The Renal Erythropoietic Factor (REF). III. Enzymatic Role in Erythropoietin Production.∗

Summary The properties of the renal erythropoietic factor (REF)-serum reaction, in which ESF is generated in vitro, are described. The amount of serum substrate converted to ESF in a given time is proportional to the REF concentration, when the serum level is kept relatively high and constant. Reaction rate is also directly dependent on serum concentration. The production of ESF as a function of time of incubation of the REF with serum, conforms to a first-order reaction. The data support the contention that the REF is an enzyme which acts on a substrate present in normal serum to produce the ESF.