THE BIOSYNTHESIS OF INSULIN AND A PROBABLE PRECURSOR OF INSULIN BY A HUMAN ISLET CELL ADENOMA

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Abstract
Tritiated phenylalanine and leuclne were incorporated into acid-alcohol soluble protein during incubation of slices from a human islet cell adenoma in vitro. When partially purified acid-alcohol extracts of the incubation mixtures were subjected to gel filtration on columns of Sephadex G-50, 2 peaks of labeled material were separated, both of which reacted with guinea pig anti-insulln serum. The more slowly eluted peak was apparently identical with normal human insulin and was more abundant than the earlier eluted component. A large proportion of the early eluting material could be converted to a form indistinguishable from authentic insulin by. digestion with small amounts of trypsin. N-terminal aunno acid analysis of phenylalanine-labeled insulin and the early component revealed the presence in both of a free N-terminal phenylalanine residue. The evidence available indicates that the immunoreactive material consists of a larger protein molecule of mol wt about, 10,800 which includes the amino acid sequences of insulin in a form which can be released by limited proteolysis.

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