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Previous studies have shown that approximately 20% of hemoglobin is lost from circulating red blood cells (RBCs), mainly during the second half of the cells' life span. Because hemoglobin-containing vesicles are known to circulate in plasma, these vesicles were isolated. Flow cytometry studies showed that most RBC-derived vesicles contain hemoglobin with all hemoglobin components present. The hemoglobin composition of the vesicles resembled that of old RBCs. RBC cohort studies using isotope-labeled glycine have been described, which showed a continuous presence of this label in hemoglobin degradation products. The label concentration of these products increased during the second half of the RBC life span, accompanied by a decrease within the RBC. It is concluded that the hemoglobin loss from circulating RBCs of all ages can be explained by shedding hemoglobin-containing vesicles. This loss occurs predominantly in older RBCs. Apparently the spleen facilitates this process since asplenia vesicle retention within RBCs of all ages has been described, accompanied by an increase in the percentage of total HbA(1). The present study shows that in old RBCs of asplenic individuals, the decrease of hemoglobin content per cell such as seen in old RBCs of control individuals is absent due to an increase in the absolute amount of HbA(1c) and HbA(1e2). It is concluded that hemoglobin-containing vesicles within old RBCs are "pitted" by the spleen.
Willekens et al. (Tue,) studied this question.