A PHYSIOLOGICAL-FUNCTION FOR ALKALINE-PHOSPHATASE - ENDOTOXIN DETOXIFICATION

Alkaline phosphatase (AP), a common enzyme present in many species inc luding humans, has been studied extensively. Although the enzyme is ro utinely applied as a marker for liver function, its biologic relevance is poorly understood. The reason for this is obvious: the pH optimum of AP in vitro, as measured with the usual test substrates (+/- 10.5), greatly exceeds the physiologic pH range as it occurs in biologic tis sues. We now hypothesize that this relatively high pH optimum in vitro is related to dissociation of acidic groups in the protein preparatio n, which leads to the formation of negatively charged groups in the vi cinity of the active site of the enzyme. These negatively charged grou ps may promote the activity of AP. We examined the possibility that en dotoxin is a natural substrate for this enzyme because this phosphoryl ated substance is able to supply multiple negatively charged residues in the microenvironment of the enzyme at a physiologic pH level. Phosp hate groups in the endotoxin molecule are known to be essential for th e biologic activities of this bacterial product. The present study dem onstrates that in intestinal and renal tissue specimens in vitro, AP i s endowed with endotoxin dephosphorylating activity at pH levels close r to the physiologic range. This is also illustrated by our experiment s in vivo showing that the toxicity of endotoxin is significantly redu ced after exposure to AP preparations, as tested by inducing a local i ntradermal inflammatory reaction in rats. Collectively, our data sugge st that the ubiquitous enzyme AP may accomplish protection against end otoxin, an equally ubiquitous product of Gram-negative bacteria that m ay cause lethal complications after an infection with these micro orga nisms.