The metabolism of recombinant erythropoietin in the isolated perfused rat liver.

Indirect evidence points to extrarenal organs, presumably the liver, as the site of degradation of erythropoietin (EPO). The metabolism of both fully glycosylated and desialated intrinsically labelled 35S-Cysteine recombinant human erythropoietin (rhEPO) was therefore studied in isolated Wistar rat livers perfused in a recirculating mode for 180 min with a hemoglobin-free medium containing rhEPO. Perfusate and bile levels of rhEPO were measured by RIA. Total 35S-radioactivity in liver, bile and perfusate as well as non-acid precipitable radioactivity in perfusate were determined. In addition, detection of 35S-radioactivity was performed after subcellular fractionation of rat livers perfused with desialo-35S-Cysteine rhEPO. While concentrations of fully glycosylated 35S-Cysteine rhEPO did not exhibit any detectable decrease during perfusion, desialo-35S-Cysteine rhEPO was rapidly cleared from the perfusate. After 60 min of perfusion, only 32% of the initial levels of both immunoreactive rhEPO and total radioactivity remained in the perfusate. Quantitative hepatic accumulation of desialated tracer was demonstrated. Subcellular fractionation showed extensive hepatic degradation of the desialated tracer. Furthermore, during perfusion progressively larger amounts of small molecular weight degradation products of the tracer were found in the perfusate. Bile excretion of both fully glycosylated and desialated tracer was negligible. The significance of hepatic metabolism of desialo-35S-Cysteine rhEPO was supported by reduced removal of desialo-35S-Cysteine rhEPO from plasma in hepatectomized rats. It is hypothesized that continuous in vivo desialation is a crucial rate-limiting step in the degradation of circulating EPO.