If {HEAD ~ the quantitatively determinable substance} contained in the cells of the reagent is {TAIL ~ enzyme} , this reagent further contains a substrate which specifically interacts with the enzyme  
is
If { HEAD ~ the quantitatively determinable substance } contained in the cells of the reagent is { TAIL ~ enzyme } , this reagent further contains a substrate which specifically interacts with the enzyme
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH