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