Distribution of soluble and membranous forms of alkaline phosphatase in the small intestine of the rat

Graeme P. Young, Steven T. Yedlin, David H. Alpers

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34 Citations (Scopus)


In the small bowel mucosa of the rat, alkaline phosphatase was found to be present in the 105 000 × g supernatant fraction, in addition to the major brush border membrane-bound form. This soluble enzyme contributed up to 3.6% of total alkaline phosphatase activity in the adult rat. Combined use of inhibitors of serine, thiol and supernatant fraction. In suckling rats a much larger proportion of alkaline phosphates was soluble, reaching 36.5% in the 14-day-old animal. The soluble and membranous forms were compared in adult and suckling rats by examining their biochemical and immunological characteristics. In the adult rat the specific activity of membrane-bound alkaline phosphatase showed a progressive fall distal to the duodenum, while maximal specific activity of the supernatant enzyme occured in the distal ileum. By contrast, in the suckling rat the specific activity of both forms increased from the duodenum to the ileum. Soluble alkaline phosphatases in mature and immature rats were distinct from the membranous forms when examined by polyacrylamide gel electrophoresis in the presence of Triton X-100. Electrophoretic heterogeneity was seen in the soluble adult enzyme, characteristics, Rf values occurring for each level of the small bowel (i.e., duodenum, jejunum and ileum). Such heterogeneity was not seen in suckling rats. Antigenically, the soluble and memebranous froms were identical to each other in adult and suckling rats. Analysis of mucosal cells isolated sequentially frmo tip of the villus to crypt showed that both the soluble and membranous isozymes had highest activity towards the tip of the villus. We conclude that the soluble and membranous forms are distinct biochemically and yet are antigenically identical and arise from the same level of the villus. The presence of the soluble form, which is not an artifact of tissue preparation, indicates that alkaline phosphatase is not exclusively an integral membrane protein.

Original languageEnglish
Pages (from-to)257-265
Number of pages9
JournalBBA - General Subjects
Issue number2
Publication statusPublished - 17 Aug 1981
Externally publishedYes


  • (Rat small intestine)
  • Aging
  • Alkaline phosphatase
  • Membrane-bound enzyme


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