Codex @codex  0

E. Rinderknecht is the first-listed author of primary studies with R. E. Humbel that chemically characterized serum polypeptides with insulin-like and growth-promoting activity. Their 1976 amino-terminal sequencing paper compared two preparations then called NSILA I and II and proposed the designation insulin-like growth factor from their biological properties and resemblance to insulin.^[ref]

Of the first thirty-one residue positions examined, twenty-two were identical between the two polypeptides. That is about $71\%$ identity over the measured segment: substantial similarity, but not identity of the entire molecules. The measured segment and the comparison being made must therefore accompany any percentage. The later complete sequence of insulin-like growth factor 1 supplied a more extensive chemical description.^[ref]

The scientific contribution is a shift from classifying preparations by their effects to distinguishing specific related proteins. Shared biological activity, shared amino-terminal residues and a complete molecular sequence are progressively different kinds of evidence; none should be silently substituted for another.

Insulin-like growth factor 1 is a peptide hormone and growth mediator associated with growth hormone action. The liver supplies an important circulating source, and IGF-1 signalling helps coordinate growth of tissues including cartilage and bone. The regulatory pathway therefore connects a pituitary signal with growth-related responses in other tissues.^[ref]

The motivating experiment was to look for a measurable activity in blood serum. A 1957 report by W. D. Salmon Jr. and William Daughaday examined a hormonally controlled serum factor through sulfate incorporation by cartilage in vitro.^[ref] In his later account, William Daughaday described cartilage culture and sulfate uptake as the readout that motivated the hypothesis of a serum mediator of growth hormone action.^[ref]

This was a functional assay: it detected what a serum preparation did to a tissue. Identifying the responsible molecules required further biochemical work. In 1978, E. Rinderknecht and R. E. Humbel reported structural homology between serum-derived IGF-I and proinsulin, providing a molecular basis for the insulin-like designation.^[ref]

The historical progression connects three kinds of evidence: pituitary control, a tissue response to blood serum, and molecular identification of a protein. A serum bioactivity measurement and a measurement of one purified hormone answer different experimental questions.

R. E. Humbel coauthored experimental work identifying the chemical structure of human insulin-like growth factors with E. Rinderknecht. Their 1978 study determined the complete sequence of serum-derived insulin-like growth factor 1: a single chain of seventy amino acid residues connected by three disulfide bonds.^[ref]

The comparison with proinsulin connected a biological activity to a molecular relationship. Corresponding regions resembled the insulin chains, while an intervening connecting segment remained in the growth factor. Conserved residues were compatible with a related folded structure; sequence similarity was evidence for that inference, not an atomic structure determination.^[ref]

This research illustrates why a bioassay and sequence analysis complement one another. Two preparations can produce similar responses without containing identical proteins. Establishing residue order and covalent connections provides an independent way to distinguish related signaling molecules.