The process as claimed in claim 2, in which is used the preproinsulin of the formula I (SEQ ID NO: 4) in whichR1 is Phe, R2 is hydrogen or a peptide with 2 to 30 natural amino acids and with C-terminal L-arginine at the end, R3 is a hydroxyl group, a natural amino acid or a peptide with 2 to 10 natural amino acids, X is L-arginine or a C chain of human, porcine or bovine proinsulin, Y is Thr, Z is