## 3 Results and Discussion  
The present study was designed to investigate the unexplored mechanism of selected fluoroquinolone drug-enzyme interactions and their binding patterns, critical for melanin-dependent responses and important for the cosmetic, medicine and agriculture industries. Our result confirmed that all the selected antibiotics (Figure 2) i.e., ciprofloxacin, enoxacin sesquihydrate, gemifloxacin, levofloxacin, moxifloxacin, ofloxacin and sparfloxacin showed excellent tyrosinase inhibitory activity with IC50 \(\pm\) SEM values 37 \(\pm\) 2, 28 \(\pm\) 4, 34 \(\pm\) 2, 39 \(\pm\) 2, 50 \(\pm\) 1.9, 40 \(\pm\) 2 and 30 \(\pm\) 2.8 \(\upmu\)M, respectively. Interestingly, all the drugs outperformed the standard hydroquinone (IC\({}_{50}\) = 170 \(\upmu\)M). Additionally, nonfloxacin was tested for comparison, and it was found to be a tyrosinase inhibitor with IC\({}_{50}\) = 820 \(\upmu\)M. The selected fluoroquinolones are known for their broad-spectrum antibacterial activity. These drugs were selected due to their excellent bioavailability, easy synthesis, low toxicity, ability to kill different clinical pathogens and, most importantly, they have been used as antibiotics for a long time. The fluoroquinolones are synthetic fluorinated analogues of naldixic acid, which are important for cosme, medicine and agriculture [41; 42; 43]. A cyclopropyl moiety at position 1 (ciprofloxacin, gemifloxacin, moxifloxacin, and sparfloxacin) is considered to be the most potent modification against anaerobic bacteria [44], and any bulky substituent at position 7 (gemifloxacinic and enoxcine) is very close to the topoisomerase IV binding site, resulting in a lower level of microbiological activity [45; 46]. Moreover, 3-carboxylate (ciprofloxacin, levofloxacinic, moxifloxacinic, ofoflaxicine and sparfloxacinicine), fluoro groups (enoxcine and gemifloxacinic) and 4-carbonyl groups are important for antimicrobial activity [47]. Substituents at position 5 (sparflaxacine) change the overall steric configuration of the molecule. The addition of an amino group, as in the case of sparfloxacine at position 5, can remarkably increase the in vitro activity against Gram-positive bacteria and _Toxoplasma gondii_[47; 48]. The addition of fluorine at position 6 (ciprofloxacin, levofloxacin, moxifloxacin, ofloxacinicine and sparfloxcine) is used widely nowadays, due to their clinical importance, which improves antimicrobial activity as compared to the original quinolone compound [49]. Any kind of substitution at positions 1, 7 and 8 plays a key role in the overall biological activities of the compounds under development [50]. It was observed that enoxacin exhibited the most potent tyrosinase inhibitory activity, with the lowest IC\({}_{50}\) of 28 \(\upmu\)M. Position 8 is important, as it affects the steric configuration, similarly to position 5, by substitution, accessing the binding sites of enzymes [51]. Thus, the presence of an ethyl group at position 1 plays a very important role in tyrosinase inhibitory activity, as it was shown to have higher activity than the reference hydroquinone. The role of the 1-cyclo propyl group in ciprofloxacin and 1,8 cyclo compounds in ofloxacinand levofloxacin was also studied, which exhibited a higher tyrosinase inhibition potential than mooxifloxacin, but lower than gemifloxacin. Hence, the IC\({}_{50}\) values of all the tested drugs were found to be 3-6 times lower than the standard hydroquinone. An important observation is that sparfloxacin exhibits comparable inhibition activity to the most potent enoxacine drug, which is because of the bulkiness at position 7, and the bulk here also increases the anti-anaerobic activity [52]. It is worth noting that antibiotics already on the market show excellent anti-tyrosine activity, which may give them an advantage over any new synthetic drug that requires a medical trial before considerations. Their interactions, binding patterns and dose-dependent effect on melanin have been studied widely [26; 27; 28; 29; 30; 31; 32].