Growth factors - Pharmaceutical Biotechnology: Concepts and Aapplications - page 275

Figure 10.3 Three-dimensional structure of EPO. Structural details courtesy of the Protein Data Bank,
http://www.rcsb.org/pdb/
synthesized at a rate of about 2.3 million cells per second and have a circulatory life of approxi-
mately 120 days, during which they travel almost 200 miles.
EPO is an atypical cytokine, in that it acts as a true (endocrine) hormone and is not synthesized
by any type of white blood cell. It is encoded by a single copy gene, located on (human) chromo-
some 7. The gene consists of four introns and fi ve exons. The mature EPO gene product contains
165 amino acids and exhibits a molecular mass in the region of 36 kDa (Figure 10.3). EPO is a
glycoprotein, almost 40 per cent of which is carbohydrate. Three N-linked and one O-linked gly-
cosylation sites are evident. The O-linked carbohydrate moiety appears to play no essential role in
the ( in vitro or in vivo ) biological activity of EPO. Interestingly, removal of the N-linked sugars,
although having little effect on EPO's in vitro activity, all but destroys its in vivo activity. The
sugar components of EPO are likely to contribute to the molecule's solubility, cellular processing
and secretion, as well as its in vivo metabolism.
Incomplete (N-linked) glycosylation prompts decreased in vivo activity due to more rapid hepatic
clearance of the EPO molecule. Enzymatic removal of terminal sialic acid sugar residues from
oligosaccharides exposes otherwise hidden galactose residues. These residues are then free to bind
specifi c hepatic lectins, which promote EPO removal from the plasma. The reported plasma t 1/2
value for native EPO is 4-6 h. The t 1/2 for desialated EPO is 2 min. Comparison of native human
EPO with its recombinant form produced in CHO cells reveals very similar glycosylation patterns.
EPO in the human adult is synthesized almost exclusively by specialized kidney cells (peritubu-
lar interstitial cells of the kidney cortex and upper medulla). Minor quantities are also synthesized
in the liver, which represents the primary EPO-producing organ of the foetus.