Rapid enzymatic degradation of [125I] (Tyr 10) FGF (1-10) by serum in vitro and involvement in the determination of circulating FGF by RIA.

In the conditions used in the RIA procedure for circulating FGF quantitation, the tracer [125I] (Tyr 10) FGF (1-10) was extensively degraded into two non immunoreactive peptides corresponding to a sequential removal of two amino acid residues at the NH2-terminus i.e. Pro and Ala. A FGF like immunoreactive fraction exists in serum the molecular weight of which was estimated to be 240 Kda. This fraction was also able to perform the same extensive degradation of (Tyr 10) FGF (1-10) than whole serum. The results presented raise the question of the validity of RIA for the determination of circulating FGF. They also present evidence that a high molecular weight serum fraction which reacts as immunoreactive FGF is an enzymatic activity responsible for biodegradation of the growth factor rather than a distinct biological entity which is related to the FGF structure.