At the end of the 19th century, it became clear that biochemical processes do not necessarily require living cells, but are driven by special substances, enzymes, formed in cells. However, it remained to be seen how pure forms of these enzymes could be isolated. After James Sumner succeeded in isolating a pure form of urease, John Northrop carried out additional research into other enzymes. Around 1929 he managed to produce pure crystals of pepsin, trypsin and chymotrypsin enzymes, which are active in the digestive process. John Northrop's research showed that these enzymes are also proteins.