The alkaline phosphatases present on isolated brush border and basal lateral membranes of rat duodenal epitheilum were examined by means of a variety of biochemical assays and physical methods. The two alkaline phosphatases have similar pH optima of 9.6-9.8, similar substrate km's for p-nitrophenyl phosphate (PNPP) of 7 1 micromolar, similar responses t o the inhibitors 2-mercaptoethanol, theophylline, phenylalanine, and ethylenediaminetetraacetic acid (EDTA), similar sensitivities t o calcium, magnesium, zinc, sodium, and potassium, and similar insensitivities t o digestion with trypsin or papain. The two enzymes also exhibit similar molecular weights on SDS-polyacrylamide gels in the range 124,000-150,000, and both enzymes show an Rf value of 0.092 on Triton X-1 00 polyacrylamide gels, indicating similar intrinsic charges. The Vmax of the brush border enzyme is ten times greater than that of the basal lateral enzyme, 140 pmoles/mg-h as opposed to 14 pmoles/mg-h. The differences in Vma, are a reflection of the known distribution of alkaline phosphatase in rat duodenum, there being more alkaline phosphatase activity present on the brush border than on the basal lateral surface. One other major difference was observed between the two enzymes, the stimulation of the basal lateral and not the brush border alkaline phosphatase by SDS, Triton X-100, or cholate. We conclude that the enzymes are very similar t o one another and probably perform similar membrane functions.
Key words: alkaline phosphatase, basal lateral