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a protein of low molecular weight that occurs in the nuclei of spermatozoa in fish and birds. Its molecular weight ranges from 4,000 to 12,000.
The basic properties of protamines are determined by their characteristically high content of basic amino acids, especially arginine (70–80 percent). Protamines are readily soluble in water and in acid and neutral media. They are precipitated by alkalies and do not denature when heated. The protamines that have been studied are mainly those found in mature fish spermatozoa, where they constitute a fraction of the basic protein but nearly the entire nuclear protein. The amino-acid composition of protamines is specific for each species of fish.
In cell nuclei, protamines, like histones, are associated with deoxyribonucleic acids to form nucleoprotamines. X-ray diffraction analysis has shown that the protamine chain is the third strand coiled around the DNA double helix. Protamines form salts in the presence of acids and complexes with acid proteins; in medicine, the slightly soluble complex of protamine and insulin is used to prolong the effectiveness of the latter.