Studies on Avian Erythrocyte Metabolism

The erythrocyte count of the adult ostrich (∼ 2.0 × 106/mm3) is about 40% of that of man (∼ 5.0 × 106/mm3) but the red blood cell is approximately three times larger (220 μm3). The total phosphate content per cm3 red blood cell is 37.7 μmol. Inositol tetrakisphosphate (inositol‐P4), a compound not previously reported in the erythrocytes of any species of animal, is the major organic phosphate in erythrocytes of the adult ostrich, accounting for 35 % of the total phosphate. Inositol pentakisphosphate (inositol‐P5), the major phosphorylated compound in the erythrocytes of most mature species of birds, is also present but in amounts approximately half that of inositol‐P4. The effects of adenosine triphosphate, 2,3‐bisphosphoglyceric acid (P2‐glycerate), inositol‐P4, inositolP5, and inositol hexakisphosphate (inositol‐P6) on the oxygen affinity of whole ‘stripped’ hemoglobin, hemoglobin A (major component), and hemoglobin D (minor component) isolated from erythrocytes of the mature ostrich have been determined. All five compounds cause a right shift in the oxygen dissociation curve of each hemoglobin and their relative effectiveness in decreasing the oxygen affinity is ATP < P2‐glycerate < inositol‐P4 < inositol‐P5 < inositol‐P6. No preferential interaction of inositol‐P4 or inositol‐P5 with either hemoglobin A or hemoglobin D was observed. The unique organic phosphate composition of the erythrocyte of the ostrich probably explains the relatively high whole blood oxygen affinity (P50= 24.5 torr) of these birds.