Sentence:  One of the <WSD>proteases</WSD> has pH optimum of about 3.7 and another of about 5.7 ( McQuillan , Stanley and Trikojus , 1954 ; Alpers , Robbins and Rall , 1955 ) . 
Meaning of the word marked with <WSD> tags:  any enzyme that catalyzes the splitting of proteins into smaller peptide fractions and amino acids by a process known as proteolysis