<prompt> C- peptide is seen:-In pre-proinsulinIn ProinsulinAs a combined entity with insulin after secretionAs a gastrointestinal proactive molecule  <response>: - Insulin is synthesized in BETA- cells of pancreas by usual cell machinery for protein synthesis. - Beginning with translation of insulin RNA by ribosomes attached to E.R to form pre proinsulin. - Pre proinsulin is cleaved in E.R to form pro-insulin. - Insulin C peptide are packaged in secretory granules secreted in equimolar amounts. - So, c - peptide levels can be measured by radioimmunoassay in insulin-treated diabetic patients to determine how much of their own natural insulin they are still producing 