Studies with [35S]methionine indicate that the 22,000-dalton [Met]enkephalin-containing protein in chromaffin cells is a precursor of [Met]enkephalin.

It has been shown that [35S]methionine is incorporated into the [Met]enkephalin sequences of a 22,000-dalton enkephalin-containing protein in the adrenal medulla. Pulse-chase experiments indicate that label is incorporated into the large polypeptide before it appears in free [Met]enkephalin and a smaller [Met]enkephalin-containing peptide. These findings provide direct evidence of a precursor-product relationship of these structurally related polypeptides.