Conversion of proinsulin to insulin occurs coordinately with acidification of maturing secretory vesicles. Orci L, Ravazzola M, Amherdt M, Madsen O, Perrelet A, Vassalli JD, Anderson RG. Proinsulin is a single polypeptide chain composed of the B and A subunits of insulin joined by the C-peptide region. Proinsulin is converted to insulin during the maturation of secretory vesicles by the action of two proteases and conversion is inhibited by ionophores that disrupted intracellular H+ gradients.