Assay reagent

An assay reagent for determining the activity of an enzyme in a metabolically active whole cell, said assay reagent comprising at least one water soluble physiologically acceptable salt of an assay compound having the ability to pass through a cell membrane, said assay compound having an unblocked leaving group selected for cleavage by an enzyme to be analyzed and a fluorogenic indicator group being selected for its ability to have a first non-fluorescent state when joined to the leaving group, and a second fluorescent state excitable at a wavelength above 450 nm when the leaving group is cleaved from the indicator group by the enzyme, wherein said fluorogenic indicator group comprises fluorescein and derivatives thereof;

• and wherein said leaving group is at least one selected from the group consisting of amino acids, peptides, saccharides, sulfates, phosphates, butyl ester, chloroacetyl ester, palmityl ester, nucleotides, polynucleotides, nucleic acids, pyrimidines, purines, nucleosides, lipids and mixtures thereof;
• and wherein said assay reagent has a fluorescence less than the auto-fluorescence of a metabolically active cell.