Oxygen-binding properties of extracellular hemoglobin from the leech, Hirudo medicinalis. Effects of pH, cations and temperature

Abstract

Electron microscopy of the extracellular hemoglobin from the medicinal leech, Hirudo medicinalis, revealed a typical annelid two-tiered hexagonal structure. The dimensions of the molecule were 30.2 ± 1.6 nm corner to corner across the hexagon and 18.9 ± 1.1 nm in height. The oxygenation properties of this high-molecular-weight hemoglobin have been thoroughly studied. The effects of hemoglobin concentration, pH, temperature and concentration of specific cations on the oxygen affinity and the degree of cooperativity have been investigated with special consideration given to physiological conditions. At near-physiological cation concentrations (6 mM Ca 2+, 0.125 M Na +), the oxygen affinity and cooperativity tend to increase with increasing alkalinity. At physiological conditions ((pH 7.5) temperature 20°C), they reach P 50 values of 5.0 mmHg and n 50 = 3.1. An alkaline Bohr effect with considerable magnitude ( Ø = -0.50) is observed at pH 7.0–8.0. The oxygen-binding properties are also affected by cations. Both oxygen affinity and cooperativity rise with increasing cation concentrations. Divalent cations, such as Ca 2+ and Mg 2+, bring about larger effects than monovalent ones. They begin to be effective at concentrations as low as 0.1 mM. The magnitude and position of the Bohr effect of H. medicinalis hemoglobin are controlled by cations to such an extent that this Bohr effect can be predominantly ascribable to the oxygen-linked binding of the physiologically available allosteric effectors Ca 2+ and Na +. The results obtained indicate that protons and cations are effective allosteric regulators of achaete (leech) extracellular hemoglobins.