The UV fading of hydrocarbon fluorescence and its prevention for observations in single living cells.

Excitation intensities used for standard microspectrofluorometric observations of natural cell fluorescence, i.e. NAD(P)H, lead to fading of hydrocarbon (polycyclic aromatic, heterocyclic) fluorescence in EL2 cells incubated with such compounds. The disappearance of hydrocarbon fluorescence under excitation at 366 nm seems to be an exponential function of time. The fading prevents studies on hydrocarbon metabolization in correlation with intracellular microelectrophoretic injection of substrate, e.g. glucose-6-P. A return to 8-10 times less intense excitation conditions used in an earlier prototype microspectrofluorometer, has allowed the observation of sequential changes in the difference spectra (after glucose-6-P minus before) of hydrocarbon-treated cells (e.g. benzo(a)pyrene, dibenzocarbazols). The possible relative contributions of NAD(P)H and hydrocarbon metabolites (or alterations) to such sequential spectra are still under consideration, but the main obstacle to their observation, fading, is removed by less intense excitation.