## 3 Results and Discussion
The most popular PPO inhibitors are ascorbic acid and citric acid. Their mechanisms of action are mainly based on decreasing pH. Therefore, these compounds have a negative effect on the texture and taste of food. Moreover, our previous studies have shown that the acidification of low-processed vegetables could stimulate the growth of molds and yeasts [8]. Food preservation without the use of chemicals is becoming more and more popular. Consumers consciously eliminate products preserved with synthetic preservatives from their diets. This is due to the increased care for one's health, but also the desire to care for the natural environment. It is worth noting that the cost of producing and utilizing natural water extracts is relatively low compared to pure chemicals. Moreover, the application of natural plant extracts can have a positive effect on human health, because they are a rich source of low-molecular-weight antioxidants. Natural plant extracts, especially received from herbs such as basil, marjoram, lavage, or oregano, are accepted by consumers because they have a positive effect on the taste and aroma of iceberg lettuce and are commonly used in the seasoning of salads. Because of the above, other inhibitors, especially of natural origin, are wanted. [PERSON] et al. [31] showed that the addition of _Rosa roxburghii_ juice effectively improved the appearance of apple juice by inhibiting the enzymatic browning and PPO activity. In turn, [PERSON] and [PERSON] [10] used waste material such as grape seeds. Their research showed that the treatment of fresh-cut lettuce (_L. sativa_) with extracts received from grape seeds could cause limited darkening (lower values in lightness values during storage). Our studies have shown that wheat bran from common wheat (_Triticum aestivum_ L.) and orange peel (_Citrus sinensis_), also being a waste material, could effectively inhibit the activity of PPO isolated from iceberg lettuce. [PERSON], [PERSON], and [PERSON] (2018) [32] confirm that natural anti-browning agents (honey, red onion extract, red chili pepper extract, and pineapple juice) have the potential to be used to control the browning of ginger as well as other vegetables and fruits. Kinetic constants (K\({}_{\rm m}\) and V\({}_{\rm max}\)) are calculated for six substrates using the Lineweaver-Burk plot. According to the Km values, as a determinant of the enzyme's affinity for its substrate, the PPO enzyme exhibited the highest affinity pyrocatechol (K\({}_{\rm m}\) = 2.39 mM) and 4-methylocatech (K\({}_{\rm m}\) = 10.01 mM). In connection with the above for further studies, pyrocatechol was selected as a substrate for iceberg lettuce PPO. Our results are in accordance with [PERSON], [PERSON], and [PERSON] [33], who reported that the broccoli PPO had the highest affinity to catechol and 4-methylcatechol. Generally, to detect the PPO activity among various plants, catechol and 4-methylcatechol are usually used as a substrate. Moreover, [PERSON] [34] showed that the K\({}_{\rm m}\) value for PPO isolated from artichoke and for catechol was 10.2 mM min\({}^{-1}\), and that for 4-methylocatech was 12.4 mM min\({}^{-1}\). The results from the Lineweaver-Burk plots (Table 3) received in the results reaction carried out with pyrocatechol indicated that, among the examined functional solutions, the extracts of oregano and wheat bran act according to the competitive model, the extract of lemon peel represents a non-competitive inhibition type, and the parsley and orange peel represent the uncompetitive type, while the extracts of basil and lovage, and marjoram represent the mixed type. In turn, L-cysteine and ascorbic acid represent competitive types of inhibition, and kojic acid is mixed. In turn, reactions carried out with an endogenous substrate showed that the orange peel extract represented a competitive mode of inhibition, the oregano extract uncompetitive, and the lovage extract non-competitive.