Patent ID: 8460862

Claim:
A method of detecting native enzyme activity in acidic organelles, comprising: a) combining a sample that comprises mammalian cells with a solution comprising a substrate of the formula: T −LINK− F ( R )−BLOCK( R ′) wherein T represents a targeting group that is a weakly basic amine containing moiety that partitions the substrate to the acidic organelle; F represents a reporter moiety comprising one or more substituents R independently selected from the group consisting of hydrogen, halogen, alkyl, alkenyl, aryl, and heteroaryl; BLOCK is a monovalent moiety adapted to be cleaved from the remainder of the substrate by action of a specific enzyme native to said acidic organelle, resulting in a visible signal at the site of enzyme reaction; R′ is selected from the group consisting of an unsubstituted carboxylic acid ester and an alkyoxy substituted carboxylic acid ester; LINK has the formula —(CH2) a (CONH(CH2) b ) z , where a is an integer from 0-5, b is 1-5 and z is 1; and b) detecting the presence of a visible signal, wherein said sample is combined with said substrate in a concentration greater than 10 μM and less than 2 mM for a time sufficient for said native enzyme to remove BLOCK and produce said visible signal.