Curcumin, the principal curcuminoid in the food spice turmeric, is insoluble in water at room temperature. We have previously solubilized curcumin in water with the application of heat (100 °C) and found that this solubilized curcumin could be used as a gel protein stain. However, heat solubilization in water solubilized only a small fraction of the curcuminoid (1.5%), making it relatively expensive to stain one gel ($1.5-2 per gel). Heat/pressure solubilization (121 °C/15 psi) helps increase the solubility of curcumin in water to only 5%. Therefore, we ventured into increasing curcumin solubility further by using detergents (Tween 20, Tween 80, or Triton X-100) combined with heat/pressure treatment. Curcumin solubilized with Tween 20, Tween 80 or Triton X-100 stained proteins efficiently as before. We could dramatically increase the solubility of curcumin by solubilizing in detergents such that we had to dilute the solubilized curcumin 1:100 with water to stain proteins. However, we had to use the soluble curcumin (extracted with heat/pressure alone) straight without dilution. Thus, the amount of curcumin solubilized with detergents was at least 100-fold, with 0.5% Tween 80 being the most efficient. Staining can be carried out in 30 min, and sensitivity of staining is similar or better than that obtained with Coomassie Brilliant Blue (CBB). It is unnecessary to destain the gel, and excess curcumin can be discarded into the sink, unlike CBB. Curcumin solubilized in Tween 20 has been found to be stable for a period of over 4 years. Therefore, curcumin solubilized in Tween 80 or Triton X-100 can serve as an efficient nontoxic and environment-friendly protein stain. Tween 80 has been used in cough syrups to make a colloidal suspension, and also in foods. Solubilizing curcumin with Tween 80 would be a useful alternative to increase curcumin bioavailability in clinical studies.
Keywords: Coomassie Brilliant Blue; Curcumin; Curcuminoid; Environment-friendly protein stain; SDS-PAGE; Triton X-100; Tween 80.