The effects of hypoxia/reoxygenation stress on activities of mitochondrial complexes I and IV and activities of protein kinases and phosphatases in marine bivalves

Hypoxia commonly occurs in aquatic habitats and puts strong pressure on the benthic sessile organisms unable to escape the oxygen fluctuations. Here we focused on the effects of hypoxia/reoxygenation (H/R) on activities of the mitochondrial Complexes I and IV in hypoxia-tolerant marine bivalves, namely, the blue mussel Mytilus edulis, the Arctic quahog Arctica islandica and the Pacific oyster Crassostrea gigas, exposing them for one or six days to extreme hypoxia (<0.1% O2) followed by one hour of reoxygenation. Furthermore, we examined the potential role of the posttranslational modifications (PTM) by reversible protein phosphorylation in regulation of the Complex I and IV activities and in the mitochondrial responses to H/R stress. The overall level of protein phosphorylation by PKA (serine/threonine protein kinase A) and PKC (protein kinase C) was measured to test for possible association between the H/R-induced changes in the PKA and PKC activities and the respective protein products of these kinases. Our results showed a strong species-specific modulation of the PKA and PKC activities by H/R stress. In the mussels and quahogs, phosphorylation by PKA and PKC led to an increase in activity of Complexes I and IV, whereas in oysters these enzymes were insensitive to PKA and PKC activation. Non-site-specific dephosphoryaltion strongly suppressed the activity of Complex I and IV in all three studied species.