I-Gastrin, eyaziwa nangokuthi i-pepsin, iyihomoni yamathumbu ekhishwa ikakhulukazi amaseli e-G e-antrum yesisu ne-duodenum futhi idlala indima ebalulekile ekulawuleni ukusebenza komgudu wokugaya ukudla kanye nokugcina ukwakheka okuqinile komgudu wokugaya ukudla. I-Gastrin ingakhuthaza ukukhishwa kwe-asidi yesisu, ikhuthaze ukukhula kwamangqamuzana e-mucosal emathunjini, futhi ithuthukise umsoco kanye nokunikezwa kwegazi kwe-mucosa. Emzimbeni womuntu, ngaphezu kwe-95% ye-gastrin esebenzayo ye-biologically i-α-amidated gastrin, equkethe ikakhulukazi ama-isomer amabili: i-G-17 ne-G-34. I-G-17 ibonisa okuqukethwe okuphezulu kakhulu emzimbeni womuntu (cishe u-80%~90%). Ukugcinwa kwe-G-17 kulawulwa ngokuqinile ivelu ye-pH ye-antrum yesisu futhi kubonisa indlela yempendulo engemihle ehlobene ne-asidi yesisu.