2.4. Tyrosinase Inhibition Assay

To determine the influence of GHK-Cu on tyrosinase activity, the following protocol [51] was used. A solution containing 100 mM sodium phosphate of pH = 6.5, 2 mM L-DOPA and 0.5 mg/cm³ of aqueous GHK-Cu solution or water was used for the analysis for the peptide sample or reference sample, respectively. The concentration of the peptide solution was identical to that used in the liposomes. The reaction was started by adding 0.2 mg/cm³ of tyrosinase solution. The mushroom tyrosinase was used in the study. Changes in the absorbance associated with the formation of dopachrome were measured at 475 nm using a spectrophotometer (Nanocolor UV/Vis, MACHEREY-NAGEL GmbH & Co. KG, Dueren, Germany). The final result was calculated as an average of three measurements. Tyrosinase inhibition (I_T) is expressed via the following Equation (3):

where AR—absorbance of the reference sample: AS—absorbance of the peptide-containing sample.