Abstract
Naturally occurring chelators of Ca 2+ and Mg 2+ have largely been unrecognized due to their low binding affinities. They include carbohydrate and cyclitol phosphates, nucleotides and nucleic acids. The calciotrophic inositol phosphates Ins(1,4,5)P 3 and Ins(1,3,4,5)P 4 form chelates within the range of Ca 2+ concentrations found in biological systems. As well as being a likely source of experimental artifact where these compounds have been investigated at unphysiological cation concentrations, chelation may have important physiological roles. The autoregulation of Ca 2+ entry into the cell cytosol is one, whereas the coupling of chelation with enzyme or receptor interactions offers a general mechanism for divalent cation control of diverse biological processes. Inositol monophosphate 1-phosphatase and inositol polyphosphate 1-phosphatase are two related enzymes which may conform to this mechanism. If so, it would provide a possible explanation for their sensitivity to divalent cations and for their non-competitive inhibition by lithium ion.
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