Trypsin-induced activation of the thiol esters in alpha 2-macroglobulin generates a short-lived intermediate ('nascent' alpha 2-M) that can react rapidly to incorporate not only methylamine or putrescine but also proteins lacking proteinase activity.

@article{SottrupJensen1981TrypsininducedAO,
  title={Trypsin-induced activation of the thiol esters in alpha 2-macroglobulin generates a short-lived intermediate ('nascent' alpha 2-M) that can react rapidly to incorporate not only methylamine or putrescine but also proteins lacking proteinase activity.},
  author={Lars Sottrup-Jensen and Torben Elleb{\ae}k Petersen and Sigurdur M Magn{\'u}sson},
  journal={FEBS letters},
  year={1981},
  volume={128 1},
  pages={123-6}
}