## 4 Conclusions  
The results suggest that using endogenous substrates may be more appropriate for studying enzyme kinetics, especially when studying enzymes isolated from plant material. This study concluded that oregano extract was the best natural inhibitor to inhibit iceberg lettuce PPO. Moreover, other natural inhibitors such as lovage, marjoram, and orange peel can be used to replace the sulfite-containing agents that could harm human health. Among metal ions, 0.5 mM MgCl\({}_{2}\) and 0.5 mM FeCl\({}_{2}\) decreased enzyme activity by nearly 30%. The addition of metal ion solutions while extending the shelf life of low-processed foods may contribute to the prevention of the most common nutritional deficiencies. The obtained research results are very promising and it is possible that, in the future, they will allow us to limit the use of synthetic preservatives. Moreover, as a result of the obtained results, it is possible to more precisely determine the kinetic constants in the reaction of PPO with the endogenous substrate and to more precisely determine the type of inhibition.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Inhibitor** & \multicolumn{3}{c}{**Relative Activity [\%] \({}^{*}\)**} \\ \cline{2-4}
**Metal Ions** & \multicolumn{3}{c}{**Concentration (in the Reaction Mixture) [mM]**} \\ \cline{2-4}  & **0.5** & **1** & **2** \\ \hline MgCl\({}_{2}\) & 72 \(\pm\) 4 g & 100 \(\pm\) 5 efg & 123 \(\pm\) 6 defg \\ \hline KCl & 82 \(\pm\) 4 fg & 100 \(\pm\) 5 fg & 130 \(\pm\) 6 defg \\ \hline ZnCl\({}_{2}\) & 97 \(\pm\) 5 fg & 123 \(\pm\) 6 defg & 197 \(\pm\) 10 bcd \\ \hline FeCl\({}_{2}\) & 73 \(\pm\) 4 g & 178 \(\pm\) 24 cde & 266 \(\pm\) 13 ab \\ \hline CuCl\({}_{2}\) & 118 \(\pm\) 6 defg & 210 \(\pm\) 10 bc & 316 \(\pm\) 16 a \\ \hline NaCl & 83 \(\pm\) 4 fg & 112 \(\pm\) 6 fg & 125 \(\pm\) 6 defg \\ \hline CaCl\({}_{2}\) & 79 \(\pm\) 4 fg & 115 \(\pm\) 6 efg & 161 \(\pm\) 8 cdef \\ \hline \hline \end{tabular} \({}^{*}\)100% activity of enzyme--activity of enzyme without inhibitors, value lower than 100%—enzyme with inhibitors, value higher than 100%—some metal ions act as activators of PPO. Means (\(\pm\)standard deviation) with different letters are statistically significant (\(n=9\); \(\alpha=0.05\), 2-way ANOVA).  
\end{table}
Table 4: Effect of selected metal ions on the iceberg lettuce PPO activity.  
**Author Contributions:** Conceptualization, M.S.; methodology, M.S.; software, M.S.; validation, M.S.; formal analysis, M.S. and M.S.; investigation, M.S. and M.S.; resources, M.S.; data curation, M.S.; writing--original draft preparation, M.S.; writing--review and editing, M.S.; visualization, M.S.; supervision, M.S. and M.S.; project administration, M.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the National Science Centre, grant number 2020/37/N/NZ9/03286, and the APC was funded by the National Science Centre.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors upon request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.