscle of whales where it represents a major O2 store during diving. In addition to its well-known roles in O2 storage and delivery, Mb when deoxygenated can reduce endogenous nitrite to nitric oxide (NO), which in turn may downregulate respiration under hypoxia and protect against tissue damage upon reoxygenation. Although these functional properties of Mb are expected to contribute to the diving capacity of whales, very little is known about the variation in Mb function among whale species with different diving abilities. In this study, we have therefore measured O2 affinities and nitrite reductase activities of purified Mbs from five whale species; three toothed whales (harbor porpoise, sperm whale, bottlenose whale) and two baleen whales (minke whale and humpback whale) with distinct diving capacities and known average dive durations. We show that differences in O2 affinities and nitrite reductase activities are present among whale Mbs, but that these do not correlate with the average dive duration of the selected species. However, a higher Mb O2 affinity found in toothed whales is significantly correlated to a higher nitrite reductase activity, an adaptation that may increase NO production per unit time in the hypoxic muscle. In addition, the higher Mb concentrations present in the extreme diving whales compared with the more moderate divers may enhance the rate of NO production from nitrite during hypoxia and this can increase the beneficial effects of NO during and after long dives.