 The difficulty in purifying proteins in large quantities made them very difficult for early protein <unk> to study . Hence , early studies focused on proteins that could be purified in large quantities , e.g. , those of blood , egg white , various toxins , and digestive / metabolic enzymes obtained from <unk> . In the 1950s , the Armour Hot Dog Co. purified 1 kg of pure <unk> <unk> ribonuclease A and made it freely available to scientists ; this gesture helped ribonuclease A become a major target for biochemical study for the following decades . 
