Proteinases were among the first enzymes to be investigated biochemi- cally, and purification and crystallization especially of proteolytic enzymes of the digestive tract has contributed much to our present knowledge of enzymic structure and mechanisms of catalysis. However, for a long time little has been known about the functional aspects of proteinases. The only exception from this have been the digestive tract enzymes responsible for extracellular catalysis of protein breakdown and supply of amino acids for new-protein assembly and nitrogen metab- olism in the respective organs. The work...