Predicting the behaviour and selectivity of fluorescent probes for lysosomes and related structures by means of...

Cultured rat fibroblasts were exposed to 50 fluorescent probes of varied physicochemical characteristics. Probe concentrations, fluorochrome excitation wavelength and period of illumination, and cell-probe contact time were varied. Structure-activity relationships defining a number of classes of fluorescent probes for lysosomes and related processes and compartments were demonstrated. Numerical specifications are now available for several familiar classes of probes: (a) acidotropic weak bases, used as markers for low pH compartments; (b) markers of adsorptive pinocytosis, involving non-specific protein binding; (c) markers for fluid phase pinocytosis; and (d) viability stains involving intralysosomal enzymic activity. Two novel classes of probes have also been specified numerically: (a) acid-precipitated weak acids, as markers for low pH compartments; and (b) lipid-binding markers of adsorptive pinocytosis. Overall, these structure-activity models provide a tool for predicting whether or not compounds enter cells; and whether they accumulate in lysosomes and related compartments. Pathways of entry are also predicted. This tool should permit design and selection of improved probes, and provide a better understanding of existing reagents. Moreover these models are expected to be applicable to interactions between any non-polymeric xenobiotic with lysosomes and related compartments.