Intestinal alkaline phosphatase in the gastrointestinal tract of fish: biology, ontogeny, and environmental and nutritional modulation

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Abstract
Intestinal alkaline phosphatase is an evolutionarily conserved system that is pivotal in controlling gut and systemic inflammation. Its main properties, essentially discovered in terrestrial mammals, include regulation of enterocyte luminal surface pH and barrier function, detoxication of pro‐inflammatory microbial components, modulation of the gut microbiota and control of nutrient absorption (e.g. calcium, phosphorus, fatty acids). In return, enzymatic activity is modulated by gut microbes, cytokines, dietary nutrients and bioactive food components. Numerous data have been published on intestinal alkaline phosphatase in fish, especially in the contexts of biological development, nutrition and environment for sustainable aquaculture. This review provides a considerable account of basic and applied knowledge pertaining to this important protective enzyme in fish, and concludes that many features of intestinal alkaline phosphatase in fish fit those reported in terrestrial mammals, especially in its modulation by nutrients and bioactives in food. Notably, food intake is a strong inducer of intestinal alkaline phosphatase activity and is often not reported in publications, thus leading to some erroneous conclusions. Gaps in knowledge, e.g. on enzyme isoforms and molecular mechanisms involved in physiological and immunological (inflammatory) responses to biotic and abiotic factors, exist in fish and need to be considered. Future work should take into account food intake and both enzymatic activities as well as isoform expressions in order to further characterise in fish the important defensive functions of this highly conserved intestinal enzyme.

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