Compositions and methods for non-invasively monitoring the intracellular processing of a biomolecule are provided. The compositions have a novel thio-luciferin moiety which can be conjugated to the biomolecule directly or via a linker such as disulfide bond wherein the luminescent (such as thio-luciferin) or pro-luminescent (such as thio-cyanobenzothiazoles) moiety is released upon reduction of the disulfide bond upon cytosolic internalization.