Protamines are proteins derived from the sperm or testes of salmon or related fish. They are added are added to insulins NPH Neutral Protamine Hagedorn and PZI protamine zinc to delay their absorption. The difference between isophane/NPH insulins and PZI insulins is the amount of protamine used; there is more in PZI than in NPH/isophane insulins. It's the extra protamine which makes the PZI a slow-acting insulin. Protamine has other medical uses as well. See also suspension.