THE EFFECTS OF HEXAVALENT CHROMIUM ON THE ACTIVITY OF ALKALINE-PHOSPHATASE IN THE INTESTINE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

This work is a contribution to the study of the effects of hexavalent chromium on intestinal alkaline phosphatase activity in fish. The enzyme activity was studied in intact tissues of rainbow trout (Oncorhynchus mykiss), incubated at 25-degrees-C with paranitrophenyl phosphate. Substrate hydrolysis has an optimum at pH 10 and is linear with time up to, at least, 1 h. Preincubation at a basic pH in the absence of substrate for 30 min or more, leads to a decrease of the enzyme activity. This inhibition is lower when phosphate is added to the preincubation medium. The mucosal layer has a special influence on V(max) which is higher when the intestinal tissue is blotted on filter paper. Chromium dichromate leads to a decrease of the enzyme activity when the tissue is first preincubated in the absence of substrate. This inhibition is dependent upon the metal concentration up to 1 mM. At higher concentrations an attenuation of this effect has been shown. Preincubation at alkaline pH stimulates the effect of chromium. The inhibition is time dependent. However, for longer preincubations (60 min) the chromium effect seems to be progressively reversed. Mucus does not protect the enzyme from the metal. Dichromate ions have a special influence on V(max) and a minor one on the K(m) of the enzyme. The substrate (paranitrophenyl phosphate) effectively protects the enzyme even at low concentrations (0.18 mM). Phosphate has the same effect but especially al a high concentration (100 mM). After a reduction by ascorbate, chromium does not inhibit phosphatase activity. The inhibition has been thought to be due to the oxidation of specific molecules. The role of SH groups has been postulated since DTT protects the enzyme, and partially restores its activity after treatment with chromium.