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Phosphorus (P) is an essential nutrient for living organisms. UnderP-limiting conditions plants and microorganisms can exude extracellularphosphatases that release inorganic phosphate (Pi) from organicphosphorus compounds (Porg). Phytic acid (myo-inositolhexakisphosphate, IP6) is an important form of Porg in manysoils. The enzymatic hydrolysis of IP6 by phytase yields availablePi and less phosphorylated inositol derivates as products. Thehydrolysis of organic P compounds by phosphatases leaves an isotopic imprinton the oxygen isotope composition (δ18O) of releasedPi, which might be used to trace P in the environment. This studyaims at determining the effect of phytase on the oxygen isotope compositionof released Pi. For this purpose, enzymatic assays with histidineacid phytases from wheat and Aspergillus niger were prepared usingIP6, adenosine 5'-monophosphate (AMP) and glycerophosphate (GPO4)as substrates. For a comparison to the δ18O of Pireleased by other extracellular enzymes, enzymatic assays with acidphosphatases from potato and wheat germ with IP6 as a substrate wereprepared. During the hydrolysis of IP6 by phytase, four of the sixPi were released, and one oxygen atom from water was incorporatedinto each Pi. This incorporation of oxygen from water intoPi was subject to an apparent inverse isotopic fractionation(ϵ ~ 6 to 10 ‰), which was similar to thatimparted by acid phosphatase from potato during the hydrolysis of IP6(ϵ ~ 7 ‰), where less than three Piwere released. The incorporation of oxygen from water into Piduring the hydrolysis of AMP and GPO4 by phytase yielded a normalisotopic fractionation (ϵ ~ −12 ‰), similar tovalues reported for acid phosphatases from potato and wheat germ. Weattribute this similarity in ϵ to the same amino acid sequencemotif (RHGXRXP) at the active site of these enzymes, which leads to similarreaction mechanisms. We suggest that the striking substrate dependency of theisotopic fractionation could be attributed to a difference in theδ18O values of the C–O–P bridging and non-bridging oxygen atoms inorganic phosphate compounds

biogeosciences

The oxygen isotope composition of phosphate released from phytic acid by the activity of wheat and &amp;lt;i&amp;gt;Aspergillus niger&amp;lt;/i&amp;gt; phytase