Cardiolipin in the spotlight: Quantitative analysis and fluorescence-based competitive binding assay

Cardiolipin (CL) is a key phospholipid responsible for mitochondrial function and cristae integrity. The CL level is associated with various diseases characterized by mitochondrial dysfunction, including ischemic heart diseases and cancer. CL is an attractive target for mitochondria-specific drugs, but unnecessary interaction with CL might lead to detrimental side effects such as heart failure and kidney dysfunction. Thus, a simple and robust method for CL quantification and a reliable assay for the determination of drug affinity for CL are desired. We report a new fluorescent CL-specific probe with impressive photophysical properties that allows CL quantification in mitochondrial fractions isolated from cell and tissue homogenates and enables estimates of drug affinity for CL in the first fluorescence-based competitive binding assay. It was found that CL concentration is elevated in mitochondrial fractions isolated from cancer cells and cells with high proliferation rate (up to 108.5 ± 16.0 nmol/mg prot in mouse colon carcinoma cells, CT-26). CL concentration in mitochondria from brain tissue (66.11 ± 5.78 nmol/mg prot) is circa twice higher than in heart and kidney mitochondria (37.49 ± 8.69 and 33.95 ± 5.32 nmol/mg prot, respectively.) Generally, positively charged substances bind with CL, but their affinity is highly variable with EC₅₀ values ranging from sub-micromolar to millimolar concentration.