An O2-Hb "paradox' in frog blood? (n-values exceeding 4.0)

O2-Hb dissociation curves have been determined for the anurans, Rana temporaria and Rana catesbeiana, and compared with human blood for the specific purpose of defining the n-value (hill's cooperativity coefficient) at high O2 saturations. The P50 values at normal conditions (see fig. 1) for each species were 24.6 mm Hg for human blood, 37.0 mm Hg for Rana temporaria blood and 53.5 mm Hg for Rana catesbeiana. For human blood the n-value was 2.7 at saturations from 36% to 98% levelling off at the highest saturations. For Rana temporaria Hill plots between saturations from 20% to 98% showed 3 segments. The average n-value increased from 1.6 to 2.4 at about 50% saturation, increasing again at about 80% saturation to 7.3. The pattern in Rana catesbeiana blood was similar with the n-value changing from 1.6 to 3.1 at about 50% saturation, averaging 3.5 between 53% and 89% saturation. Additionally the Bohr effect in Rana temporaria blood more than doubled when compared at 50% and 90% saturation. The data show that for R. temporaria blood the free energy of interaction associated with the binding of 02 to Hb is displaced to the upper segment of the dissociation curve.