Aminomethyl coumarin acetic acid: a new fluorescent labelling agent for proteins.

A new fluorescent protein labelling agent, 7-amino-4-methyl coumarin-3-acetic acid (AMCA), emits in the blue region (440-460 nm) on activation with UV light (350 nm). The active reagent is the N-hydroxysuccinimide ester which reacts with lysine residues under mild conditions to form photostable amide links. The Stokes shift of 100 nm compared to 30 nm for Fluorescein isothiocyanate (FITC) allows easy filter discrimination of exciting and emitting radiation. The agent has been demonstrated in use for fluorohistochemical examination of human kidney glomeruli, using the sandwich technique and compared with the same procedure using FITC-labelled antibodies. The good quantum yield coupled with convenient emission lines in the mercury spectrum allows photographic exposure time of fluorescent labelled sections to be reduced to a quarter of that required for a corresponding FITC conjugate. AMCA-immunoglobulin conjugates were not susceptible to photobleaching and have a storage life at -20 degrees C of more than two years.