The oxygen isotope composition of phosphate released from phytic acid by the activity of wheat and Aspergillus niger phytase

[摘要] Phosphorus (P) is an essential nutrient for living organisms. UnderP-limiting conditions plants and microorganisms can exude extracellularphosphatases that release inorganic phosphate (P_i) from organicphosphorus compounds (P_org). Phytic acid (myo-inositolhexakisphosphate, IP₆) is an important form of P_org in manysoils. The enzymatic hydrolysis of IP₆ by phytase yields availableP_i and less phosphorylated inositol derivates as products. Thehydrolysis of organic P compounds by phosphatases leaves an isotopic imprinton the oxygen isotope composition (δ¹⁸O) of releasedP_i, which might be used to trace P in the environment. This studyaims at determining the effect of phytase on the oxygen isotope compositionof released P_i. 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 P_ireleased 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 sixP_i were released, and one oxygen atom from water was incorporatedinto each P_i. This incorporation of oxygen from water intoP_i 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 P_iwere released. The incorporation of oxygen from water into P_iduring 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.

[学科分类] 地球化学与岩石