The characterization of somatomedin A, isolated by microcomputer-controlled chromatography, reveals an apparent identity to insulin-like growth factor 1

Abstract

The polypeptide termed somatomedin A (SMA) was isolated from outdated human plasma by a new purification procedure, not using acid ethanol extraction. Fractions containing SMA were monitored by a placenta radioreceptorassay and a radioimmunoassay for SMA. The purification method utilized a microcomputer‐controlled chromatography system, yielding both SMA (identified as insulin‐like growth factor 1 (IGF‐1) or a deamidated derivative) and insulin‐like growth factor 2 (IGF‐2). The first step of CM‐Affigel blue adsorbed at neutral pH the majority of somatomedins detectable by the radioreceptorassay for SMA. Exclusion chromatography on Sephadex G‐50 in 0.1 M acetic acid separated this active material from albumin and NaCl. Separation between SMA and IGF‐2 was achieved on two different cation‐exchange columns, but not in the final high‐performance liquid chromatography step. The isoelectric points, determined by chromatofocusing, were 8.0 for SMA and 6.2 for IGF‐2. The amino acid compositions of the two isolated peptides were indistinguishable from the known compositions of IGF‐1 and IGF‐2. Sequence analysis up to position 39 of the peptide with a pi of 6.2 also proved identity with IGF‐2 for all positions examined. The peptide with a pi of 8.0, corresponding to SMA, was degraded directly as well as after CNBr cleavage. The results show that it is identical to IGF‐1, with the possible exception of an acid/amide assignment, which could correspond to a deamidation. If occurring in the native preparation before analysis, it could explain the chromatographic properties and isoelectric point of SMA versus IGF‐1 isolated by other techniques.