Determination of interdependent ligand effects on human red cell oxygen affinity

Abstract

Human whole blood oxygen affinity was determined as P₅₀, i.e. Po₂ for oxygen saturation (So₂) = 0.5, in a new system which allows the measurement of So₂, Po-2, and pH continuously and independently, with control of Pco₂ and temperature. The influence of pH on P₅₀ expressed as the H⁺ factor (∂log P₅₀/∂pH)_pco2 was measured under conditions of varying Pco₂, temperature and concentration of 2,3-diphosphoglycerate (2,3-DPG), resulting in a set of data expressing second-order inter-ligand interactions. The H⁺ factor appeared to be only slightly dependent on Pco₂. Similarly, the CO₂ factor (∂log P₅₀/∂log Pco₂)_pH shows only a minor dependence on pH. The H⁺ factor is linearly related to the temperature: at 17°C and 42°C the H+ factor is about -0.53 and -0.36, respectively. Likewise, the temperature factor (∂log P₅₀/∂T)_pco2, pH is linearly related to pH. A pilot study on the effect of varying intra-erythrocytic 2,3-DPG concentrations on the oxygen affinity showed that a very low 2,3-DPG/Hb₄ ratio apparently does not influence the H⁺ factor. A high ratio, however, seems to lower the H+ factor considerably.

Key-words:

• Bohr effect
• CO₂ factor
• H⁺ factor
• haemoglobin oxygen affinity
• temperature factor