DEPHOSPHORYLATION OF ENDOTOXIN BY ALKALINE-PHOSPHATASE IN-VIVO

Natural substrates for alkaline phosphatase (AP) are at present not id entified despite extensive investigations. Difficulties in imagining a possible physiological function involve its extremely high pH optimum for the usual exogenous substrates and its localization as an ecto-en zyme. As endotoxin is a substance that contains phosphate groups and i s usually present in the extracellular space, we studied whether AP is able to dephosphorylate this bacterial product at physiological pH le vels. We tested this in intestinal cryostat sections using histochemic al methods with endotoxin from Escherichia coli and Salmonella minneso ta R595 as substrate. Results show that dephosphorylation of both prep arations occurs at pH 7.5 by AP activity. As phosphate residues in the lipid A moiety determine the toxicity of the molecule, we examined th e effect of the AP inhibitor levamisole in vivo using a septicemia mod el in the rat. The results show that inhibition of endogenous AP by le vamisole significantly reduces survival of rats intraperitoneally inje cted with E. coli bacteria, whereas this drug does not influence survi val of rats receiving a sublethal dose of the gram-positive bacteria S taphylococcus aureus. In view of the endotoxin-dephosphorylating prope rties of AP demonstrated in vitro, we propose a crucial role for this enzyme in host defense. The effects of levamisole during gram-negative bacterial infections and the localization of AP as an ecto-enzyme in most organs as well as the induction of enzyme activity during inflamm atory reactions and cholestasis is in accordance with such a protectiv e role.