Tissue carboxylesterase activity of rainbow trout

The activity of carboxylesterase (CaE), a class of nonspecific serine hydro lases, was evaluated in vitro in tissues and microsomes of rainbow trout an d compared to esterase activity in rats, other fish species, and embryo to adult life stages of trout. Trout gill and liver microsomes exhibited subst antial CaE activity and limited variation over the range of 2 to 40 degrees C, with a temperature optimum of approximately 22 degrees C. Trout sera an d rat liver microsomes exhibited a temperature optimum of approximately 35 to 40 degrees C. The CaE of trout liver (maximum reaction rate [V-max] = 67 2 nmol/min/mg microsomal protein) was four times less than in rats. Apparen t Michaelis constant (K-m) values ranged from 28 (trout liver) to 214 (trou t sera) mu M. Values of V-max/K-m suggested that in vivo CaE activity of tr out liver would be about three times higher than serum, 135 times higher th an gill, and three times lower than rat liver. The CaE activity in whole ra inbow trout homogenates significantly increased 300% per gram of tissue to 1,200% per milligram of protein between the yolk-sac and juvenile stages. T he CaE activity of whole fish homogenates was not significantly different i n juvenile rainbow trout, channel catfish, fathead minnows, and bluegill. T he results demonstrate that rainbow trout had high esterase activity over a broad range of temperatures, that CaE activity significantly increased bet ween the yolk-sac and juvenile life stages, and that variation between the CaE activity in trout and three other families of freshwater fish was limit ed. The CaE activity In fish is expected to substantially influence the acc umulation and toxicity of pesticides and other esters entering the aquatic environment.