Similar results are reported for Mugil cephalus ( Guizani, Rolle, Marshall, & Wei, 1991) and S. s. caerulea ( Castillo-Yáñes et al., 2005), both with an optimal temperature of 50 °C, and for C. macropomum, with an optimal temperature of 60 °C. The high optimal temperature may be due to the fact that D. rhombeus lives in warm waters, whereas most species analysed thus far live in cold waters. With regard to thermostability, the trypsin from the fish cited proved also to be sensitive to temperatures above 45 °C, which is similar to the results found in the present study ( Fig. CDK and cancer 2D). Kishimura et al. (2008) reported a direct correlation between the temperature of the habitat and
the thermal stability of fish trypsin. The effects of metal ions (1 mM) on the activity of trypsin from D. rhombeus are displayed in Table 1. Enzyme activity was higher than the control (100%) when incubated in the
presence of K+ (34%), Li+ (46%) and Ca2+ (83%). Calcium was shown to be a positive effector for D. rhombeus trypsin. In fact, this ion is known as a classic activator for mammal trypsins. However, Bezerra et al. (2005) and Souza et al. (2007) found that trypsin from the Nile Bcl 2 inhibitor tilapia and spotted goatfish were inhibited by calcium. These results suggest that there are differences in calcium dependence amongst the trypsins from mammal and some fish. The activity of trypsin from the Nile tilapia and spotted goatfish was also inhibited in the presence of Mn2+ and Ba2+, but trypsin isolated from the species analysed in the present study exhibited no traces of enzyme inhibition with these ions. Fe2+, Cd2+, Cu2+ and Al3+ decreased enzyme activity by about 20–35%, whereas Hg2+ and Zn2+ inhibited trypsin activity Thiamine-diphosphate kinase by 53% and 71%, respectively. However, these inhibition values are less expressive than those described for the spotted goatfish. In the presence of Pb2+, there was total inactivation of the trypsin purified from D. rhombeus. Ions such as Cd2+ and Hg2+ are known to act on sulfhydryl residues in proteins ( Aranishi
et al., 1998) and, according to Bezerra et al. (2005), inhibition caused by these metal ions suggests the importance of sulfhydryl residues to the catalytic action of this peptidase. This relevance was also reinforced by the inhibition (approximately 35%) of the D. rhombeus trypsin activity by 2-mercaptoethanol. Moreover, the influence of metals ions or other inhibitory compounds over trypsin activity has been employed as a means to detect xenobiotics in a solution containing commercially available trypsin ( Šafařik et al., 2002). The influence of some synthetic inhibitors on the activity of the enzyme purified from the viscera of the D. rhombeus is displayed in Table 1. The trypsin from D. rhombeus was completely inhibited in the presence of TLCK. Similar results are reported for the Nile tilapia ( Bezerra et al., 2005), bluefish ( Klomklao et al.