prothrombin: a protein in blood plasma that is the inactive precursor of thrombin
protein: any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat and eggs and milk and legumes
assignee: (law) the party to whom something is assigned (e.g., someone to whom a right or property is legally transferred)