Enzymatic production of p-nitrophenol in liver of Piaractus mesopotamicus Holmberg, 1887 was consistently assayed at pH 8.5 using 7.5 mM paraoxon as substrate. This pacu liver paraoxonase activity was activated by NaCl. Apparent values of K M were 2.42 x 10-3 M in the presence of 0.5 M NaCl and 8.99 x 10-3 M without NaCl. Apparent maximum velocity values calculated in the absence and presence of 0.5 M NaCl were 1.09 x 10-3 µmoles/min/mg of proteins and 1.29 x 100-3 µmoles/min/mg of proteins, respectively. These Vmax values are fifty-fold the value described for trout (Salmo trutta Linnaeus, 1758) liver paraoxonase. Paraoxonase activity of pacu liver homogenates was recovered as much in cytosolic as in particulate cellular subtractions, but the particulate subtractions showed higher specific activities. The data presented here indicate that hepatic hydrolysis of organophosphorous pesticides may not be an important detoxification process in pacu.

A paraoxonase activity present in serum of two Brazilian fish species was consistently assayed at pH 8.5 using 7.5 mM paraoxon final concentration. The paraoxonase activity was more activated by 0.5 M NaCl in serum of Piaractus mesopotanricus Holmberg, 1887 (pacu) than in serum of Hypostomus punctatus Valenciennes, 1840 (cascudo). Apparent values of K M were 3.3 x 10-3 M for cascudo and pacu paraoxonase activity in the presence of 0.5 M NaCl. Apparent maximum velocity values calculated in the presence of 0.5 M NaCl were 6.1 and 6.5 nmole/min/mL of serum for cascudo and pacu, respectively. Vmax/K M ratio values of determinations in the presence and absence of 0.5 M NaCl showed that NaCl had a more evident effect on increasing the affinity of serum paraoxonase for paraoxon in pacu serum. Young specimens of pacu showed a marked decreased paraoxonase serum activity when kept in tanks treated with 0.25 ppm methyl-parathion.