Interaction of molecular probes with living cells and tissues. Part 2. A structure-activity analysis of mitochondrial staining by cationic probes, and a discussion of the synergistic nature of image-based and biochemical approaches.

Cultured rat fibroblasts were exposed to 41 cationic fluorescent probes of very varied hydrophilicity/lipophilicity. Outcome of probe-cell interaction fell into one of the following categories: probe failed to enter the cells; probe accumulated on cell surfaces; probe accumulated in mitochondria, and/or in other intracellular regions. The observations were analysed using a Simplistic Chinese Box (SCB) approach, and the following conclusions were reached. It was the hydrophilic probes which failed to enter cells, whilst extremely lipophilic probes were retained on the cell surfaces. Only the slightly lipophilic cationic probes were permeant, and accumulated in mitochondria. Using the probes log P values to model hydrophilicity/lipophilicity, effective cationic mitochondrial stains can be specified numerically so: 0 less than log P probe less than +5. This SCB model was used to rationalise a variety of earlier observations on the action of mitochondrial probes. The applicability of the SCB approach to integrate image-based and biochemical investigations was demonstrated by using the action of chlorpromazine on mitochondrial action as a case example.