Erythropoietin is a glycoprotein hormone that supports production of red blood cells, or erythropoiesis. It acts through the erythropoietin receptor on responsive precursor cells; the signal is not itself an oxygen-carrying substitute for hemoglobin. Experiments showed association of the receptor with JAK2 and activation of that protein kinase after erythropoietin stimulation.[ref]
JAK2 associates with the erythropoietin receptor and is tyrosine phosphorylated and activated following stimulation with erythropoietin by B. A. Witthuhn and others. Cell 74, pp. 227–236; Abstract, receptor association and kinase activation
Purifying the hormone made its activity experimentally tractable. In 1977, researchers used seven separation steps on human urine, including ion-exchange chromatography and gel filtration, and reported -fold purification with activity yield. The preparation had activity units per milligram of protein.[ref]
Purification of human erythropoietin by T. Miyake and others. Abstract; purification, specific activity and yield
These quantities answer different questions. Specific activity asks how much biological effect remains per unit protein mass; yield asks how much starting activity survives purification. A preparation can become much purer while losing most of the original material.
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