Heparin is an anticoagulant drug - it inhibits blood clotting. Its effects may need to be reversed acutely if, for example, a patient is bleeding or a surgical procedure requiring anticoagulation has ended. Protamine sulfate can be used in such situations. Protamine sulfate is a mixture of polypeptides originally isolated from Salmon sperm and present in the sperm of various fish. There, it is believed to play a role in the stabilisation of DNA in the sperm head. These peptides also happen to bind heparin and prevent its action. In higher doses, protamine can have an anticoagulant effect and therefore should be given in the minimum dose required to neutralise the Heparin in the system. Anaphylactic reactions are uncommon but can occur in upto 1% of patients with Diabetes who have received Protamine-containing Insulin and have therefore been sensitised. Trout sperm ('Milt') was first added to Insulin in 1936 by Danish Physician and Pharmacologist Hans Christian Hagedorn who found it prolonged the duration of action. You may be relieved to know that the drug is now synthesised using recombinant DNA technology. This means the gene sequence for the protein is inserted into other organisms such as bacteria. We can then harvest the protein when they start churning it out. Beats collecting fish sperm.
British National Formulary v.54 Brunton et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics 11th edition. | 
