Modification of peptide structure of hormones take place in:

Traslation of m-RNA leads to the production of preprohormones and it reaches into endoplasmic reticulum. After processing and modification of peptide structure in golgi apparatus the hormone get packaged in secretory granules and is released into blood at the stimulus. So modification pof peptide structure occurs in golgi apparatus. eg: Sulfation of CCK. Ref: SLeisenger and Fordtrans, Edition-9, Page-5