The process of hypoxic induction of Daphnia magna hemoglobin: subunit composition and functional properties

The process of oxygen-dependent hemoglobin induction in Daphnia magna was studied over an 11-day period of hypoxia (ambient oxygen partial pressure: 3 kPa). Along with the increase of hemoglobin concentration in the hemolymph, hemoglobin became the dominant protein fraction in gel filtration experiments using extracts of whole animals. The size of the native aggregates was constant. However, subunit composition depended on the duration of hypoxia: the pattern of predominantly expressed subunits under hypoxia deviated from that of normoxic individuals. The varying degree of hypoxic induction for different hemoglobin subunits was confirmed by autoradiography. Along with changes in hemoglobin subunit composition, oxygen affinity of the respiratory protein increased. The dynamics of the hemoglobin induction process was analysed. Newly synthesized hemoglobin can be detected within 18 h after the onset of hypoxia. A marked increase in hemoglobin concentration is evident from the third day of hypoxia, and a steady state of hemoglobin concentration is reached within 11 days. The changes of hemoglobin subunit expression in response to hypoxia form the structural basis for the observed adjustments of hemoglobin function leading to enhanced oxygen transport at low ambient oxygen concentrations.