Oxygen Affinity, P50
Oxygen Affinity, a red cell characteristic determined by both hemoglobin, and cytosolic composition is measured with the Hemox analyzer. The P50 is a conventional measure of the affinity of hemoglobin for oxygen. The oxyhemoglobin dissociation curve relates oxygen saturation (SO2) and partial pressure of oxygen in the blood (PO2). The PO2 at which the hemoglobin is 50% saturated, typically about 26-28 mmHg for a healthy person, is known as the P50. In the presence of disease or other conditions that change the hemoglobin's oxygen affinity and, consequently, shift the curve to the right or left, the P50 changes accordingly. An increased P50 indicates a rightward shift of the curve, which means that a larger partial pressure is necessary to maintain a 50% oxygen saturation. This indicates a decreased affinity. Conversely, a lower P50 indicates a leftward shift and a higher affinity.
Hemoglobin's affinity for oxygen increases as successive molecules of oxygen bind. More molecules bind as the PO2increases until the maximum amount that can be bound is reached. As this limit is approached, very little additional binding occurs and the curve levels out as the hemoglobin becomes saturated with oxygen. Hence the curve has a sigmoidal shape. At pressures above about 60 mmHg, the dissociation curve is relatively flat, which means that the oxygen content of the blood does not change significantly even with large increases in the PO2. Although binding of oxygen to hemoglobin continues to some extent for pressures below about 60 mmHg, as the PO2 decreases in this steep area of the curve, the oxygen is unloaded to peripheral tissue readily as the hemoglobin's affinity diminishes.
The effectiveness of hemoglobin-oxygen binding can be affected by several factors. The curve is shifted to the right by an increase in temperature, 2,3-diphosphoglycerate, PCO2, or a decrease in pH. The curve is shifted to the left by the opposite of these conditions, the presence of carbon monoxide, methemoglobinemia, and fetal hemoglobin. Typically, fetal arterial oxygen pressures are low, and hence the leftward shift enhances the placental uptake of oxygen.
The Hemox analyzer is an automatic system for the recording of blood oxygen equilibrium curves utilizing fresh whole blood based on dual wavelength spectrophotometry for the measurement of the optical properties of hemoglobin and an electrode for measuring the PO2 in mmHg. The resulting signals from both measuring systems are fed to the computer, which plots the resulting curve.
The production of 2,3 Diphosphoglycerate( 2,3 DPG) is an important function of the Embden-Meyerhoff pathway in the red cell. Together with ATP this phosphate easter is a regulator of the oxygen affinity of hemoglobin. A decrease in 2,3 DPG will shift the oxygen affinity curve to the left (lowers the P50) while an increase in 2,3 DPG will shift the oxygen affinity curve to the right (increase the P50). The measurement of 2,3, DPG levels is therefore important to properly evaluate the oxygen affinity of the red cell. Since the state of red cell metabolism will affect 2,3 DPG levels, fresh samples are required for measurement and special care needs to be taken in collection and shipment.
Human 2,3-Diphosphoglycerate (2,3-DPG) Assay
The 2,3-DPG Assay is a UV-detection for the determination of 2,3-DPG in blood samples in the range of 0.02–0.15 µmol in life science research applications. The 2,3-DPG in the sample is split by phosphoglycerate mutase to form phosphoglycerate, which is then detected at 340 nm.
This assay is for research use only and is not for use in diagnostics.
The normal range for adults is 4.8 ± 0.2 mM of 2,3-DPG/l erythrocytes and for children 5.3 ± 0.6 mM 2,3-DPG/l erythrocytes. The 2,3-DPG content within blood samples will change rapidly after collection. For this reason, the deproteinization procedure should be carried out immediately.