CELLULAR ACTIONS OF INOSITOL PHOSPHATES AND OTHER NATURAL CALCIUM ANDMAGNESIUM CHELATORS

Naturally occurring chelators of Ca2+ and Mg2+ have largely been unrec ognized due to their low binding affinities. They include carbohydrate and cyclitol phosphates, nucleotides and nucleic acids. The calciotro phic inositol phosphates Ins(1,4,5)P-3 and Ins(1,3,4,5)P-4 form chelat es within the range of Ca2+ concentrations found in biological systems . As well as being a likely source of experimental artifact where thes e compounds have been investigated at unphysiological cation concentra tions, chelation may have important physiological roles. The autoregul ation of Ca2+ entry into the cell cytosol is one, whereas the coupling of chelation with enzyme or receptor interactions offers a general me chanism for divalent cation control of diverse biological processes. I nositol monophosphate 1-phosphatase and inositol polyphosphate 1-phosp hatase 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 i on.