Sentence:  One of the proteases has pH <WSD>optimum</WSD> 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:  most desirable possible under a restriction expressed or implied