There are no well-known endogenous proteins in mouse heart tissue around 70 kDa that have intrinsic fluorescence at Excitation 488 nm. However, there are some possibilities to consider:

1. Autofluorescent Molecules

• Lipofuscin: A common autofluorescent pigment in aged tissues, can fluoresce at 488 nm (broad spectrum emission). However, it is not a single protein and accumulates with age.
• Flavoproteins: These electron transport proteins (like ETF, ~68 kDa) have flavin cofactors (FAD, FMN) that fluoresce around 488 nm.

2. Potential Proteins Near 70 kDa

• Albumin (~67 kDa): Present in blood and heart tissue but weak intrinsic fluorescence.
• Heat Shock Proteins (HSP70, ~70 kDa): Can be found in heart tissue under stress conditions but typically don’t fluoresce significantly on their own.
• Myosin Light Chains (MLC): Some isoforms are close to 70 kDa, but they do not fluoresce naturally.
• NADH-dependent proteins: Complex I (~75 kDa subunits) has NADH-related fluorescence (~460 nm emission, weaker at 488 nm).

3. Post-translational Modifications & Cofactors

• Some proteins may bind fluorescent cofactors (like NADH, FAD, or FMN) that can be excited at 488 nm.
• Oxidized flavoproteins have fluorescence in this range.

Would you like me to refine this based on a specific application (e.g., microscopy, FACS, Western blot)?