Sentence:  <WSD>One</WSD> of the proteases 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:  having the indivisible character of a unit