In situ biochemical studies of intestinal alkaline phosphatase in normal and phosphate-depleted rats by microdensitometry.

A quantitative microdensitometric study has been designed to characterize in situ intestinal brush border-bound alkaline phosphatase of rat duodenal villosities. Intestinal slices were incubated with beta-glycerophosphate as substrate. Free phosphate liberated was precipitated in presence of a lead reagent as lead sulfide. The precipitate was quantified in situ by scanning and integrating microdensitometry. Kinetic parameters of the reaction were determined at 37 degrees C, pH 8.8, in the middle part of the villosities. Apparent Michaelis constant (Km) for beta-glycerophosphate was found to be 8.16 +/- 0.56 mM (mean +/- S.E.). Maximal enzyme activation was obtained at pH 8.5. Maximal inhibition of enzyme activity was observed in the presence of L-phenylalanine (30 mM) or theophylline (5 mM). Along the villosity axis, enzyme activity rose from the crypt up to the midportion of the villosity and finally decreased at the tip region. In phosphate-depleted rats, enzyme activity was increased in all portions of the villosity, with conservation of the same activity gradient. In this situation, kinetic analysis showed a marked decrease of Km, i.e. 4.56 +/- 0.39 mM (mean +/- S.E.) as compared to normal rats.